romanian fusion research activities

Association EURATOM / MEdC

ANNUAL REPORT 2007

Romanian Fusion Research Activities

Institute of Atomic Physics Fusion Research Unit

2007 Annual Report of the EURATOM-MEdC Association

CONTENTS

EXECUTIVE SUMMARY ES-1 FUSION PLASMA PHYSICS • Interpretation and control of helical perturbations in tokamaks 1 • Statistical physics for anomalous transport in plasmas 8 • Anomalous transport in turbulent plasmas 16 • Tokamak neutron diagnostics based on the superheated fluid detector (SHFD) 24 • Sheath properties and related phenomena of the plasma wall interaction in

magnetised plasmas. Application to ITER. 28 • Upgrade of gamma-ray cameras – neutron attenuators 34 • Gamma-ray spectrometry 44

UNDERLYING TECHNOLOGY • Evaluation of the irradiation effects on passive/active components of optical fiber systems

for control and sensing 48 • The development of MOD-TFA precursors for the deposition of thick YBCO films on

metallic substrates for the superconducting coated conductors 53 EFDA TECHNOLOGY WORKPROGRAMME 2007 • Development of calculation tools: Calculation of cross sections for

50,52Cr up to 60 MeV 59 • ITER-like Wall Project at JET: Optimization and Manufacturing of 10 μm W-coatings for the CFC tiles to be installed in JET 65 • Update of ITER ISS-WDS Process Design – 2 71 • Production of Beryllium Coatings for Inconel Cladding and Beryllium Tile Markers

for the ITER-like Wall project 76 • Characterization of fuel retention in ITER relevant mixed materials. 83 • Development of an Inside-Gap Plasma Generator for wall cleaning applications 89 • Be coatings of CFC and W EU targets for exposure to ITER-relevant Type I ELM,

disruption and mitigated disruption loads in plasma-gun facilities 96 • Endurance tests for WDS components 101

Publications in scientific journals & Contributions to conferences and workshops 111 Contact information 118

2007 Annual Report of the EURATOM-MEdC Association ES-1

EXECUTIVE SUMMARY 1. Introduction

The national frame for the fusion research activities is the National Programme for International Collaboration “CORINT” of the National Plan for Research Development and Innovation of the Ministry of Education and Research (MEdC) for the period 2000-2008.

All the fusion research activities carried out in Romania in the frame of the European Fusion Programme is mainly financed by MEdC and partly by EURATOM.

The Association EURATOM/MEdC was established in 25 December 1999 when the Contract of Association between EURATOM and MEdC was signed. At present the following contracts between Euratom and MEdC are extended to the end of 2007: the Contract of Association, the European Fusion Development Agreement, the JET Implementing Agreement and the Staff Mobility Agreement.

The Fusion Research Unit is the Institute of Atomic Physics with research groups in the National Institutes for Physics and the Universities participating in the European Fusion Programme as follows: the National Institute for Laser, Plasma and Radiation Physics (NILPRP), Magurele-Bucharest, the "Horia Hulubei" National Institute of R&D for Physics and Nuclear Engineering (IFIN-HH), Magurele-Bucharest, the National Institute of R&D for Cryogenics and Isotope Technologies (ICIT), Ramnicu Valcea, the University of Craiova (UCv), Craiova, the Technical University of Cluj-Napoca (TUCN), Cluj-Napoca and the “Al. I. Cuza” University (UAIC), Iasi. The research activities of the Association are directed by the Steering Committee, that comprises the following members in 2007: Chairman: Yvan.Capouet, EU Commission, Research DG Members: Francesca Sinischalchi, EU Commission, Research DG Steven Booth, EU Commission, Research DG Dan Popescu, Nuclear Agency Gheorghe Mateescu, “Horia Hulubei” National Institute of R&D for Nuclear Physics and Engineering Voicu Lupei, National Institute for Laser, Plasma and Radiation Physics Head of Research Unit: Theodor Ionescu Bujor, Institute of Atomic Physics The Steering Committee had one meeting in 2007 on 3 October. The Romanian Members in the EU Fusion Committees: Consultative Committee for the Euratom Specific Research and Training Programme in the Field of Nuclear Energy-Fusion (CCE-FU): T. Ionescu Bujor - Institute of Atomic Physics M. Chis - Ministry of Education and Research-National Authority for Scientific Research

EFDA Steering Committee:

Olivia Comsa - Ministry of Education and Research-National Authority for Scientific Research

M. Chis - Ministry of Education and Research-National Authority for Scientific Research

ES-2 2007 Annual Report of the EURATOM-MEdC Association

2. Research activity in 2007

The 2007 Annual Report of the Association EURATOM/MEdC presents our main results obtained in the frame of the European Fusion Programme in a number of 18 tasks as follows: Fusion Plasma Physics (8), Underlying Technology (2), Technology Tasks 5.1a (4), Technology Tasks 5.1b (4).

In the framework of the Fusion Plasma Physics our Association contributes to:

- Theoretical and numerical studies of MHD stability and plasma control (Resistive Wall Modes);

- Statistical physics for anomalous transport;

- The anomalous transport in turbulent plasmas;

- A neutron diagnostics technique based on the super-heated fluid detectors (SHFD’s or “bubble detectors”) has been successfully tested at JET on various types of discharges during Campaigns C17-C19. It provided new information about the following characteristics of the neutron field at the end of the KM11 line-of-sight: fluence, beam profile, broadband energy distribution. This technique for determination of the neutron field characteristics technique should be applicable for high performance discharges (neutron yields of 5x1016). It is proposed to be used in the next campaigns together with other two different and independent methods: bubble detectors, neutron activation and time-of-flight spectrometry.

- Numerical investigation on the formation of the floating space-charge sheath in the Pilot-PSI plasma and obtaining the floating potential of the target

- The design activities for the JET KN3 Gamma-Ray Cameras (KN3 GRC) have been continued with the scheme design phase where the drawing of assemblies and parts were produced.

A fully functional neutron attenuator prototype was manufactured and tested. The system, as a whole, performed according to the specifications. Integrity tests proved the neutron attenuator casing is suitable (from the mechanical point of view) for use.

Techniques for the reconstruction of the radiation profiles provided by the JET KN3 neutron/gamma-ray cameras have been successfully and tested.

The work on the KM6T tangential gamma-ray spectrometer upgrade continued with a conceptual design of the full diagnostics system.

In the framework of the EFDA technology workprogramme some contributions are related to:

- A suitable description of all activation cross sections for the Cr stable isotopes, with a good agreement of the calculated cross sections with the more recent data between 14 and 21 MeV has been obtained.

- The new W-coatings facilities for 10 μm W coating of CFC tiles required for the JET relevant large scale production and quality control, based on the Combined Magnetron Sputtering and Ion Implantation (CMSII) technology was designed, manufactured and commissioned. The HHF tests carried out at IPP Garching in GLADIS machine, at power densities up to 23.5 MW/m2 for 1.5 s and cycling loading at 16.5 MW/m2 for 2s,

2007 Annual Report of the EURATOM-MEdC Association ES-3

on the first lot of samples, proved that the CMSII method for 10 μm W coating can be successfully applied at the industrial scale.

- The Block diagrams were developed in the Piping and Instrumentation Diagram application from the CATIA V5 software, having as reference the process diagrams from the DDD _32_E report [2], DDD_32_B report [1] and FMEA report .A 3D layout of the WDS and ISS systems in the building has been developed and plot plans were generated with equipment arrangement for each floor.

- The 8 μm Beryllium layers deposited on Inconel samples by thermal evaporation method were exposed to high power load in JUDITH machine. The tests carried out in the range from 0.4 MW m-2 to 2.6 MW m-2 in pulses lasting of up to 11 s proved no damage (melting or exfoliation) caused by energy loads exceeding at least three times the level characteristic for a regular plasma operation.

- The HHF tests of the Beryllium marker samples, produced by TVA method, carried out in JUDITH machine proved that the markers survived without noticeable damage power loads of 4.5 MW m-2 for 10 s (energy density 45 MJ m-2) and fifty repetitive pulses performed at 3.5 MW m-2 each lasting 10 s, i.e. corresponding to the total energy deposition of 1750 MJ m-2 .

- High quality beryllium films on tungsten, graphite and CFC substrates using thermionic vacuum arc method were prepared to be used to investigate deuterium retention.

- Be coatings of 10 μm & 100 μm performed on: CFC and W TARCAR target plate parts, CFC coupons and W disk samples were characterized at FZJ Juelich by e-beam loads and will be the basis of the final technical specifications for the coatings of the EU W and CFC targets.

- A laboratory scale Inside-Gap Plasma Generator (IGPG) setup for wall cleaning applications, compatible with scanning operations, was designed and built-up. Experiments performed on a castellated assemble proved that the cleaning process is effective, the removal rate depending on deepness: the cleaning proceeds faster at the upper gap margins (about 0.24 μm/min for the first 2 mm), where in fact the co-deposited layers on real tokamak tiles are thicker.

3. Visit of Dr. Jerome Pamela, EFDA Leader, to the EURATOM / MEdC Association

On Monday 12 March 2007 Dr. Jerome Pamela visited Romanian Euratom Association in Magurele. A welcome meeting was organized in Magurele, at the Faculty of Physics, with Dr. Aldea Alexandru, Vice-President of the National Authority for Scientific Research, Prof. Stefan Antohe, the Dean of The Faculty of Physics and T. Ionescu Bujor, Head of Research Unit.

In the Scientific session organized in the Conference Room of the Faculty of Physics Dr. Pamela presented a very interesting invited talk “Fusion Research-new horizons”. Also, Romanian participants to the European Fusion Programme presented the last results obtained in ten tasks.

Dr. Pamela visited also five laboratories of the National Institute for Laser Plasma and Radiation Physics involved in JET tasks.

ES-4 2007 Annual Report of the EURATOM-MEdC Association

4. Mobility Actions of the Staff of our Association

In the framework of the Mobility Agreement 22 scientists were seconded to the EURATOM partners: JET (16), Université Libre de Bruxelles (5), CEA (3), FZK (1), FOM (3), OAW (2).

5. The 4th Association Days Meeting

In this year the Days of the EURATOM/MEdC Association took place in Ramnicu Valcea on 1-2 October at the National Institute of R&D for Crayogenics and Isotope Technologies (ICIT). The invited guests at this meeting were: Dr. Yvan Capouet, Head of Unit and Dr. Steven Booth from the European Commission, Directorate-General for Research, Dr Francesco Romanelli, EFDA Associate Leader for JET and Daniele Carati, Head of Research Unit-ULB. They participated with very interesting invited talks, chaired sections of the meeting and visited the Tritium Pilot Plant and the ICIT laboratories.

This two days meeting was devoted to fifteen oral presentations of Romanian research groups involved in the European Fusion Programme. They reported on the last results obtained during 2007 in the fields: Physics, Underlying Technology and Technology Tasks.

The session was a very good opportunity for participants to meet and discuss with our guests.

6. The expenditure and the staff of the Association

In 2007 the eligible expenditure was as follows:

• Physics: 463,065 Euro of which 254,074 Euro – JJET Notification;

• Underlying Technology: 75,000 Euro;

• Technology Tasks (5.1a): 300,035 Euro;

• Technology Tasks (5.1b): 304,094Euro;

• Art.6.3 Orders: 269,692 Euro.

The Staff of the Association presented a slowly decrease because a few technological tasks were completed in 2006.

Staff of the Association 2000 - 2007

010

203040

506070

8090

2000 2002 2004 2006

Professionals

Non-professionals

2007 Annual Report of the EURATOM-MEdC Association 1

INTERPRETATION AND CONTROL OF HELICAL PERTURBATION IN TOKAMAKS

C.V. Atanasiu, A. Moraru

National Institute for Laser, Plasma and Radiation Physics, Magurele

During the period January-December 2007, the theoretical and modelling research activity of the “Mathematical Modelling for Fusion Plasmas Group” of the National Institute for Laser, Plasma and Radiation Physics (NILPRP), Magurele - Bucharest, Romania was focalized on:

• Plasma models for feedback control of helical perturbations (Resistive wall modes) This activity was performed in collaboration with the Max-Planck - Institut für Plasmaphysik (IPP), Tokamakphysics Department, Garching, Germany, and represents a continuation of our activity from 2006.

1. Resistive wall modes

It is known that the maximum achievable β in "advanced tokamaks" is limited by the pressure gradient driven ideal external-kink modes (10-6s). When a tokamak plasma is surrounded by a close fitting resistive wall, the relatively fast growing ideal external kink modes (EKM) is converted into the far more slowly growing "resistive wall mode" (RWM) which grows on the characteristic RLw /=τ time of the wall and has virtually identical stability boundaries to those

of the EKM in the complete absence of a wall. Note that the stabilization of RWM in ITER, where it is probably not possible to maintain a very fast plasma rotation is still an open problem.

The objective of our common research was the Development of plasma models for feedback control of helical perturbations. To accomplish this objective, we have considered as necessary to develop a 2D numerical model for the RWM investigation, consisting of a real axisymmetrical tokamak model with arbitrary cross-section and plasma parameters, and with 3D poloidal and toroidal disposals of wall (in its thin wall approximation).

1.1 The 2D numerical model

For this case, we have considered a 2D axisymmetrical equilibrium with a 3D perturbation given by an m/n=3/2 external kink mode.

The following milestones have been achieved:

• description of the vacuum field given the normal component of the perturbed field on the plasma boundary for a 2D axisymmetrical tokamak;

• calculation of the surface current corresponding to a given plasma mode;

• calculation of the patterns of the induced eddy currents in the resistive wall.

In contrast to the usual ideal MHD treatment with the extended energy principle, the present analysis is generally non-selfadjoint and includes the necessary perturbed magnetic pressure for satisfying pressure balance. One solving method could be to found a new basis of orthogonal eigenvectors to ensure the self-adjointness property of the energy operator - the normal mode approach [1-2], or to replace a perfect conducting wall with one of finite

Atentie 2 2007 Annual Report of the EURATOM-MEdC Association

conductivity and to deduce a modified energy principle [3]. Numerical MHD stability studies in the presence of toroidal rotation, viscosity, resistive walls and current holes, by using the CASTOR FLOW code are presented in Ref. [4]. Modelling of RWMs has been made by using the VALEN code [5] with an equivalent surface current model for the plasma. Extensive study and theoretical development has also been presented by Bondeson and his co-workers [6-7] by using the MARS code. In a first stage, we have adopted the second approach [3], based on the assumption that the plasma displacement trial function, in the situation where a conducting wall located at finite distance from the plasma is considered to be identical to the one used in the evaluation of the potential energy when the conducting wall is moved infinitely far away. We considered such assumption too limiting and have developed another approach.

Writing the expression for the potential energy in terms of the perturbation of the flux function, and performing an Euler minimization, we have obtained a system of ordinary differential equations in that perturbation [8]. This system of equations describes a tearing mode or an external kink mode, the latter if the resonance surface is situated at the plasma boundary. Usually, vanishing boundary conditions for the perturbed flux function at the magnetic axis and at infinity are considered. From single layer potential theory, we have developed an approach to fix "natural" boundary conditions for the perturbed flux function just at the plasma boundary, replacing thus the vanishing boundary conditions at infinity [8]. Now, in the presence of a resistive wall, the boundary conditions of the external kink mode at the plasma boundary are determined by the reciprocal interaction between the external kink perturbation of the plasma and the toroidal wall (in its thin wall approximation). A general toroidal geometry has been considered. By using the concept of a surface current [9], the description and calculation of the influence of the modes outside the plasma were greatly simplified. The normal component of the magnetic field perturbation, at the plasma boundary, has been considered as excited by toroidally coupled external kink modes and it is that component that gives the normal to the wall component of the exciting field causing the wall response via the induced eddy currents.

Our results are devoted to diverted tokamak configurations and will be applied to the ASDEX Upgrade, JET and ITER tokamaks.

General expression of the potential energy

By writing, in a coordinate system with straight field lines (a, θ, ζ) the general expression of the potential energy W in terms of two test functions u(a, θ, ζ) and λ(a, θ, ζ)

( ) ( ) (( ) , ( ) , ( ) ,i m N i m N i m Nm m m

m m mY a e u U a e a eθ ζ θ ζ θ ζ )ψ λ

∞ ∞ ∞− −

=−∞ =−∞ =−∞

= = = Λ∑ ∑ ∑ −

instead of the displacement ξ, one obtains the expression of the potential energy [8]:

0

1 ( , , , , , , , , )2

rikW dad d f g u p F Jθ ζ ψ λ

μ= Ψ Φ∫ (1)

where ψ is the perturbation of the flux function Ψ, are the metric coefficients, Φ is the

toroidal flux, u and

rikg

λ correspond to the perpendicular and parallel displacements, respectively, while are the pressure, poloidal and toroidal current densities, respectively. After developing the perturbed values in Fourier series, performing an Euler minimisation of the energy functional and then integrating with respect to the angles θ and ζ the result of that minimisation, one obtains a system of coupled ordinary differential equation of the form

, ,p F J


where f , V and G are matrices, and Y is the flux function perturbation vector, with the non-diagonal terms representing both toroidicity and shape coupling effects. Close to the magnetic axis (a→0), we have found the following behavior of the amplitude of the flux function perturbation

where am/n is the resonance radius corresponding to the wave numbers m and n. We have to consider a "natural" boundary condition just at the plasma boundary [8]. To our knowledge, we were the first to develop the methodology to consider, in a flux coordinates system, the boundary conditions just at the separatrix,. From potential theory we know that a continuous surface distribution of simple sources extending over a not necessarily closed Liapunov surface ∂D and of density σ(q), generates a simple-layer potential at p, in ∂D. After some tedious calculations, the boundary condition becomes

with I the unit matrix and h the "radial" integration mesh, D and F [M x M] complex matrices, with the elements given by the metric coefficients, normal and tangential magnetic field components. αk is a known [M x M] coefficient matrix and βk a known [M] coefficient vector, both resulting from a forth-order Runge-Kutta integration scheme. For unit perturbations Y2/1 (m = 2, n = 1) and Y3/2 (m = 3, n = 2), the corresponding surface charge distributions are given in Fig.1.

Figure 1. The surface charge distribution along the plasma boundary for unit flux perturbations Y2/1 and Y3/2, respectively. The plasma configuration of the ASDEX Upgrade tokamak corresponding to the shot no. 13476 at 5.2 s has been considered. Determination of the vacuum field due to a helical perturbation

The perturbation field B=grad Φ outside the plasma can be assumed to be produced

by the surface current equations


where means the jump across the plasma surface. κ(θ, ζ, t) being the time-dependent stream function of the surface current, and g an arbitrary vector. In terms of the external and internal magnetic scalar potentials (with respect to the plasma boundary) one has

The normal component of the perturbed magnetic field has been considered as excited

by a flux function perturbation ψ of unit amplitude Y(1) on the plasma boundary and resulting from an external kink mode. BBn can be calculated with the relation

Determination of the diffusion equation in the wall

In an orthogonal curvilinear coordinate system (u, v), with hu and hv the Lamé coefficients, the diffusion equation for the eddy current stream function U(u, v, t) in a thin wall looks like [10, 11]

(8)

1 ( ) ( )ext

v u ss

u vu v

h U h U BUu h u u h v d t th h

μσ σ∂ ∂⎡ ⎤∂ ∂ ∂− =⎢ ⎥∂ ∂ ∂ ∂ ∂ ∂⎣ ⎦+ n∂

with the initial and the boundary conditions

U(u, v, t) = 0, F(u, v, Ut, Uu, Uv, t) = 0. (9)

Considering the following input data: d = 10-3 m, μ = 4π10-7 H/m, σv = 107 1/Ω/m, σs = 104 1/Ω, for a toroidal wall geometry with holes, the function U(x, y, t) at different time, excited by an m=n =3/2 external kink mode in a thin wall with holes is presented in Fig. 2 and Fig. 3.

Figure 2. Perturbed magnetic field and stream function U of the induced eddy currents given by an EKM.


0

0.5

1

0 1 2 3 4 5

U(x,y,t) - m/n=3/2 at t=0. s

0

0.5

1

0 1 2 3 4 5

U(x,y,t) - m/n=3/2 at t=0.16 s

0

0.5

1

0 1 2 3 4 5

U(x,y,t) - m/n=3/2 at t=0.36 s

0

0.5

1

0 1 2 3 4 5

U(x,y,t) - m/n=3/2 at t=0.56 s

0

0.5

1

0 1 2 3 4 5

U(x,y,t) - m/n=3/2 at t=0.76 s

0

0.5

1

0 1 2 3 4 5

U(x,y,t) - m/n=3/2 at t=1. s

Figure 3. The eddy current stream function U(x, y, t) at different time, excited by an m/n =3/2 external kink mode in a thin wall with arbitrary holes.

Numerical treatment of holes singularities in time dependent problems

Special attention has been given to the accurate calculation of the influence of the eddy currents on the boundary conditions of the system of equations describing the RWM. If for an elliptical type problem, this is a “classical” task, this is not for a parabolic one.

We have developed a method for treating singularities which occur in solutions of parabolic partial differential equations due to sharp corners in the boundary. This method is used in conjunction with the simple explicit finite-difference scheme and subsequently the overall method is explicit. The standard finite-difference in such a neighbourhood was replaced by a truncated series representation of the exact solution at points close to the corner. The coefficients of this truncated series are estimated at each time step in terms of the solution values at points where the influence of the singularity is neglected and which have been derived by an explicit finite-difference scheme from the previous time step.

On the left hand side of Fig. 4, the dependence of the relative error of the numerical solution Vn with respect to the analytical one Va for different numbers of discrete meshes of the domain N are given. On the right hand side of the same figure, the relative error with removal of the corners singularities is reported for the same numbers of discrete meshes.


Figure 4. On the left hand side, the relative errors distribution around a re-entry corner for different numbers of discrete meshes with respect to the analytical solution without correction. On the right hand side, the relative errors distribution around a re-entry corner for the same numbers of discrete meshes but with analytically removed singularity.

This work has been carried out in close collaboration with our German colleagues

from the Tokamak Physics Department of the Max-Planck-Institut für Plasmaphysik, during the mobility 27.07.07-05.10.07 at IPP Garching and at our home institute NILPRP. Part of this work has been performed in cooperation with Dr. L.E.Zakharov from PPPL, Princeton.

Next steps:

• to introduce, according to our semi-analytical model, feedback coils and detector sensors;


• to consider a toroidal wall with gaps and holes, compatible with the geometry of the ASDEX Upgrade and JET tokamaks;

• to continue the investigation of different dissipation mechanisms in the plasma.

References

[1] Chu M.S., Chan V.S., Chance M.S., et al., “Modelling of feedback and rotation stabilization of the resistive wall mode in tokamaks”, Nuclear Fusion, 43, 196 (2003).

[2] Chu M.S., Chance M.S., Glasser A.H. and Okabayashi M., “Resistive wall stabilization of high-beta plasmas in DIII---D’’, Nuclear Fusion, 43, 441 (2003).

[3] Haney S.W. and Freidberg J.P., “Variational methods for studying tokamak stability in the presence of a thin resistive wall “, Physics of Fluids B 1, 1637 (1989).

[4] Strumberger E., Günter S., Merkel P. et al, “Numerical MHD stability studies: toroidal rotation, viscosity, resistive walls and current holes”, Nuclear Fusion 45, 1156 (2005)

[5] Bialek J., Boozer A.H., Mauel M.E. and Navratil G.A., “Modeling of active control of external magnetohydrodynamic instabilities“ Physics of Plasmas 8, 2170 (2001).

[6] Bondeson A., Ward D., “Stabilization of external modes in tokamaks by resistive walls and plasma rotation”, Physical Review Letters, 72, 718 (1994).

[7] Liu Y.Q. and Bondeson A., “Active Feedback Stabilization of Toroidal External Modes in Tokamaks“, Physical Review Letters 84, 907 (2000).

[8] Atanasiu C.V., Günter S., Lackner K., et al., “Linear tearing modes calculation for diverted tokamak configurations”, Physics of Plasmas 11, 5580 (2004).

[9] Atanasiu C.V., Boozer A.H., Zakharov L.E. et al., “Determination of the vacuum field resulting from the perturbation of a toroidally symmetric plasma”, Physics of Plasmas 6, 2781 (1999).

[10] Atanasiu C.V., Günter S., Moraru A., et. al., “Resistive Wall Modes Stabilization in the Presence of 3D Wall Structures”, EPS34, Warsaw, Poland, 2-6 July 2007.

[11] Atanasiu C.V., Günter S., Moraru A., Zakharov L.E., “Resistive Wall Modes Investigation in the Presence of 3D Wall Structures”, EFTC12, Madrid, Spain, 24-27, 2007.

[12] Neu R., Balden M., …, Atanasiu C.V., et al, “Plasma wall interaction and its implication in all tungsten divertor tokamak“, Plasma Physics and Controlled Fusion, 49, 12 B, B59 (2007).

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STATISTICAL PHYSICS FOR ANOMALOUS TRANSPORT IN PLASMAS

Florin Spineanu, Madalina Vlad

Plasma Theory Group, National Institute of Laser, Plasma and Radiation Physics, Magurele

1. Overview

The main research topics for this year comprise two objectives: the development of a model for the enhanced plasma resistivity due to generation of entanglement of the magnetic flux tubes and the study of nonlinear effects of the ExB drift in turbulent tokamak plasmas. Our research program was focused in 2007 on five milestones with strong theoretical and experimental relevance. We present below the two objectives with their milestones and the main results. The detailed description of the results is presented in Section 2.

Objective 1: Model for the enhanced plasma resistivity due to generation of entanglement of the magnetic flux tubes

Milestone 1

Analytical method for the evaluation of the linking number generated by local fluctuation of the magnetic helicity.

Milestone 2

Calculation (in the drift-kinetic framework) of the enhanced resistivity in the magnetic stochastic regions by evaluating the effect of curvature related to the topological linking of the magnetic lines.

This research is a development of the original ideas proposed by the Plasma Theory Group for the quantitative description of the constraints imposed by the topology of the magnetic field configuration to the phenomenon of magnetic reconnection.

The main results

1. We have described the topological constraints on the stochastic magnetic configuration when a transient increase of helicity occurs in a finite plasma volume. Via bounds related to the magnetic energy that can be stored in that volume (i.e. statistical stationarity can be attained) a scaling was derived between the helicity and the average curvature of a generic magnetic line in the volume.

2. The particles' curvature-drift induces a new dissipation. We have estimated the contribution of this mechanism to the resistivity and we have shown that it is not substantial. However the new instruments that imply the topology of magnetic field are useful for quantifying the trapping of a magnetic line and for determining the reduction of the classical magnetic diffusion.

Objective 2: Nonlinear effects of the ExB drift in turbulent tokamak plasmas

Milestone 1

Effect of trajectory trapping on density and impurity peaking.


Milestone 2

Analytical method for evaluation of the transport coefficients of test particles in electromagnetic turbulence.

Milestone 3

Assessment of the effect of trajectory trapping on structure generation in turbulent confined plasmas and calculation of the nonlinear growth rate of drift modes.

We have shown that the ExB stochastic drift produces strong non-linear effects in the regime characterized by trajectory trapping. There is memory in the motion, which determines a class of anomalous diffusion regimes. Trajectory quasi-coherent structures are produced. An average velocity is produced in turbulent plasmas through a ratchet-type process due to the space variation of the confining magnetic field [1]. These effects have been found by means of the semi-analytical statistical methods that we have developed (the decorrelation trajectory method [2] and the nested subensemble approach [3]).

These results were developed in three directions contained in the milestones of this objective.

The main results

1. We have developed the model for the ratchet pinch in turbulent magnetically confined plasmas by introducing particle collisions and an average velocity Vd that describes plasma poloidal rotation. Strong effects on the ratchet velocity and on the peaking factor were shown to appear. Large peaking factors (of the order of the experimental ones) for impurities and for plasma density are produced only through nonlinear effects at large Kubo numbers and in the presence of a weak poloidal rotation with a velocity smaller than the amplitude of the stochastic ExB velocity.

2. The decorrelation trajectory method was extended to the study of test particle transport in electromagnetic turbulence. We have obtained strong effects on both electrons and ions in the nonlinear regime. The magnetic component of the field has a decorrelation effect for the ions that leads to the increase of the diffusion coefficient. The effect on the electrons is to reduce the transport coefficient due to the stochastic magnetic islands that appear and trap the electrons.

3. The connection between the trajectory trapping and the structure generation in confined plasmas was analysed. We have shown that the ExB drift in turbulent plasma determines trajectories that have a high degree of coherence and long time memory. They form transitory structures with characteristics that depend on the turbulence. We have determined the effect of these structures on the growth rate of drift modes on turbulent plasma and we have shown that the evolution to large scale is produced by the ion trajectory structures. A different perspective on the inverse cascade is obtained. It does not appear as wave-wave interaction but as the effect of ion ExB motion on the drift wave stability.

These results are presented in 8 publications [P1]-[P8].

2. Results obtained in 2007

2.1. Model for the enhanced plasma resistivity due to generation of entanglement of the magnetic flux tubes

The energy of the stochastic magnetic field is bounded from below by a topological quantity expressing the degree of linkage of the field lines. When the bound is saturated one can assume that the storage of a certain magnetic energy requires a minimal degree of topological


complexity. It is then possible to infer a connection between the helicity content and the average curvature of the magnetic field lines. The random curvature induces random drifts leading to an additional dissipation and modified resistivity [P1]. Extensions to statistical properties of linking were analysed.

When the Chirikov criterion is verified for several chains of magnetic islands (developing at closely neighbour resonant magnetic surfaces in a volume of plasma) the magnetic field becomes stochastic. In general the magnetic stochasticity is taken into account in transport processes due to the high efficiency of energy spreading through the stochastic region. However from the point of view of the structure of the magnetic field it is difficult to say anything more than we know from Hamiltonian chaotic systems: there is a stochastic (exponential) instability, local Lyapunov exponents and Kolmogorov length and the test particles move diffusively or have various sub- and supra-diffusive behaviours.

However in a stochastic region the field must still obey some constraints. These constraints arise from the relation between the energy stored in the magnetic field and the topological complexity of the field. The constraints can be briefly expressed in this way: it is not possible to support a certain energy in a volume spanned by transiently stochastic magnetic field lines if these magnetic field lines do not have a certain minimal degree of topological complexity.

This should be seen in relation with the equation that expresses the topological link in terms of writhe and twist and in relation with the dynamics of a twisted flux tube. If an initial amount of link is stored exclusively as twist, then beyond a certain level of twist the flux tube deforms and acquires writhe, thus distributing the higher amount of link into the two kinds of topological deformations: twist and writhe. In plasma free from strong magnetic background (as in astrophysical plasma or solar corona) generation of writhe means a coiling or super-coiling instability, a large spatial deformation. In a tokamak the stochastic flux tubes are also subject to the writhing instability when a local fluctuation of the parallel electric field occurs, but they are more constraint by the confining and cannot perform large spatial displacements. Instead, as

a result of small deformations originating from local writhing (coiling) instability, they will reconnect such that, from elements of tubes, effectively new strings are created, with a new effective entanglement. It is reasonable to assume that these new, episodic, flux tubes, by their mutual linking, satisfy on the average the energy-topology constraints. Therefore we assume that the field flux tubes inside the stochastic region reconnect to generate transiently configurations that exhibit a certain topological entanglement. Together with the dynamical nature of the stochasticity phenomena, the formation of these entangled structures is transient and we may suppose that the higher topological content results from a statistical average. At large time the topological reduction occurs with suppression of relative linking via tube merging, a process called by Parker topological dissipation.

0B

The curvature of magnetic flux tubes induces drifts of particles. Electrons and ions flowing along curved magnetic lines will have opposite drifts and local charge separations produce random transversal electric fields. For a finite collisionality this is a source of additional dissipation. We have estimated the negative current perturbation due to curvature.

2.2 Nonlinear effects of the ExB drift in turbulent tokamak plasmas

2.2.1. Effect of trajectory trapping on density and impurity peaking

We have developed an alternative model for particle pinch in tokamak plasma based on test particle approach. We have shown that an average velocity is produced in turbulent plasmas


through a ratchet-type process due to the space variation of the confining magnetic field [1]. We have shown that the inhomogeneity of the confining magnetic field determines a directed transport (an average velocity), although the average ExB drift velocity is zero.

We have developed this model for the direct transport in turbulent magnetically confined plasmas by introducing particle collisions and an average velocity Vd that describes plasma poloidal rotation. These two processes are always present in plasmas and thus it is important to know their effects. Studies of the transport for constant magnetic field have shown that both collisions and average velocity have very strong effects and determine anomalous diffusion coefficients, which appear in a turbulence with K>1 in the condition when particle eddying motion persists. The average velocity and the diffusion coefficients were determined using our semi-analytical statistical approach, the decorrelation trajectory method. The combined action of both perturbations was determined. Strong effects on the ratchet velocity and on the peaking factor were shown to appear. We have shown that the collisional displacements lead to a correlation between the stochastic potential and the inhomogeneity of the magnetic field, which increases the effects of collisions on the ratchet pinch. It also determines the change of the direction of the ratchet pinch at large Kubo numbers. This work is submitted for publication [P2].

The ratchet pinch appears in the frame of test particle approach and thus we determine the effects of the non-homogeneity of the magnetic field on particle trajectories. We note that additional effects appear when a density of particles is considered: the divergence of the velocity determines the modulation of the density along trajectories and the concentration in the regions where the divergence of the velocity is negative. This density concentration produces a pinch velocity, the curvature or turbulent equipartition pinch. This effect does not appear in the test particle approach. We have studied the relation between these two pinches and between test particle and scalar field advection approaches. We have determined the average velocity produced by the gradient of the magnetic field on the density and we have shown that it is a combination of the ratchet and curvature pinch. We have determined the effect of ratchet pinch on the density (passive scalar) and we have extended the studies of passive scalar transport to the nonlinear regime. The preliminary results were presented at “Festival de Theorie”, Aix-en-Provence [P3] and at the 49th Annual Division of Plasma Physics Meeting [P4].

2.2.2. Analytical method for evaluation of the transport coefficients of test particles in electromagnetic turbulence

We have studied in previous work test particle transport both in electrostatic and magnetic turbulence. The decorrelation trajectory method was extended to the study of test particle transport in electromagnetic turbulence. A numerical code was developed, which determines the diffusion coefficients for electrons and ions for given spectrum of the electromagnetic field. The results show strong effects on both electrons and ions in the nonlinear regime. The magnetic component of the field has a decorrelation effect for the ions that leads to the increase of the diffusion coefficient. The effect on the electrons is the decrease of the diffusion coefficient. This is produced due to the stochastic magnetic islands that appear at large magnetic Kubo numbers and consists in solenoidal segments of the magnetic lines. The magnetic islands have a trapping effect that determines the decrease of the diffusion coefficient. The perturbed magnetic lines also determine for the electrons an effective parallel decorrelation time that depends on the magnetic Kubo number.

We have developed in collaboration with Université de Provence, Marseille a model of Hasegawa-Wakatami type for the electromagnetic turbulence in non-homogeneous magnetic field. A computer code for the simulation of the turbulence is developed. Detailed comparison


of the numerical and analytical results will contribute to better understanding these nonlinear effects on the transport.

2.2.3. Assessment of the effect of trajectory trapping on structure generation in turbulent confined plasmas and calculation of the nonlinear growth rate of drift modes

Detailed statistical information about particle trajectories was obtained using the nested subensemble method [3]. This method determines the statistics of the trajectories that start in points with given values of the potential. This permits to evidence the high degree of coherence of the trapped trajectories. Their average displacement, dispersion and probability distribution function saturate in a time τs. The time evolution of the square distance between two trajectories is very slow showing that neighbouring particles have a coherent motion for a long time, much longer than τs. They are characterized by a strong clump effect with the increase of the average square distance that is slower than the Richardson law. These trajectories form structures, which are similar with fluid vortices and represent eddying regions. The statistical parameters of these structures (size, build-up time, dispersion) were determined. In time dependent potentials the structures with τs> τc are destroyed and the corresponding trajectories contribute to the diffusion process. The average size of the structures S in a time dependent potential was determined as function of the Kubo number K. For K<1 the structures are absent (S~0) and they appear for K>1 and continuously grow as K increases.

These results obtained for the statistics of test particle trajectories were used for the study of test modes on turbulent plasmas by developing a Lagrangian approach that takes into account trajectory trapping. Studies of plasma turbulence based on trajectories were initiated by Dupree and developed especially in the seventies. These methods do not account for trajectory trapping and thus they apply to the quasilinear regime or to unmagnetized plasmas. A very important problem that has to be understood is the effect of this non-standard statistical behaviour of the test particle trajectories on the evolution of the instabilities and of turbulence in magnetized plasmas. We have studied linear test modes on turbulent plasma for the drift instability in slab geometry with constant magnetic field. The combined effect of the parallel motion of electrons (non-adiabatic response) and finite Larmor radius of the ions destabilizes the drift waves. We consider a turbulent state of the plasma with known statistical characteristics of the electrostatic potential. The perturbations of the electron and ion distribution functions are obtained from the gyro kinetic equation as integrals along test particle trajectories of the source terms determined by the density gradient.

The background turbulence produces two modifications of the equation for the linear modes. One consists in the stochastic ExB drift that appears in the trajectories and the other is the fluctuation of the diamagnetic velocity. Both effects are important for ions while the response of the electrons is approximately the same as in quiescent plasma. The average propagator of the modes is evaluated using the above results on trajectory statistics. In the first order it depends on the size S(K) of the structures. The trajectory trapping process has a complex influence on the mode. The ion trajectory structures (the quasi-coherent component of their motion) determine the S-dependent exponential factor in the frequency ω. Its effect is the displacement of the unstable k-range toward small values. The random component in the ion motion determines a diffusive damping term in the growth rate γ that produces the stabilization of the large wave numbers. The fluctuations of the diamagnetic velocity determine the last term in the growth rate. The tensor Rij contributes to the growth of the modes.

A different perspective on the inverse cascade is obtained. It does not appear as wave-wave interaction but as the effect of ion ExB motion on the drift wave stability. Namely, the quasi-coherent motion of the trapped ions produces the destabilization of the modes with


wavelengths of the order of the average size of the trajectory structures. This decreases the frequency of the modes and produces the increase of the size S. The non-standard statistical properties of the trajectories, namely the formation of trajectory quasi-coherent structures, are shown to be associated with order and structure formation in turbulent magnetized plasmas. These results are presented in [P5] and [P6].

3. Collaborative actions

3.1 Collaboration with EFDA JET

We have continued the collaboration with EFDA JET included in the Transport Task Force initiated last year. The main topics and results are presented below.

T-3.3.2. Determination of parametric dependences of ITG and TEM thresholds

• We have participated to the identification of physical conditions appropriate for the experimental study of the threshold for the onset of Ion Temperature Gradient driven turbulence. In particular, the examination of the role of poloidal rotation in the threshold value. Considering the fast variation of the linear growth beyond the threshold we have started an analytical study of the effect of fluctuations (small scale turbulence) on the precision with which the threshold can be identified. The effect of plasma rotation has been included. A paper is in preparation.

• Evaluation of the free-energy source available for the density pinch in the stationary regime. We have derived the general expression of the energy for a stationary vortex in two-dimensional plasma [P7, P8]. We proved that there are neighbouring states of lower energy, which are accessible if the system is weakly driven by vorticity input. We have proved that a minimum energy is reached when the vortex is quasi-singular, with all vorticity concentrated on the magnetic axis. This provides a new contribution to the density pinch, independent of turbulence.

T-3.3.3. Study of impurity transport and control

• We have participated at the experiments on the study of impurity transport and control using He3.

• We have developed our pinch model based on a ratchet type process, which includes particle collisions and poloidal average flows, by taking into account the mass-charge dependence of the collisional diffusivity and its anisotropy. The anisotropy is determined by the Larmor radius dependence on the R dependent toroidal magnetic field. A new program based on the decorrelation trajectory method was developed and tested. An interesting effect was obtained that consists in the change of the sign of the pinch velocity from inward to outward at large Kubo numbers.

• A series of calculations for the range of parameters that are of interest for JET plasmas provide the following preliminary results:

- The ratchet pinch is compatible with the Z dependence obtained in experiments, including the change of sign.

- The absolute values of the peaking factor are smaller due to the effective diffusion that has values larger than in experiments.

After the Campaigns, this work was continued by improving the treatment of collisions in the frame of the decorrelation trajectory method. A correlation between the stochastic


potential and the space dependent magnetic field was shown to appear due to collisions. This determines a stronger effect of collisions on the pinch velocity. Also, the relation between the ratchet pinch and the curvature pinch, both determined by the gradient of the magnetic field, was understood. These theoretical results are presented in two papers, one is submitted for publication and the other is in preparation.

Secondments:

- Dr. F. Spineanu at JET for 28 days (22/01/2007 – 16/02 /2007).

- Dr. M. Vlad at JET for 28 days (22/01/2007 – 16/02 /2007).

3.2 Participation at ITM-Project 4: Microinstabilities and Turbulence.

We have proposed the study of the following topics in the ITM-Project 4.

1. Fast particle turbulent transport

We have found that, in some conditions, turbulence induced transport at the very large Larmor radii corresponding to fusion produced particles can be important. This happens when the turbulence has slow time variation that corresponds to large Kubo numbers. We think that this problem is very important for ITER and we have discussed it with several participants to TF T and ITM-Project 4. We have begun a series of studies with the aim of better understanding this fundamental process. We have studied the effect of the poloidal rotation of plasma on fast particle turbulent transport. A code was developed based on decorrelation trajectory method that includes the parallel motion, and preliminary results were obtained.

2. Density peaking induced by the dynamics of the vorticity

We have developed the idea (which is proposed for collaborative work) that the self-organization of the vorticity of the background equilibrium plasma is as important as turbulence (which has captured most of the present-day preoccupations). Density pinch, momentum transport, impurity accumulation or expelling and profile resiliency are signatures of the fact that the background plasma reaches a quasi-coherent state, with a balance of drive and turbulent transport [8], [9]. We have derived a differential equation that should describe the vorticity equilibria.

The experimental verification is proposed, consisting of observation of the large scale (not turbulent) vorticity dynamics in various conditions.

3.3 Collaboration with CEA Cadarache and Universite de Provence

3.3.1. Effect of trajectory trapping on density and impurity peaking

We have continued the collaboration on this topic, which started in the year 2005. We have developed our model for average velocities produced in turbulent plasma, the ratchet pinch.

The main results of this collaboration are presented in Section 2.1.

Mobility Secondment:

- Dr. M. Vlad at Universite de Provence – CEA Cadarache for 68 days (4/05-10/07/2007)

3.3.2. Calculation of the enhanced resistivity due to the higher topological content of the stochastic magnetic fields in tokamak plasmas

We have developed explicit analytical procedures for the evaluation of the effect of the topological properties of the field, based on the concept of invariant magnetic helicity. This


allows the investigation of the bound in the energy of the magnetic field connected with the total topological link in the plasma volume. We have estimated the amount of link that is generated when a quantity of energy is injected into a volume of plasma. Connecting the geometry of a line intersecting a surface and the link between that line with respect to the board of the surface we have estimated the curvature of the line and its relation with the total helicity. The difference between the electron and ion curvature drifts represents a current and actually induces an effective turbulent resistivity which we have calculated.

Mobility Secondment:

- F. Spineanu at Universite de Provence – CEA Cadarache for 87 days (3/10-30/12/2007)

References

[1] M. Vlad, F. Spineanu, S. Benkadda, “Impurity pinch from a ratchet process”, Phys. Rev. Letters 96 (2006) 085001.

[2] M. Vlad, F. Spineanu, J.H. Misguich, R. Balescu, Phys. Rev. E 58 (1998) 7359.

[3] M. Vlad, F. Spineanu, “Trajectory structures and transport”, Phys. Rev. E 70 (2004) 056304.

List of publications in 2007

[P1] F. Spineanu, M. Vlad, “Helicity fluctuations, generation of linking number and effects on resistivity”, International Review of Physics (IREPHY) 1 (2007) 65. [P2] M. Vlad, F. Spineanu, S. Benkadda, “Collisions and average velocity effects on ratchet pinch”, Physics of Plasmas, submitted 2007. [P3] M. Vlad, F. Spineanu, S. Benkadda, “Ratchet versus curvature pinch”, Festival de Theorie, Aix-en-Provence, 2007. [P4] M. Vlad, F. Spineanu, S. Benkadda, „Ratchet and curvature pinch in turbulent plasmas”, 49th Annual Division of Plasma Physics Meeting, November 2007, Orlando, Florida. [P5] M. Vlad, F. Spineanu, “Trajectory trapping and structure generation in turbulent magnetized plasmas”, 49th Annual Division of Plasma Physics Meeting, November 2007, Orlando, Florida, oral presentation. [P6] M. Vlad, F. Spineanu, ”Test particles and test modes in plasma turbulence”, Annals of the University of Craiova, special issue in honor of R. Balescu (2007). [P7] F. Spineanu, M. Vlad, S. Benkadda, „The basic evolution of the angular momentum density in a field-theoretical model of vorticity transport”, 49th Annual Division of Plasma Physics Meeting, November 2007, Orlando, Florida. [P8] F. Spineanu, M. Vlad, “The large scale two-dimensional stationary vortex in magnetized plasma” Annals of the University of Craiova, special issue in honor of R. Balescu (2007).


ANOMALOUS TRANSPORT IN TURBULENT PLASMAS

Gy. Steinbrecher*, N. Pometescu*, M. Negrea*, Dana Constantinescu**, Iulian Petrisor*

*Department of Physics, University of Craiova, Craiova **Department of Mathematics, University of Craiova, Craiova

Results obtained in 2007

1. ELM modelling. (Provision of support to the advancement of the ITER Physics Basis)

Reduced models of ELM’s provide a possibility to obtain statistical information on rare, intermittent and large amplitude events, where ab-initio numerical simulations of large tokamaks require huge parallel computing resources to collect reliable statistics. Intermittent events in ELM are related to heat load problems on plasma facing components in large tokamaks.

Milestone 1.1: ELM modeling by stochastic differential equations, study of intermittence effects.

The results of previous heuristic results on the mechanism of on-off intermittency from the work S. Aumaitre et all, P.R.L. 95, 064101 (2005) was generalized with rigorous proofs, by using the results from [1]. From the new model [2-4] a new stochastic mechanism for noise–driven relaxation oscillations, including ELM’s was obtained. In this framework, the exact form of the asymptotic behavior of the stationary probability density was obtained and the study of the stochastic linear stability analysis was started. New mechanism for generation of the coherent structures was proposed and stochastic numerical approximation for the collision term in gyrokinetic codes was obtained. The work was done in collaboration with Dr. X. Garbet from C.E.A. Cadarache and will continue in 2008.

Milestone 1.2: Discrete time reduced deterministic models for relaxation oscillations.

The purpose of the study was to investigate the possibility of generating the same chaotic effects as with the external noise terms in the previous ELM models, by deterministic chaotic mechanism. The conclusion is that on-off intermittency behavior can be reproduced qualitatively by discrete time, 2 component versions of reduced ELM models, without external noise. This result is a new support for reduced stochastic models for on-off intermittency effects. The results were obtained in collaboration with Dr. X. Garbet from C.E.A. Cadarache.

2. Stochastic test particle dynamics.

New mechanism for particle transport in tokamak was studied in the framework of stochastic mechanism of instability growth in the edge plasma turbulence. From the mathematical study a new method for the reduction of particle loss from large tokamaks was derived. The improved numerical methods will be used in the optimisation of the gyrokinetic codes for ab-initio simulation of the large tokamaks.


Milestone 2.1: Test particle dynamics in stochastic electrostatic field, mathematical methods.

By new mathematical methods from [1] the large time asymptotic properties of the mean square displacement of test particles in random electromagnetic field generated by instabilities was studied. A new kind of transport, “extreme transport”, was discovered. In this

class of transport the mean square displacement increases according to ,

where the typical value of the Hurst exponent is . It follows that by reducing the value of the exponent H in tokamaks, the particle loss is largely reduced. The isotope effect paradox is explained and new limits on the guiding center approximation were obtained [5-7] in the frame of collaboration with Dr. B. Weyssow from ULB Belgium.

))(exp( 2HtconstO ×75.0≈H

Milestone 2.2: Test particle dynamics in stochastic electrostatic field, elaboration of improved numerical methods.

The problem of extending the mean field approximation methods, by adding stochastic components determined from self-consistency conditions, for the study of interacting charged particles was considered. On simplified models the problem of replacing the deterministic electrostatic field in the Vlasov equation by a self –consistent field with stochastic component was studied. In the case of linear models the stochastic term restored the exact result. In order to perform extensive numerical test of the self-consistent stochastic approximation, the first step was the elaboration of specialized stochastic Runge-Kutta methods. The numerical stability of these schemes was studied and robustness criteria were derived in ref. [8]. The work was partially done in collaboration with Dr. B. Weyssow from ULB Belgium.

3. Development of a Hamiltonian map for particle trajectories.

The advantage of the mapping methods consists in the possibility to discover fine structures in the phase space of dynamical systems, like fractal structures, closely connected to the anomalous transport in tokamak. These fractal structures are related to the impurity transport, important for the optimisation of the large tokamaks.

Milestone: Formulation of a Hamiltonian map for particle trajectories. Application of the Hamiltonian map for particle trajectories to impurity transport.

This study was devoted to the transverse particle motion in turbulent electrostatic field with fixed magnetic field. The tokamak structure was modeled in the cylindrical approximation. The turbulent electrostatic field was modeled by linear superposition of Fourier components. The Poincare map was derived by using the non-commutative effects in the classical decomposition method in numerical integrations. The robustness of this approximation results from ref. [8]. The dependence of the impurity trapping effects on the spectrum of the turbulent electric field and specific charge was studied. The fractal structure, similar to Julia sets, of the domains of the trapped particles was observed. The reduction of the fractal dimension of the domain of trapped particles in tokamak, with the increase of the intensity of short wave components was observed. The work was partially done in collaboration with Dr. B. Weyssow from ULB Belgium.


4. Anomalous transport of impurities in tokamak plasma.

Milestones: Evaluation of the α-particle flux. Influence of the radio frequency heating on the quasineutrality condition.

The transport of impurities driven by the instabilities is modified in the presence of radio-frequency heating [6, 9-11]. The quasi-neutrality condition contains besides characteristics of instability wave, the characteristics of radio-frequency waves used for plasma heating and current drive. First we have evaluated the density perturbation given by the passing particles in the presence of ion cyclotron resonant heating (ICRH) in tokamak plasma with electromagnetic turbulence. For two different radial localization of the resonant layer we evaluated the radial variation of the density perturbation [12-13]. The results indicate a significant modification of the density perturbation for parallel heating wave number greater than 1/cm. It was also observed a disparity in the position of the resonant layer absorption compared with the radial position of a significant density perturbation. We used plasma parameters corresponding to ITER design. The obtained results are important for a better understanding of the turbulent particle transport in core tokamak plasma in ICRH regime. The work was partially done in collaboration with Dr. B. Weyssow from ULB Belgium and will continue in 2008.

5. Electron transport in the presence of RF heating. Milestones: Evaluation of the electron density perturbation in plasma with rf heating. Perturbation of electron density due to electrostatic turbulence in presence of radio-frequency waves (ECRF) was evaluated in the case of TEM (trapped electron mode) using kinetic formalism. This will be used in future study of the quasi-neutrality condition in presence of radio frequency heating.

6. Theory related to specific models for particles and energy transport in SOL.

Milestones 6.1: The study of the diffusion of a stochastic anisotropic sheared magnetic field lines using the decorrelation trajectory method.

The anisotropy in the magnetic fluctuation spectrum (stochastic anisotropy) and magnetic shear induces variations of global averaged quantities such as the running and the asymptotic diffusion tensors that can be investigated using a semi-analytical method [14]. The study considers ranges for the anisotropy parameter, magnetic Kubo number and shear parameter leading to contrasting dynamical behaviors. In particular, the trapping of the stochastic magnetic field lines is analyzed. An asymptotic ‘poloidal’ velocity larger for stronger anisotropy is obtained for the wandering of the magnetic field lines for different values of the parameters. The averaged Lagrangian poloidal velocity is represented in Figure 1 for different values of the y-stochastic anisotropy and for a single value of the Kubo number and the shear parameter Ks = 0 and Ks = 2. For Ks = 2 and for a short time interval the averaged poloidal velocity is negative whatever the anisotropy. The magnitude of the averaged velocity is constant and non-zero in the shearless cases and a fixed x-anisotropy (Λ= 5) when the magnetic Kubo number takes the values 2 > Km > 0.5. The presence of the shear (Ks = 2) changes the feature of the averaged velocity: the magnitude of the final velocity is greater than in the shearless case and grows when the level of magnetic turbulence grows. The space variation of the magnetic field, i.e. the magnetic shear, creates the average poloidal velocity even in absence of an initial velocity of the field lines. Thus the inhomogeneity of the magnetic field determines a directed transport because the magnetic potential depends on the ‘z’ coordinate, which plays here the


role of time. In absence of the magnetic shear, for relatively small magnetic turbulence and different values of the stochastic anisotropy, the average poloidal velocity is zero.

This study shows that the mean magnetic shear and the stochastic anisotropy have an important role in the self consistently generated poloidal flow, known as reducing the radial turbulent transport. This mechanism contributes in large tokamaks like ITER to a rapid transition in an enhanced confinement regime, H-mode.

Figure 1: Average poloidal velocity for different values of the y-anisotropy. Other parameters are set to Km=0.5 and Ks=0 or Ks=2.

Milestones 6.2: Theory related to specific models for particles and energy transport in SOL. Zonal flow case.

We have demonstrated that the growth of the diamagnetic Kubo number produces a suppression of the generation of the zonal flow in weak electrostatic turbulence [19]; it is an effect of the combined variation of the gradient length and of the stochastic characteristics of the electrostatic weak anisotropic turbulence. A real situation (e.g. a tokamak) is complex, but our model indicates that, under certain circumstances, the drift wave turbulence generates spontaneously a shear flow in the poloidal direction. The latter is random and is characterized by a correlation length that is much longer in the poloidal direction than in the radial one. We have also calculated global Lagrangian average of k, vg and ω(θ) – Figure 2; their time evolutions have confirmed the tendency of suppression of zonal flow when the initial wave-vector is k0x = (0.8)1/2, k0y = (0.2)1/2. Our results give a contribution to the role of zonal flows in anomalous transport and creation of transport barriers, which are important for ITER.


Figure 2: The Lagrangian global average of the normalized drift wave frequency for k0x= (0.2)1/2, k0y= (0.8)1/2, K=0.2, Λ=0.1 (subplot (a)) and Λ=0.4 (subplot (b)) and four values for the Kd. Milestones 6.3: Particle dynamics and anomalous diffusion in stochastic electromagnetic field.

The electron diffusion induced by a two-dimensional electrostatic turbulence, in a sheared slab approximation of the toroidal magnetic geometry, is studied by direct numerical simulation. The ‘radial’ and the ‘poloidal’ running and asymptotic diffusion coefficients of electrons are obtained for physically relevant parameter values and the existence of an enhanced diffusion in the poloidal direction is observed in the presence of magnetic shear. The global effects of Ksim and KS

sim on the running and asymptotic diagonal diffusion tensor components are exhibited using direct numerical simulation for a guiding center system in a first order of drift approximation. The radial running diffusion coefficient (Figure 3) starts with a linear part characteristic to a ballistic regime, Dxx(τ) ~ τ. In all of the cases a trapping effect appears for large enough values of Ksim and/or KS

sim. We can conclude that an enhancing of the diffusion on poloidal direction and a relatively reduction on the radial one is caused by the presence of the magnetic shear for the same level of electrostatic turbulence. In our paper we have calculated the diffusion coefficients for the electrons by direct numerical simulation and we have found a very good qualitative agreement with the results obtained by the decorrelation trajectory method [15]. This conclusion gives a relatively certitude in order to apply DCT to other problems of interest where a Langevin treatment can be done.


Figure 3: Diagonal running diffusion coefficients for and different values for magnetic shear 5.2Ksim =

We have used in our various analyses the decorrelation trajectory method and the direct

numerical simulations, which were performed mostly using the computing facilities of ULB-VUB Belgium and in collaboration with Dr. Boris Weyssow from ULB Belgium. Some comparisons between the results obtained by these two different methods were done and a good qualitatively agreement was emphasized [14-19]. In many cases, the magnetic shear is a key parameter. This feature has been shown in JET, ASDEX-Upgrade, TCV, FTU and TORE SUPRA, and is utilized to gain plasma stability. In consequence, all the reported results and the numerical simulations are important for “Validation of physics-based transport models” and “Plasma edge characterization and modelling”, objectives for ITER.

7. Internal transport barriers, magnetic reconnection and anomalous transport in tokamaks.

Milestone 7.1: The study of the diffusivity of some magnetic fields using mathematical models generated by symplectic maps

The accuracy of some general mathematical models (discrete Hamiltonian systems) used for the description of the magnetic field configuration and of the charged particles’ dynamics in tokamaks was systematically studied. The magnetic diffusivity of the magnetic field specific to ASDEX-Upgrade tokamak was computed directly from the mathematical models generated by symplectic maps. The conclusion was that the diffusivity is larger in the region where the dynamics of the magnetic field lines is chaotic (also called stochastic) and it is closely related to the chaotic indicators of the orbits [20-21]. The technique presented in [20] was applied in order to obtain a realistic model for the description of the charged particles’ motion [22]. These results were obtained during the mobility at Universite Libre de Bruxelles. These dynamical features could be used in the theoretical description and mathematical simulation of internal transport barriers in large tokamaks, like ITER. Milestone 7.2: The study of the influence of the reconnection on the transport.

By using the Tokamap model (specific to Tore Supra) we studied the influence of the safety factor on the formation of the magnetic transport barriers and on the main properties of the magnetic transport. It was observed that the stochastization of the magnetic field, generated by the reconnection, increases the diffusivity of the magnetic field and drastically modifies the transport properties [23-24]. The statistical aspects of the anomalous transport related to


contamination of the tokamak plasma with multiply ionized impurities, elimination of the helium ash were studied during the mobility at ULB Bruxelles. The results are relevant for the estimation of the particle loss, heat load on the ergodic divertor plates, phenomena, which are partially caused by the anomalous transport generated by chaotic particle dynamics. References

[1] G. Steinbrecher, W. T. Shaw. “Quantile Mechanics”, European Journal of Applied Mathematics, 19, 87, (2008); http://mathematics.verticalnews.com/articles/593923.html. [2] G. Steinbrecher, X. Garbet, “On-Off Intermittency and Heavy Tail Exponent in Random Multiplicative Processes”, submitted to Phys. Rev E. [3] X. Garbet, G. Steinbrecher, “Noise-Driven On-Off Intermittency”, Annals of the University of Craiova, (Physics AUC), 17, Part II, 22-28 (2007). [4] X. Garbet, G. Steinbrecher, „Bifurcations in Reduced ELM Model“, Annals of the University of Craiova, (Physics AUC), 17, Part II, 28-35 (2007). [5] B. Weyssow, G. Steinbrecher, “Extreme Anomalous Particle Transport in the Random Linear Amplification Model of the Edge Plasma Turbulence“, Conference on Stochasticity in Fusion Plasmas, Jülich, March 05-07, 2007. http://www.fzjuelich.de/sfp/talks/2007/Poster/05_Weyssow.pdf. [6] N. Pometescu, G. Steinbrecher, “Anomalous transport of particles in tokamak plasma”, 4th Association EURATOM/MEdC Days Meeting, October 1st-2nd, 2007, Ramnicu Valcea. [7] B. Weyssow, G. Steinbrecher, “Extreme Anomalous Transport Driven by Fractional Brownian Motion”, Annals of the University of Craiova, (Physics AUC), 17, Part I, 172-189 (2007). [8] B. Weyssow, G. Steinbrecher, “Ergodicity and Robustness”, Annals of the University of Craiova, (Physics AUC), 17, Part II, 11-21 (2007). [9] N. Pometescu, Weyssow B., “Radial and poloidal particle and energy fluxes in a turbulent non-Ohmic plasma: An ion-cyclotron resonance heating case”, Physics of Plasmas, Vol.14, 022305 (2007). [10]N. Pometescu, “Radial Turbulent Transport of Ions in Tokamak Plasma with ICRH”, 6-th School on Fusion Physics and Technology, University of Thessaly – Volos, Greece – 26-31 March 2007, invited lecture. [11] N. Pometescu, Weyssow B., “Turbulent Transport in non-Ohmic plasma: an ion-cyclotron resonance heating case”, European Fusion Theory Conference, Madrid – September 24-27, 2007, poster session. [12] N. Pometescu, “Ion density perturbation driven by electromagnetic turbulence and ICRH”, 14-th International Conference on Plasma Physics and Applications, September 14-18, 2007, Brasov, Romania. [13] N. Pometescu, “Ion Density Perturbation in Turbulent Plasma with ICRH”, lucrare prezentata la: International Working Session on “Statistical Physics for Anomalous Transport in Plasmas”, Craiova, October 7 – 12, 2007.

[14] M. Negrea, I. Petrisor, B. Weyssow, “Diamagnetic effects on zonal flow generation in weak electrostatic turbulence”, European Fusion Theory Conference, Madrid – September 24-27, 2007, poster session. [15] M. Negrea, I. Petrisor, B. Weyssow, “Influence of magnetic shear and stochastic electrostatic field on the electron diffusion”, 14-th International Conference on Plasma Physics and Applications, September 14-18, 2007, Brasov, Romania – poster presentation (accepted for publication in Journal of Optoelectronics and Advanced Materials). [16] M. Negrea, I. Petrisor, B. Weyssow, “Role of stochastic anisotropy and shear on magnetic field lines diffusion”, Plasma Physics and Controlled Fusion 49, 1767 (2007). http://dx.doi.org/10.1088/0741-3335/49/11/002

http://mathematics.verticalnews.com/articles/593923.html

http://dx.doi.org/10.1088/0741-3335/49/11/002


[17] I. Petrisor, M. Negrea, B. Weyssow, “Electron diffusion in a sheared unperturbed magnetic field and an electrostatic stochastic field”, European Fusion Theory Conference, Madrid – September 24-27, 2007, poster session. [18] M. Negrea, I. Petrisor, B. Weyssow, “Influence of magnetic stochastic drift on ion diffusion in magnetic turbulence”, Physics AUC, 17 (Part I) 2007, 263-286 (http://cis01.central.ucv.ro/pauc/vol/2007_17_part1/2007_part1_20.pdf). [19] M. Negrea, I. Petrisor, B. Weyssow, “Characterization of zonal flow generation in weak electrostatic turbulence”, Phys. Scr. 77 (2008), 055502 (10pp). http://stacks.iop.org/PhysScr/77/055502[20] D Constantinescu, O Dumbrajs, V Igochine, B Weyssow, “On the accuracy of some mapping techniques used to study the magnetic field dynamics in tokamaks”, presented at the 3th international workshop “Stochasticity in Fusion Plasmas”, Julich, March 4-7, 2007, published in Nuclear Fusion 48 (2008) 024017 (9pp). [21] D. Constantinescu, “Intrinsic versus numerical chaos in discrete models used for the study of 1 ½ degrees-of-freedom Hamiltonian systems” presented at International symposium of quantum field theory and symmetries, Valladolid, Spain, July 22-28, 2007. [22] D Constantinescu, B. Weyssow, “On guiding centre map”, presented at 12-th European Fusion Theory Conference, Madrid, 24-27 Sept, 2007. [23] D. Constantinescu, J. H. Misguich, J.-D. Reuss, B.Weyssow, “The influence of the safety factor on the formation of the internal transport barriers in the Tokamap-model”, International working session “Statistical Physics for Anomalous Transport in Plasmas, Craiova, October 7 – 12, 2007, in Annals of the University of Craiova, (Physics AUC), 17 (2007) pp 190-200. [24] D. Constantinescu, B Weyssow, O. Dumbrajs, V. Igochine, “Anomalous transport. Internal transport barriers and magnetic reconnection in tokamak plasma”, presented at 4th Association EUTATOM/MEdC Days Meeting, October 1st-2nd, 2007, Ramnicu Valcea.

http://cis01.central.ucv.ro/pauc/vol/2007_17_part1/2007_part1_20.pdf

http://stacks.iop.org/PhysScr/77/055502

24 2007 Annual Report of the EURATOM-MEdC Association

TOKAMAK NEUTRON DIAGNOSTICS BASED ON THE SUPER-HEATED FLUID DETECTOR (SHFD)

V. Zoita*, M. Gherendi*, A. Pantea*, S. Soare**

*National Institute for Laser, Plasma and Radiation Physics, Magurele **National Institute for Cryogenics and Isotopic Technologies, Rm. Valcea

1. Introduction

Neutron diagnostics techniques based on a new type of detector (the super-heated fluid detector – SHFD) have been tested on the JET tokamak for the characterisation of the neutron emission. The super-heated fluid detectors (also known as “bubble detectors”) were found to be of particular interest due to their particular characteristics: immediate, visible response; high neutron efficiency, (practically) zero gamma sensitivity; lightweight, rugged and compact; broad energy range spectrometric capability [1,2].

2. Superheated fluid detectors (SHFDs)

Super-heated fluid detectors are suspensions of metastable droplets, which readily vaporise into bubbles when they are nucleated by radiation interactions. The active detecting medium is in the form of microscopic (20-50 μm) droplets suspended within an elastic polymer (Fig. 1) [3].

Figure 1. The basic operation of Super-Heated Fluid Detector(Bubbles detector)

The process of neutron detection by a SHFD resides in a mixture of nuclear interactions (neutron collisions with nuclei of the active medium), thermodynamic behaviour of the detecting medium (the super-heated fluid), and the mechanical response of the elastic polymer.

If sufficient energy is transferred from the colliding neutron to the nucleus of one of elements in the composition of the active medium, the recoil nucleus will initiate the generation of a vapour embryo of sub-micron dimensions. Under proper conditions (that depend on the thermodynamics of the active medium) the vapour embryo will lead to the vaporisation of the super-heated droplet with the subsequent expansion into a macroscopic (0.2 – 0.5 mm) bubble. The bubbles generated in the detector are counted by various means: eye counting for up to a


few tens of bubbles per detector, automatic counting through processing of the detector image, acoustical detection of the bubble formation. The number of bubbles generated within a given volume of the detector is simply and directly related to the neutron fluence (neutrons per unit area).

The SHFD’s have a threshold-type energy response. The threshold energy depends on: droplet composition, detector operating temperature, and detector operating pressure. For a standard bubble detector like the BD-PND(*) type the energy response is approximately flat within the range 0.3-10 MeV (Fig. 2). Using detectors with different energy thresholds, a bubble detector spectrometer (BDS(*)) is obtained. The BDS covers a broad energy range (0.01 – 20 MeV) and provides six energy thresholds in that range (Fig. 3).

Figure 2. Energy response of BN-PND –type detector

Figure 3. Energy response of BDS-type detector

3. SHFD neutron measurements at JET

Three types of SHFD’s (BD-PND, BDS and DEFENDER(∗)) (Fig. 4) have been used for neutron measurements at JET during the 2007 experimental campaigns (C18-C19). The BD-PND’s type detectors have been used for neutron fluence measurements, high sensitivity DEFENDER-type detectors for neutron beam imaging and the BDS type detectors have been used for neutron energy distribution measurements.

(a) (b) (c)

Figure 4. (a) BD-PND-type detectors for neutron fluence measurements; (b) BDS-type detectors for neutron energy distribution; (c) DEFENDER-type detectors for neutron beam imaging.

All measurements have been done at the end of the KM11 diagnostics line-of-sight,

above the TOFOR neutron time-of-flight spectrometer. The SHFD detectors have been placed in front of the vertical NaI(Tl) gamma-ray spectrometer.

By using a set of four DEFENDER detectors the profile of the neutron beam propagating along the collimated vertical line-of-sight of the KM11 diagnostics was obtained. (*) All detectors used in this work were manufactured and calibrated by Bubble Technology Industries, Chalk River, Canada


The radial distribution of the neutron fluence in the neutron beam at a distance of about 3 m from the exit of the 40 mm diameter KM11 collimator was obtained with a spatial resolution of less than one centimetre (Fig. 5). As an immediate effect of this measurement a better alignment of the vertical NaI(Tl) gamma-ray spectrometer was obtained.

~25 mm

~50 mm

~34 mm

~18 mm

DEFENDER(*)-type detector Figure 5. The radial distribution of the neutron fluence in the KM11 neutron beam.

The cross-section of the neutron beam is determined by the diameter of the floor-collimating hole. The Full Width at Half Maximum (FWHM) of the beam profile is approximately 40 mm [4].

The neutron energy distribution at the end of the KM11 line-of-sight was obtained over a broad energy range (six energy bins, defined by the energy thresholds: 0.01; 0.1; 0.6; 1.0; 2.5; 10.0 MeV).

Neutron fluence in energy bin

0

10

20

30

40

50

60

70

80

1 2 3 4 5 6

Energy bin

Frac

tion

(%)

1: 10 – 100 keV 2: 100 – 600 keV 3: 600 – 1000 keV 4: 1000 – 2500 keV 5: 2500 – 10000 keV

Figure 6. Neutron energy distribution determined by bubble detector spectrometer (BDS).

The measurement was done during a Ripple H-mode Study session (JET pulse numbers:

70656-70660). The energy distribution (Fig. 6) shows an energy component around the 2.5 MeV value (DD fusion neutrons) and a large low energy component, most probably generated by the scattering of the fusion neutrons in the collimating structures.


3. Conclusion

A neutron diagnostics technique based on the super-heated fluid detectors (SHFD’s or “bubble detectors”) has been successfully tested at JET on various types of discharges during Campaigns C17-C19. It provided new information about the following characteristics of the neutron field at the end of the KM11 line-of-sight: fluence, beam profile, broadband energy distribution.

The results (although quite encouraging) are of preliminary nature and they should be checked and confirmed in better-defined and suitably controlled measurements during Campaigns C20-C25. This technique for determining the neutron field characteristics could be applicable for high performance discharges (neutron yields of 5x1016) and it was proposed to be used in the next campaigns together with other two different and independent methods: bubble detectors, neutron activation and time-of-flight spectrometry.

Acknowledgements

The reported work includes contributions from the following people outside the MEdC Association: V. Kiptily, S. Popovichev, T. Edlington (Association EURATOM-UKAEA, Culham Science Centre, Abingdon, UK), A. Murari (Association EURATOM-ENEA, RFX, Padova, Italy), and S. Conroy (Association EURATOM-VR, Uppsala University, Uppsala, Sweden). References:

[1] A. Murari, T. Edlington, M. Angelone, L. Bertalot, I. Bolshakova, G. Bonheure, J. Brzozowski, V. Coccorese, R. Holyaka, V. Kiptily, I. Lengar, P. Morgan, M. Pillon, S. Popovichev, P. Prior, R. Prokopowicz, A. Quercia, M. Rubel, M. Santala, A. Shevelev, B. Syme, G.Vagliasindi, R.Villari, V.L. Zoita and JET-EFDA Contributors, “Recent Diagnostic Related Radiation Hardness Studies at JET”, International workshop on ITER-LMJ-NIF components in harsh environments, Aix-en-Provence France, 2007.

[2] A. Murari, T. Edlington, M. Angelone, L. Bertalot, I. Bolshakova, G. Bonheure, J. Brzozowski, V. Coccorese, R. Holyaka, V. Kiptily, I. Lengar, P. Morgan, M. Pillon, S. Popovichev, P. Prior, R. Prokopowicz, A. Quercia, M. Rubel, M. Santala, A. Shevelev, B. Syme, G.Vagliasindi, R.Villari, V.L. Zoita and JET-EFDA Contributors, “Measuring the Radiation Field and Radiation Hard Detectors at JET: Recent Developments”, sent to Nuclear Instruments and Methods in Physics Research Section A.

[3] F. d’Errico and M. Matzke, Rad. Prot. Dosimetry, Vol. 107, pp. 111-124 (2003).

[4] M. Gherendi, V. Kiptily, V. Zoita, S. Conroy, T. Edlington, D. Falie, A. Murari, A. Pantea, S. Popovichev, M. Santala, S. Soare and JET-EFDA contributors, “Super-heated fluid detectors for neutron measurements at JET”, 14th International Conference on Plasma Physics and Applications (CPPA 2007), Sept 14-18, 2007, Brasov, Romania, accepted for publication in Journal of Optoelectronics and Advanced Materials.


SHEATH PROPERTIES AND RELATED PHENOMENA OF THE PLASMA WALL INTERACTION IN MAGNETIZED PLASMAS. APPLICATION TO ITER

G. Popa, C. Costin, C. Agheorghiesei, C. Lupu, V. Anita, M. L. Solomon

“Al. I. Cuza” University Iasi, Iasi 1. Introduction

The activity covered two main parts: i) experiments and data processing; ii) modelling.

Experiments and data processing. Experiments were made on Pilot-PSI machine at FOM-Institute for Plasma Physics “Rijnhuizen”, The Netherlands and in Plasma Laboratory of the “Alexandru Ioan Cuza” University (UAIC), Iasi, Romania. Moreover, the main components used for plasma diagnostics were designed and manufactured at UAIC, Iasi.

Modelling. It should be mentioned that the modelling activity was carried out mainly at UAIC, Iasi but almost permanent contact and exchange of information were realized with our partners from the University of Innsbruck, FOM and IPP Prague, respectively.

2. Plasma diagnostics

2.1 Ion multi-channel analyser measurements in magnetised plasmas as Pilot-PSI

A new multi-channel analyzer with multi-collector system has been realized in order to measure time resolved distribution of the ion fluxes in the cross section of the plasma column of Pilot-PSI machine. The analyzer faces the plasma with a carbon plate of 26 mm diameter and 4 mm thickness in which 61 holes have been drilled. The holes are arranged in a concentric regular form, each hole having 0.5 mm in diameter. A ceramic plate fixes 61 collectors behind the carbon plate. Each collector is a 0.6 mm tungsten wire and is placed to correspond to a hole of the analyser carbon plate. The analyzer will be placed on the axis as target of the Pilot-PSI plasma column. It will provide 2D spatial distribution (polar-plane coordinates) of the ion flux as well as information about instabilities and rotation process of the plasma column. Measurements with the ion analyser are planned to be later performed also in Magnum-PSI. A period of documentation was considered to find good solutions for both a new data acquisition system and the corresponding software for simultaneously registering the 61 electrical signals provided by the analyzer.

Moreover, the microwave generator (2.56 GHz frequency and 1.5 kW power) has been considered for designing and manufacturing, at the Plasma Laboratory of the UAIC Iasi, a magnetized plasma device to produce a rather dense plasma column for testing the multi-channel analyser. The device will also be exploited for testing a new data acquisition system envisaged to be used for measuring the plasma parameters of the Pilot-Psi machine. 2.2 Experiments and results concerning diffusion model for Katsumata type probe and validation of the theoretical model

At the beginning of 2007 Castor Tokamak was decommissioned. Nevertheless, we have continued to analyse the large amount of data that were already taken in collaboration with IPP Prague and Innsbruck University groups to conclude if one can speak about diffusion process of the plasma particles inside the Katsumata type probe. The results were presented at the 7th International Workshop on Electrical Probes in Magnetized Plasmas, Prague, Czech Republic,


22-25 July 2007 and later published within Contribution to Plasma Physics Journal. We also transferred the new Katsumata probe to Pilot-PSI to perform similar measurements concerning the diffusion model. The probe was used either as cylindrical probe (when the collector of the probe is outside the ceramic tube) or as Katsumata probe (when the collector is inside the ceramic tube). The interest is to measure the plasma potential and to compare the results with those obtained by a cold cylindrical probe and/or an emissive one.

Before performing Katsumata probe measurements, a cold cylindrical probe and an emissive one were used to obtain preliminary results on plasma parameters as: plasma potential, radial distribution of the electric field and ion density. The plasma source of Pilot-PSI (a cascaded arc) was operated in argon, hydrogen or argon-hydrogen mixture with a gas flow rate of 1 or 2slm (1slm ~ 4.48×1020molec/s) and a total discharge current Id of 80 to 150 A. The target was positioned at 56 cm from the nozzle of the arc source, the gas pressure in the vessel was in the range of 3 to 5 Pa and the magnetic field strength was 0.4 T for all measurements.

The cylindrical probe made of tungsten wire of 9 mm length was introduced at 3.5 cm in front of the target. The probe was movable in the radial direction with its axis parallel to the magnetic field lines. A linear ramp voltage of ±28 V sets the probe bias and the probe current was measured as a function of voltage across a 10 or 100 Ω resistor in the probe circuit.

At present there is no satisfactory model of the probe characteristic obtained in such experimental conditions, but ion saturation current Isi, floating potential Vf and plasma potential Vp might be obtained from these characteristics at least as relative values. These parameters can be used to evaluate, within the simplest model, at least two rotational drift velocities of the plasma beam: the electric drift velocity (calculated as vE = Er/B) and the diamagnetic drift velocity (calculated as vn = (kT/eB)*∇rn/n).

The electric drift velocity vE was estimated using the radial electric field Er derived from the radial distribution of the measured floating potential of the cold and hot probes. This velocity (Er/B) characterizes the azimuthal drift of a collisionless or weakly-collisional plasma for which the ion collision frequency is much smaller than the ion cyclotron frequency. Assuming that the ion saturation current of the probe is proportional with the plasma density, one can write ∇rn/n = ∇rIsi/Isi and the diamagnetic drift velocity vn can also be evaluated. Comparing the radial profile of the calculated vE with the plasma jet rotation velocity profiles measured by optical methods in hydrogen learns that the magnitude, the radial position of the maximum velocity and the profile width are similar, but the electric drift velocity is greater. The latter result appears because several effects as collisionality, viscosity, particles density and electric field gradients are not considered when calculating vE. The diamagnetic drift velocity has the same order of magnitude as vE in argon, while it is one order of magnitude lower for hydrogen plasma.

The fluctuations of the current intensity measured either on the probe or on the target can be associated with possible instabilities of the drift motion previously described. Also, the fundamental frequencies in the power spectrum of the current fluctuations (approximately 6.5 MHz for H2 and approximately 80 kHz for Ar) are comparable to the ion-cyclotron frequency.

The emissive probe (tungsten wire of 0.35 mm diameter) in form of a rectangular loop is also radially moveable and it was inserted into the vessel at about half the distance between the arc-source and the target. The active part of the loop, of 3 mm length, was oriented parallel to the magnetic field lines. The experiments showed that inside the plasma column (r ≤ 5 mm), both emissive and cold probe measure the same floating potential, which is approximately equal


to the plasma one. This result proves that in such dense plasmas (approx. 1020 m–3) even a cold probe becomes emissive under charged particles bombardment and the measured floating potential can be a good approximation of the plasma potential. The effect of what we called “self-heated” (or “self-emissive”) probe-plasma system was investigated in Plasma Laboratory of UAIC, Iasi using dense and magnetised plasma of a magnetron discharge. The results were used for designing and manufacturing the system for the diagnostic of the plasma potential of the Pilot-PSI experiment.

Electrical measurements in Pilot-PSI allow the estimation of the power density transferred from the plasma to the target. The obtained power values (approx. 0.6 MW/m2) confirm that Magnum-PSI will be able to operate at ITER relevant parameters for plasma surface interaction at the divertor. Probe measurements show that plasma parameters are significantly different for Ar and H2. The presence of the radial electric field determines the appearance of a drift instability, which may enhance radial particle losses and consequently diminish the energy transferred from the plasma to the target.

The Katsumata probe realized within the UAIC has been installed in the same plasma region as the emissive probe. This time the collector of the Katsumata probe was placed inside the ceramic tube, so that two series of measurements have been realized: the first series was related to the plasma instability and the second one to the plasma potential. The former data are used within our proposed diffusion model, while the latter data are related to radial distribution of the plasma potential. These data are under evaluation.

2.3 Secondary electron emission at the probe (and wall) surface. Final form of the experimental model and device using multi-polar confinement system

The final form of the experimental device for the investigation of the secondary electron emission (SEE) induced by electron bombardment was finished in 2006. The performances of the data acquisition system have been expanded at the beginning of 2007, by using a new acquisition board and by enlarging the range limits for data registering. The secondary electron emission was studied on the current-voltage (I-V) characteristics of a plane probe bombarded by a mono-energetic electron beam. The beam energy was adjustable in the range of 50 to 400 eV. The contribution of the secondary electron emission was evidenced by the increase of the probe current intensity in the ion branch of the I-V characteristics. The electrons bombarding the probe with low energy (less than the threshold energy of the primary electrons that may produce SEE) could not be relieved on the I-V characteristics. This is due to an electrostatic mirror effect that appears between the electron gun exit and the strongly negative probe surface, which has to be considered in the quantitative estimation of the SEE effect on the probe characteristic.

On the purpose of exceeding this experimental inconvenient, the next step is to modify the construction of the electron gun to obtain a better collimated primary electron beam. The experiments were realized in residual gas (no gas inserted in the vessel). Further measurements will be realized in the presence of argon or helium plasma.

3. Modelling of the formation of the space charge sheath in front of a conductive wall

3.1 Computer set-up for plasma numerical simulation

A dual processor computer was installed at UAIC, Iasi for plasma numerical simulations. This computer is able to support remote access and handling from all over the world using a secure Internet network for all registered users. A parallel compiler named LAM/MPI, a high-quality open-source implementation of the Message Passing Interface


specification (http://www.lam-mpi.org/), was implemented for future plasma simulation codes. Kinetic (PIC) simulations algorithms, based on Bit1 PIC-MCC code developed at the Institute for Theoretical Physics, University of Innsbruck, were written in C for Unix/Linux platforms aiming the code parallelisation. The Unified Modelling Language (UML) will be used to identify within the algorithms further possible locations where parallel programming can improve the computation time.

3.2 Numerical investigation on the formation of the floating space-charge sheath in the Pilot-PSI plasma and obtaining the floating potential of the target

Kinetic (PIC) studies of plasma-wall interaction in the Pilot–PSI machine working with H2 plasma were performed. The previous linear 1D-PIC-MCC code was written only for atomic hydrogen plasma, so the main aim of this task was to complete the code with elementary processes specific for molecular hydrogen plasma. Fourteen new elementary processes involving H2 were implemented in the Monte-Carlo collision routine.

For the initial investigations, the model included a floating solid conductive target interacting with a plasma column having similar parameters to the above-mentioned experimental device: plasma density n0 = 4×1019 m-3, electron temperature Te = 12 eV, ion temperature Ti = 2 eV, H2 neutral density nn = 1021 m-3 and magnetic field B = 0.4 T.

The injection conditions corresponding to Pilot-PSI plasma source are imposed to one boundary of the modified PIC 1D-code while the floating space-charge sheath is investigated at the second boundary. The calculated value of the steady-state floating potential is about -50 V with respect to the plasma potential. Other numerical simulations were performed considering the secondary electron emission at the floating boundary as an additional process.

A parallel code that can run on multiprocessor machines is envisaged in order to improve the numerical performances. The first step has been prepared by recompiling the old code with a parallel compiling tool to support multiple CPUs. This work is in progress and preliminary tests give a reduction of the computing time of about 20% on 2 dual core processors machine (total of 4 CPUs) from Iasi.

3.3 Kinetic (PIC) in the process of floating space-charge sheath formation

The plasma parameters used for the numerical modelling correspond to SOL (Scrape of Layer) region of a tokamak device (Te = Ti = 20 eV, ne = ni = 1018 m-3). The magnetic field strength B varied between 0 and 1T. The Φ angle between the magnetic field direction and the normal to the conductor surface varied from 0 to 90 degrees. Additionally, simulations for Pilot-PSI plasma parameters were also conducted.

Three different cases have been considered: (a) B = 0 T; (b) B = 1 T and Φ = 0 deg and (c) B = 1 T, Φ = 45 deg. The case without magnetic field and the one with magnetic field perpendicular to the wall are similar. In contrast, the potential distributions within the sheath are considerably perturbed in the case of a magnetic field inclined at 45 deg. This is due to the fact that the plasma flow is no longer in the direction of the observation. During sheath formation different types of waves are excited, crossing the magnetic field lines. The spectrum of these waves reveals the electron plasma oscillation. In the case of the inclined magnetic field, two frequencies are evidenced: the lower and the upper hybrid frequency. The results of the PIC simulation and the analytic computation of these frequencies agree up to the third digit.

The observations and conclusions resulted from our floating-sheath formation investigations are applicable for a large variety of plasma parameters. Currently we investigate more complex models that include other collisions and surface processes.


4. Development of a 2D fluid model for Pilot-PSI

4.1 Writing and solving the fluid equations for electrons, positive ions and neutrals

The numerical code is first applied for argon. Three types of particles are considered in the model: neutrals, electrons and positive ions (Ar+). Fluid equations were written for electrons and ions, considering that the neutrals spatial distribution is homogeneous in the vessel. This first approach neglects the neutrals flow. The discharge is supposed to be azimuthally symmetric, thus only radial and axial treatment is considered.

The solving of the fluid equations requires proper choice of the boundary conditions (in our case for potential and for particle fluxes). The spatial domain chosen for modelling has four types of boundary: the chamber walls, the target, the entrance of the gas and plasma in the vessel which corresponds with the exit of the cascaded-arc source and the gas exit to the pumping system. While the conditions for the first two boundaries are clear, the conditions for the last two boundaries had to be investigated.

4.2 Testing different boundary conditions for the potential at the gas entrance

Regarding the potential boundary conditions, the chamber walls are grounded and the target can be biased with respect to the ground. The cascaded-arc plasma source fixes the discharge current by adjusting the applied voltage. The radial distribution of the potential at the source exit is unknown (no experimental data are available) and thus a boundary condition had to be found for the model. For that, a stationary plasma beam was considered in the discharge chamber (corresponding to the imposed discharge current) and different potential distributions were tested for the exit of the cascaded-arc source. The general conclusion is that the spatial distribution of the potential in the discharge chamber strongly depends on the charged particles distribution. The latter one is related with both charged particle fluxes and the potential boundary condition at the plasma source exit.

4.3 Testing different boundary conditions for the particle fluxes

By neglecting the neutrals flow, as a first approach, no boundary conditions have to be imposed for the neutrals flux. Electron and ion fluxes have to be chosen at the gas entrance with respect to the discharge current. Knowing that the plasma expands from the source into the vessel, it is expected that both electron and ion fluxes are inward directed. Yet, two different conditions can be imposed: (1) equal electron and ion fluxes; (2) different electron and ion fluxes. At the gas exit both electron and ion fluxes are outward directed. A first approach is to write both electron and ion fluxes as they are for the case of a wall: each flux has two components, a drift and a thermal one. This task is under development.


The research topics described in the sub-paragraph 2.2 and in the paragraph 4 were realised in collaboration with FOM-Institute for Plasma Physics “Rijnhuizen”, The Netherlands, Association EURATOM/FOM. The experimental results and some numerical developments were obtained during our team member missions/mobilities at FOM.

A part of the research topics described in the sub-paragraphs 3.2 and 3.3 were realised during our team member mobilities at Theoretical Plasma Physics Group, Innsbruck University, Austria, Association EURATOM-ÖAW.


Publications

[1] J. Brotankova, E. Martines, J. Adamek, J. Stockel, G. Popa, C. Costin, C. Ionita, R. Schrittwieser, G. Van Oost, “Novel Technique for Direct Measurement of the Plasma Diffusion Coefficient in Magnetized Plasma”, Contributions to Plasma Physics 48(5-7) (2008), pp. 418-423

[2] J. Adamek, M. Kocan, R. Panek, J. P. Gunn, E. Martines, J. Stöckel, C. Ionita, G. Popa, C. Costin, J. Brotankova, R. Schrittwieser, G. Van Oost, “Simultaneous Measurements of Ion Temperature by Katsumata and Segmented Tunnel Probe”, Contributions to Plasma Physics 48(5-7) (2008), pp. 395-399

[3] M. L. Solomon, Steluta Teodoru, G. Popa, “Secondary electron emission at Langmuir probe surface”, Journal of Optoelectronics and Advanced Materials 10(8) (2008), pp. 2011–2014 Conferences

[1] C. Costin, V. Anita, R. S. Al, B. de Groot, W. Goedheer, A. W. Kleyn, W. R. Koppers, N. J. Lopes Cardozo, H. J. van der Meiden, R. J. E. van de Peppel, R. P. Prins, G. J. van Rooij, A. E. Shumack, M. L. Solomon, W. A. J. Vijvers, J. Westerhout, G. Popa, “On the power balance at the end plate of the plasma column in Pilot-PSI”, 34th EPS Conference on Plasma Physics, Warsaw, Poland, 2-6 July 2007

[2] J. Adamek, M. Kocan, R. Panek, J. P.Gunn, J. Stöckel, E. Martines, R. Schrittwieser, C. Ionita, G. Popa, C. Costin, J. Brotankova, G. Van Oost, L. van de Peppel, “Comparison of Ion Temperature Measurements by Katsumata and Segmented Tunnel Probes”, 7th International Workshop on Electrical Probes in Magnetized Plasmas, Prague, Czech Republic, 22-25 July 2007

[3] J. Brotankova, E. Martines, J. Adamek, J. Stockel, G. Popa, C. Costin, R.Schrittwieser, C. Ionita, G. Van Oost, “Novel technique for direct measurement of the plasma diffusion coefficient in magnetised plasma”, 7th International Workshop on Electrical Probes in Magnetized Plasmas, Prague, Czech Republic, 22-25 July 2007

[4] M. L. Solomon, R. S. Al, V. Anita, C. Costin, B. de Groot, W. Goedheer, A. W. Kleyn, W. R. Koppers, N. J. Lopes Cardozo, H. J. van der Meiden, R. J. E. van de Peppel, R. P. Prins, G. J. van Rooij, A. E. Shumack, W. A. J. Vijvers, J. Westerhout, G. Popa, “On the self excited instabilities in the plasma column of Pilot-PSI”, 14th International Conference on Plasma Physics and Applications (CPPA), Brasov, Romania, 14 – 18 September 2007

[5] M. Solomon, V. Titon, C. Andrei, G. Popa, “High density magnetised plasmas by self emissive probe”, 14th International Conference on Plasma Physics and Applications (CPPA), Brasov, Romania, 14 – 18 September 2007

[6] C. Lupu, C. Agheorghiesei, D. Tskhakaya, S. Kuhn, G.Popa, “Modelling by PIC code the time evolution of the floating potential of the plate electrode ending a high density and magnetized plasma column”, 14th International Conference on Plasma Physics and Applications (CPPA), Brasov, Romania, 14 – 18 September 2007

[7] M. L. Solomon, Steluta Theodoru, G. Popa, “Secondary electron emission at Langmuir probe surface”, 14th International Conference on Plasma Physics and Applications (CPPA), Brasov, Romania, 14 – 18 September 2007.


UPGRADE OF GAMMA-RAY CAMERAS – NEUTRON ATTENUATORS

V. Zoita*, M. Anghel**, V. Braic***, M. Constantin**, T. Craciunescu*, M. Curuia**, E. David**, M. Gherendi*, A. Pantea*, S. Soare**, I. Tiseanu*

*National Institute for Laser, Plasma and Radiation Physics, Bucharest **National Institute for Cryogenics and Isotopic Technologies, Rm. Valcea

***National Institute for Optoelectronics, Bucharest 1. Introduction

The JET KN3 gamma-ray camera diagnostics system has already provided valuable information on the fast ion evolution in JET plasmas [1,2]. The applicability of gamma-ray imaging diagnostics to high power deuterium pulses and to deuterium-tritium discharges is however strongly dependent on the fulfilment of rather strict requirements for the control of the neutron and gamma-ray radiation fields. The upgraded diagnostics should fulfill these requirements and it should, at the same time, observe the very hard design restrictions on JET (e.g., the requirement of minimum effects on the co-existing neutron camera diagnostics).

The main objective of the JET EP2 diagnostics upgrade project “Gamma-Ray Cameras – Neutron Attenuators” (GRC) is the design, construction and testing of neutrons attenuators for the two sub-systems of the KN3 gamma-ray imaging diagnostics [3-5]:

- KN3 gamma-ray horizontal camera (KN3 HC)

- KN3 gamma-ray vertical camera (KN3 VC)

This diagnostics upgrade should make possible gamma-ray imaging measurements in high power deuterium JET pulses, and eventually in deuterium-tritium discharges.

A second objective of the GRC project is the design (scheme design level) of the KM6T tangential gamma-ray spectrometer upgraded diagnostics.

An important objective of the GRC project is to develop and test design solutions of relevance to ITER. Eventually the GRC diagnostics upgrade should validate design solutions of interest for ITER.

2. KN3 Gamma-ray cameras neutron attenuators

The locations of the neutron attenuators are shown schematically in Figure 1 together with the detector lines of sight of each of the two KN3 cameras. The attenuators are placed within the KN3 diagnostics system in Octant 1 between the vacuum port and the collimator body (also called “radiation shield”) both in the case of the horizontal camera (HC) and vertical camera (VC) (Figure 1).

The KN3 neutron attenuators consist of metal casings filled with pure light water. Two materials were considered for the attenuators casings: INCONEL 600 and aluminium alloy 6061. The final choice was for INCONEL 600 (3mm thick sheet) and it was determined both by


considerations related to the interaction with pure water and by considerations related to the manufacturing process flow (effects of welding, bending, mechanical and electrical behaviour).

Figure 1. Horizontal and Vertical Camera Neutron Attenuator (HC-NA, VC-NA)

2.1 Horizontal Camera Neutron Attenuator

The horizontal camera neutron attenuator is designed to function as a neutron filter when in working position (in the plane determined by the gamma-ray detectors lines of sight). To remove the neutron attenuator from the working position two movements are required: first a 90o rotation (to the right when looking to plasma) and second a 630 mm translation as shown in Figure 2.

Figure 2. a) Horizontal Camera Neutron Attenuator in working position; b) HC-NA system


Figure 3. Horizontal Camera Neutron Attenuator system – drawing and 3D model

To place the attenuator into the working position the same actions are to be taken in

reverse order. The attenuator is steered and controlled by a commercially available electro-pneumatic system that includes additional several custom-tailored parts.

The neutron attenuator consists of the pure light water (used as attenuating material) casing and a U-shaped profile that provides the structure with mechanical strength and connects with the steering and control system.

The Horizontal Camera Neutron Attenuator system was developed to the detailed design level. The components of the system are: attenuator casing, mounting frame, pneumatic linear/rotating drives, filling/draining assembly, connecting parts, and fittings. All of them are assembled into the functional system shown as drawings (drawing no. F50710000 sheet 1 and sheet 2) and 3D model in Figure 3.

2.2 Vertical Camera Neutron Attenuator

The vertical camera neutron attenuator is positioned on Octant 1, inside the KS3 optical (H alpha) diagnostics box, mounted on the diagnostics metal frame. The vertical camera neutron attenuator casing was developed into two versions: long (assembly drawing no. F50721000, Figure 5) and short (assembly drawing no. F50720000, Figure 4).

To move in and out of the working/parking location the attenuator is translated 100 mm by a steering and control electro-pneumatic system (not as complex as that from the horizontal camera neutron attenuator), Figure 4.

Both vertical camera attenuator casings (short and long version) have a quasi-trapezoidal shape with internal reinforcements parallel and between the lines of sight.


Figure 4. a) Vertical Camera Neutron Attenuator in working position; b) Cross-section of the attenuator casing

The vertical camera neutron attenuator system was developed to the detailed design level. The assembly drawing shows the middle and upper frames (parts F50722000 and F50723000 respectively from Figure 5) of KS3 diagnostics box; these parts are required to be prepared for the attenuator system subsequent installation. The pneumatic steering and control devices will be mounted onto an assembly jig (part F50724000 from Figure 5). The assembly jig will then be fixed onto the KS3 diagnostics box with bolts.

Figure 5. Assembly drawing of the Vertical Camera Neutron Attenuator

The vertical camera neutron attenuator casing long version (assembly drawing no. F50721000) is shown in Figure 6. It consists of the same components, the only difference being its height of 600 mm (compared with 240 mm for the short version casing).


Figure 6. Assembly drawing of the long version of the vertical camera neutron attenuator casing

2.3. Steering and control system for KN3 gamma ray camera neutron attenuators

The KN3 gamma-ray camera neutron attenuators will operate in a very harsh electromagnetic environment. It was thus recommended to avoid as much as possible the use of electrical and electronic components for the attenuator steering and control. Therefore a design solution based on pneumatic components was developed for attenuator steering.

A block-diagram of the KN3 neutron attenuator steering and control system is presented in Figure 7. The components are grouped into three sub-systems:

- LUC-1 (Local Unit Cubicle 1) contains a programmable logic controller, an operator unit and a power supply. The programmable logic controller that is permanently connected to CODAS receives electrical signals from the pneumatic-electric converter from Local Unit Cubicle-2, (LUC 2) and sends electrical signals to the directional control valves from LUC-2.

Figure 7. Block-diagram of the KN3 neutron attenuator steering and control system

- LUC-2 (Local Unit Cubicle 2) contains the directional control valves, the flow control valves, the pressurized air supply and the pneumatic-electrical converter.

There are two pressurized air circuits for each actuator. The pneumatic-electric converter (PEC) receives pneumatic signals (air pressure changes) from the pneumatic limit switches and converts them into electrical signals that will be sent to the Local Unit Cubicle 1 (LUC-1). The pressurized air supply will be connected to the JET pressurized system that will provide 5-7 bar air pressure.


-PS (Pneumatic System) consists of pneumatic actuators, pneumatic limit switches and blocking devices. The pneumatic actuators perform the translation and/or rotation movements necessary to place the attenuators either in the working or in the parking positions. The role of the pneumatic limit switches is to indicate whether the attenuators have reached their pre-set positions (either the working or the parking position, with no intermediate position indications). The blocking devices will hold firmly the attenuators into their pre-set positions.

3. Vertical Camera Neutron Attenuator prototype

A fully functional prototype based on the vertical camera neutron attenuator was developed, manufactured and electro-mechanically tested. It is similar in size, shape and used materials as the KN3 vertical camera neutron attenuator; additionally a support frame was manufactured to mimic part of the KS3 optical diagnostic box. Shown in Figure 8 are the 3D model and a drawing (isometric view) of the prototype system.

Figure 8. Prototype casing, support and pneumatic linear actuators

Figure 9 shows the manufactured prototype system (the mechanical part) and the casing.

Figure 9. a) Manufactured neutron attenuator prototype; b) manufactured attenuator casing

The steering and command system is similar to that of the KN3_NA in terms of components. The differences lay with the software that commands and controls the translations and end-stroke positions. All the three main blocks (LUC 1, LUC 2 and PS) are shown in figure 10 a), b), and c) – see Chapter 2.3.


a) b)

c)

Figure 10. Main block of the prototype steering and command system (FESTO components); a) Local Unit Cubicle 1 (LUC 1) b) Local Unit Cubicle 2 (LUC 2) c) Pneumatic System (PS)

The casing was designed and manufactured according to the legal requirements currently in-force: ISCIR, (Romanian) Office for the Assessment of Lifting and High Pressure Equipment and CNCAN, (Romanian) National Commission for the Control of Nuclear Activities. Subsequently the casing was subjected to several integrity tests: helium leak test, pressure test, and penetrating liquid. The casing passed all tests it was subjected to. 4. Profile reconstruction techniques for the JET neutron camera diagnostics (KN3)

The JET neutron profile monitor ensures coverage of the neutron emissive region that enables tomographic reconstruction. However, due to the availability of only two projection angles and to the coarse sampling in each projection, tomography is a limited data set problem. In consequence, appropriate reconstruction methods must be developed in order to ensure good reconstructions.

This work on profile reconstruction techniques for the JET neutron camera diagnostics (KN3) started in 2007 with the development and implementation of a method based on the maximum entropy principle (ME). It continued during the last quarter of 2007 with work on the implementation of a Monte Carlo Back-Projection (MCBP) algorithm. The algorithm starts with an empty image and tries to allocate a small quantity “d0” in a randomly chosen pixel:

. If the allocation is compatible with the existent projections,

i.e., the allocation remains permanent, and ; otherwise it is

discarded: .

( ) ( ) doff iteri

iteri +=+1

( ) 0,1 ≥− +

kiiter

ik wfp ( ) (iteri

iteri ff =+1 )

kiikk wfpp ,−→


Figure 1 – Illustration of the Monte Carlo algorithm

principle

Figure 2 – Projection resampling implies improved domain coverage

Figure 3 - Reconstruction of the neutron emissivity for shot 61161 at 51.52 s

The same smoothing principle as in the case of the ME method was used. It assumes smoothness on magnetic flux surfaces. We implemented the smoothing operator as a one-dimensional median filtering, using a sliding window which moves on the magnetic flux contours. This operator works on the tomographic projection/back-projection weighting matrix. In order to introduce some additional degree of smoothing the experimental projections were transformed by resampling, using spline interpolation (Fig. 2). Projection resampling implies the introducing of virtual lines of sight (Fig. 2) which ensures an improved coverage of the reconstruction domain. The work was continued with the evaluation of the performances of this method. This evaluation is performed using numerically generated phantoms and experimental data. Encouraging results were obtained (Fig. 3). 5. KM6T tangential gamma-ray spectrometer

Regarding the upgrade of the JET tangential gamma-ray spectrometer (KM6T) a long series of scientific and technical evaluations carried out over a period of about two years lead to the conclusion that the full diagnostics system should be upgraded in order to fulfil the requirements of high power DD and DT JET discharges (i.e. not only the replacement of the neutron attenuators).

A Conceptual Design phase for the KM6T diagnostics upgrade is under way and it is to be completed by end of January 2008, with a conceptual design review meeting at JET planned for February 2008.

The structure proposed for the KM6T diagnostics upgrade contains:

-a neutron and gamma-ray collimating system

-neutron and gamma-ray shielding components

-a system of three neutron attenuators using lithium hydride as the attenuating material

The KM6T field of view was defined by the new diagnostics configuration. A preliminary evaluation of the radiation (neutron and photon) performance was done by means of


the MCNP transport code. The effect of the parasitic gamma-ray sources falling within the new KM6T field of view was also evaluated.

A high performance data acquisition system for the existing BGO gamma-ray detector was also considered as a necessary upgrade, but this is no longer realistic under the present financial difficulties in the lead Association, MEdC.

In order to provide the necessary information for a realistic evaluation of the upgrade of the KM6T tangential gamma-ray spectrometer it was proposed (at the Project Board no. 4, 16.07.2007) to carry out a Scheme Design phase until June 2008.

In order to test the new technology for the manufacture of the LiH attenuators for the KM6T upgrade an attenuator prototype was proposed to be developed. For this propose our experimental device for the manufacturing of the LiH discs from LiH powder was constructed. The discs will be produced by a technology based on ultrasonically assisted hot sintering.

6. Conclusions

The neutron attenuator system for the JET KN3 Gamma-Ray Cameras (KN3 GRC) diagnostics has the following main components (sub-systems): horizontal camera neutron attenuator (HC-NA), vertical camera neutron attenuator (VC-NA), and neutron attenuators steering and control. Work during 2007 progressed with the detailed design phase where the drawing of assemblies and parts were produced.

A fully functional neutron attenuator prototype was manufactured and tested. The system, as a whole, performed according to the specifications. Integrity tests proved the neutron attenuator casing is suitable (from the mechanical point of view) for use.

Techniques for the reconstruction of the radiation profiles provided by the JET KN3 neutron/gamma-ray cameras have been successfully developed and tested.

The work on the KM6T tangential gamma-ray spectrometer upgrade continued with a conceptual design of the full diagnostics system.

Acknowledgements

The reported work includes contributions from the following people outside the MEdC Association: T. Edlington, V. Kiptily, P. Prior, S. Sanders, B. Syme (Association EURATOM-UKAEA/JOC, Culham Science Centre, Abingdon, UK), K. Kneupner (Association EURATOM-FZJ, Julich, Germany), G. Gros (Association EURATOM-CEA, Cadarache, France), I. Lengar (Association EURATOM-MHST, Jozef Stefan Institute, Ljubljana, Slovenia), A. Murari (Association EURATOM-ENEA, RFX, Padova, Italy), and L. Rios (Association EURATOM-CIEMAT, CIEMAT, Madrid, Spain).

References:

[1] V.G. Kiptily, F.E. Cecil and S.S. Medley, “Gamma ray diagnostics of high temperature magnetically confined fusion plasmas”, Plasma Phys. Control. Fusion, 48 (2006) R59–R82.

[2] V.G. Kiptily, D. Borba, F.E. Cecil, M. Cecconello, D. Darrow, P.C. de Vries, V. Goloborod’ko, K. Hill, T. Johnson, A. Murari, F. Nabais, S.D. Pinches, M. Reich, S.E. Sharapov, V. Yavorskij, I.N. Chugunov, D.B. Gin, G. Gorini, A.E. Shevelev, V. Zoita, “Fast ion JET diagnostics: confinement and losses”, International Conference on Burning Plasma Diagnostics, 24-28 September, 2007, Varenna, Italy.


[3] S. Soare, V. Zoita, T. Craciunescu, M. Curuia, V. Kiptily, I. Lengar, A. Murari, P. Prior, M. Anghel, G. Bonheure, M. Constantin, E. David, T. Edlington, D. Falie, S. Griph, F. Le Guern, Y. Krivchenkov, M. Loughlin, A. Pantea, S. Popovichev, V. Riccardo, B. Syme, V. Thompson, I. Tiseanu and JET EFDA contributors, “Mechanical Design of the Upgraded JET Gamma-Ray Cameras”, 14th International Conference on Plasma Physics and Applications (CPPA 2007), Sept 14-18, 2007, Brasov, Romania, accepted for publication in Journal of Optoelectronics and Advanced Materials.

[4] S. Soare, V. Zoita, T. Craciunescu, M. Curuia, V. Kiptily, I. Lengar, A. Murari, P. Prior, M. Anghel, G. Bonheure, M. Constantin, E. David, T. Edlington, D. Falie, S. Griph, F. Le Guern, Y. Krivchenkov, M. Loughlin, A. Pantea, S. Popovichev, V. Riccardo, B. Syme, V. Thompson, I. Tiseanu and JET EFDA contributors, “Upgrade of the JET Gamma-Ray Cameras”, International Workshop on Burning Plasma Diagnostics, September 24-28, 2007, Varenna, Italy.

[5] S. Soare, V. Zoita, T. Craciunescu, M. Curuia, M. Anghel, M. Constantin, E. David, A. Pantea, I. Tiseanu, et all, ”Upgrade of the Jet Gamma-Ray Cameras”, A 13-a Conferinta „Progrese in criogenie si separarea izotopilor”, Calimanesti-Caciulata 7-9 noiembrie 2007, Romania.

Acknowledgements

The reported work includes contributions from the following people outside the MEdC Association: V. Kiptily, S. Popovichev, T. Edlington (Association EURATOM-UKAEA, Culham Science Centre, Abingdon, UK), A. Murari (Association EURATOM-ENEA, RFX, Padova, Italy), and S. Conroy (Association EURATOM-VR, Uppsala University, Uppsala, Sweden).

References:

[1] A. Murari, T. Edlington, M. Angelone, L. Bertalot, I. Bolshakova, G. Bonheure, J. Brzozowski, V. Coccorese, R. Holyaka, V. Kiptily, I. Lengar, P. Morgan, M. Pillon, S. Popovichev, P. Prior, R. Prokopowicz, A. Quercia, M. Rubel, M. Santala, A. Shevelev, B. Syme, G.Vagliasindi, R.Villari, V.L. Zoita and JET-EFDA Contributors, “Recent Diagnostic Related Radiation Hardness Studies at JET”, International workshop on ITER-LMJ-NIF components in harsh environments, Aix-en-Provence France, 2007.

[2] A. Murari, T. Edlington, M. Angelone, L. Bertalot, I. Bolshakova, G. Bonheure, J. Brzozowski, V. Coccorese, R. Holyaka, V. Kiptily, I. Lengar, P. Morgan, M. Pillon, S. Popovichev, P. Prior, R. Prokopowicz, A. Quercia, M. Rubel, M. Santala, A. Shevelev, B. Syme, G.Vagliasindi, R.Villari, V.L. Zoita and JET-EFDA Contributors, “Measuring the Radiation Field and Radiation Hard Detectors at JET: Recent Developments”, sent to Nuclear Instruments and Methods in Physics Research Section A.

[3] F. d’Errico and M. Matzke, Rad. Prot. Dosimetry, Vol. 107, pp. 111-124 (2003).

[4] M. Gherendi, V. Kiptily, V. Zoita, S. Conroy, T. Edlington, D. Falie, A. Murari, A. Pantea, S. Popovichev, M. Santala, S. Soare and JET-EFDA contributors, “Super-heated fluid detectors for neutron measurements at JET”, 14th International Conference on Plasma Physics and Applications (CPPA 2007), Sept 14-18, 2007, Brasov, Romania, accepted for publication in Journal of Optoelectronics and Advanced Materials.


“GAMMA RAY SPECTROMETRY” GRS

Silviu Olariu, Agata Olariu

“Horia Hulubei” National Institute for Physics and Nuclear Engineering, Magurele

The project GRS is part of the project of diagnostic enhancements package to be implemented during the Framework Programme 7.

The objective for the year 2007 was the installation and the availability of intense gamma ray and neutron sources at the tandem accelerator of IFIN-HH, Magurele-Bucharest, in order to carry out testing of three new spectrometers, with complementary performance: a high energy resolution HPGe spectrometer and two high efficiency, high rate spectrometers with large detection crystal.

In order to attain this objective an irradiation experiment of an aluminum target with a 10 MeV proton beam was carried out.

The target was placed between the optical elements of the beam line so that a maximum value of the current of approximately 500 nA on the target was obtained. The detection of the gamma rays was carried out using an HPGe placed at a distance of approximately 6 m of target. The recording of gamma spectra was performed using the data acquisition system of the Department of Nuclear Physics of IFIN-HH. The off-line processing of spectra gave a rate of approximately 1.4 x 109 gamma rays/s. The obtained gamma-ray spectrum is shown in Fig. 1

Figure1. Gamma ray spectrum obtained by bombarding an aluminium target with a proton beam of 10 MeV.

The lines of 843 keV and 1014 keV are obtained from the reaction 27Al(p,n)27Si. The half life for the nucleus 27Si is of 4 s and its decay feeds the transitions of the nucleus 27Al, as it is shown in Fig. 2.


Figure 2. Level scheme for the nucleus 27Al.

The gamma ray of 1368 keV was obtained from the reaction of 27Al(p,α)24Mg which feeds the level of 1368 keV of nucleus 24Mg.

The number of photons of gamma rays was obtained using the formula:

Area_photopeak/ t_measuring_time=detector_ efficiency x geometric_ factor x Λ,

where Λ is the number of gamma rays/t.

In Table 1 we present the processed experimental data.

Table 1. Number of photons generated by a beam of protons with an energy of 10 MeV and the current of 500 nA,on a Al target Energy Area Error Efficiency

of detector x geometric

factor

No gamma rays

Isotope

170.68 keV 7.6E-7 27Si 511keV 166110 627 6.3E-7 3.7E8 27Si 597 keV 5.9E-7 781 keV 5.3E-7 795.4 keV 5.2E-7 843.74 keV 48790 403 5.0E-7 1.4E8 27Si 1014.42 keV 100500 452 4.4E-7 3.3E8 27Si 1368.63 keV 78451 421 3.1E-7 3.7E8 24Mg 1720.3 keV 23124 314 1.7E-7 1.9E8 27Si A total of approximately 1.4 x 109 gamma ray photons / s was obtained.

The reaction rates expected in this (type of) experiment were calculated according to the formula :

Number of (p,n) reactions / second = σ x ρ x d x N / A x J /q , where

σ, cross section of the reaction 27Al9(p,n),

ρ, density for pentru aluminium, 2.7 g/cm3

d, the thickness of the target

N, Avogadro number,

A, molar mass, 26.9815 g,

J, current of protons, 500 nA,


q, elementary charge, 1.6 x 10-19 C.

We have considered the stopping power for protons in aluminum target and we made calculations with the code SRIM2006

We have also considered the cross-sections data for the reaction 27Al(p,n) from the data base EXFOR.

Table 2. Calculated values for the number of p,n/s reactions for a beam line of protons of 500 nA on an aluminium target Energy of protons, MeV

Cross section, mb

Delta energy

Stopping power,

Stopping power, MeV/mm

D, mm No. p,n reactions/s

5.53 0 0.24 0.0533 14.44531 0.01661 0 5.77 23.4 0.24 0.05168 14.00626 0.01714 7.55206E6 6.01 32 0.24 0.05007 13.56992 0.01769 1.06597E7 6.25 35.8 0.24 0.04998 13.54553 0.01772 1.1947E7 6.5 34.3 0.25 0.04719 12.78939 0.01955 1.26283E7 6.74 37.6 0.24 0.04596 12.45603 0.01927 1.36452E7 6.98 44.6 0.24 0.04472 12.11997 0.0198 1.66343E7 7.22 56.4 0.24 0.04367 11.8354 0.02028 2.15411E7 7.46 72.3 0.24 0.04264 11.55625 0.02077 2.82809E7 7.7 77.2 0.24 0.0416 11.27439 0.02129 3.09525E7 7.94 86.3 0.24 0.04057 10.99524 0.02183 3.54795E7 8.18 92 0.24 0.03968 10.75403 0.02232 3.86713E7 8.43 98.5 0.25 0.03882 10.52096 0.02376 4.40841E7 8.67 98 0.24 0.03798 10.2933 0.02332 4.30371E7 8.91 96 0.24 0.03715 10.06836 0.02384 4.31007E7 9.15 102 0.24 0.0364 9.86509 0.02433 4.67381E7 9.39 102 0.24 0.03571 9.67809 0.0248 4.76412E7 9.63 103 0.61 0.03502 9.49109 0.06427 1.24684E8

Thus, the total number of (p,n)/s reactions which occurred during the stopping for the beam of protons of 10 MeV and intensity of 500 nA on an aluminium target is approximately 5.7 x 108 (p,n) reactions / s.

This value is consistent with the number of photons per second experimetally obtained, since the multiplicity of photons of gamma rays per reaction is higher than 1.

This experiment shows that the using of a beam of protons of 10 MeV with the intensity of 500 nA on a aluminium target allows the generation of high fluxes of gamma rays and neutrons and therefore is appropriate for testing of the new detectors of germanium and scintillators, mentioned above.

Moreover the data acquisition system was optimized. The acquisition system dMCApro was installed. It is composed of a multichannel analyzing card which could be integrated into a computer, an amplifier, an analog-to-digital convertor, a high voltage supply and a computer code winTMCA32.

This data acquisition system integrated into a computer can be transported to the place where the irradiation experiments are to be conducted. It is to be designed at the IRASM facility, for intense gamma ray fluxes and irradiation from a high activity 60Co source, of approximately 150.000 Ci.


Implementation of acquisition system MIDAS

In order to develop a data acquisition system for high rates a Hytec 5331card was also considered. We studied the possibilities of this card interfacing with the data acquisition system of Tandem, based on CAMAC modules, from the Department of Nuclear Physics

It was studied the data acquisition system MIDAS (Maximum Integration Data Acquisition System), which is a system for small and medium-scale experiments.

This data acquisition system can be used in operation systems based on TCP/IP protocol and allows high transfer rates.

A Hytec 1331 card is also considered for the command of the Hytec 5331 controller. This will allow the measuring on gamma spectra at high rates which are produced in intense fluxes generated by the irradiation of targets with beams of protons with the energy of 10 MeV.

For measurements of neutron spectra a neutron detector was considered, which is based on a ionizing chamber with 3He and has a good energetic resolution as well as a sufficient efficiency for measuring the neutrons in intense beams

The energetic resolution is of 20 keV for thermic neutrons and 30 keV for neutrons having the energy of 1 MeV and the efficiency of 3 x 10-4 at 1 MeV.

The development of the methodology of measuring gamma and neutrons spectra with energetic resolution at high counting rates will be used for testing of a new detector of HPGe and of two scintillation detectors which will be used at JET for upgrading both the instrumentation of gamma radiation detection and the discrimination methods between gamma and neutron emission.

Atentie 48 2007 Annual Report of the EURATOM-MEC Association

EVALUATION OF IRRADIATION EFFECTS ON PASSIVE/ACTIVE COMPONENTS OF OPTICAL FIBER SYSTEMS FOR CONTROL AND SENSING

D. Sporea*, Adelina Sporea*, C. Oproiu*, Ion Vata**, Rodica Georgescu**

*National Institute for Lasers, Plasma and Radiation Physics, Magurele

** National Institute of R&D for Physics and Nuclear Engineering – “Horia Hulubei”, Magurele

Underlying Technology: Irradiation Effects in Ceramics for Heating and Current Drive, and Diagnostics Systems Deliverable: UT07; January – December 2007 Our research subject within the subtask is to evaluate:

The objective of this investigation was the study of gamma-rays and neutron irradiation on passive components for optical fibers systems, and on active components for laser diodes control. 1. Introduction

Optical fibers are expected to play an important role in plasma diagnostics, distributed sensing and communication systems within the ITER infrastructure, as different types of signals have to be transmitted in radiation environments, under high temperatures and high electromagnetic noise. Apart from the optical fiber itself, such optical signal transmission systems include various types of passive components (i.e. splitters, couplers, connectors, attenuators, etc.). Our investigation is focused on the degradation of two splitters and two attenuators, under gamma and neutron irradiation, either at specific wavelengths of interest or over a spectral band, in order to evaluate the wavelength dependence of the phenomena. These components are also tested to determine the change of the polarization state of optical radiation they guide. On the other hand, laser diodes applied in sensing/ communication systems have to be controlled both in current and temperature. For this reason, we investigated the degradation of the operational parameters for a laser diode driving circuit and a Peltier cooling structure (TEC – thermo-electric cooler), under gamma-ray, neutron irradiation and electron beam irradiation.

2. Experimental set-up 2.1 Tested components

The investigated components are: a) two 1 x 2 beam splitters/ combiners 50% / 50% (denoted by OFS-1 and OFS-2), with the

common path - SM, 2 m long, FC/APC connector, and the separate arms - SM, 2 m long, FC/PC connectors;

b) two on-line attenuator patch-cords (denoted by OFA-1 and OFA-2), 2 dB, 2 m long, SM, FC/APC & FC/PC connectors;

2007 Annual Report of the EURATOM-MEC Association 49

c) three compact, circuit boards for laser diodes current driving (denoted by LDC-g; LDC-n; LDC-e), softstart, max. driving current 200 mA, photodiode current monitor included with a max. photodiode current of 1.2 A, common laser diode anode and photodiode cathode;

d) three TEC modules (denoted by TEC-g; TEC-n; TEC-e), 1 stage cooler, 7 couplers, Imax = 5A, V = 0.85 V, Pmax = 2.8 W, ΔTmax = 67OC.

2.2 The measuring set-up

In the case of the two splitters/ combiners, the measurements before the irradiation were done at a fixed wavelength of 1310 nm, over a narrow spectral band (1510 nm - 1620 nm), and over a wider spectral band (700 nm - 1700 nm). The set-up for the fixed wavelength measurements includes a high stability, narrow-bandwidth DFB laser diode emitting at 1310 nm and a calibrated power meter. For the spectral band 1510 nm – 1620 nm we used a calibrated wavelength meter having also power measurement capabilities and a tunable laser diode. The laser source was tuned automatically over the entire spectrum, while data are acquired by a calibrated wavelength meter, both for optical power and wavelength, using a software program developed in the Laboratory. Each branch of the optical fiber splitter and the fixed attenuator were tested separately, and for the evaluation of the set-up stability and reproducibility three full runs were performed. The collected data are saved in Excel-like files for further processing. For the measurements over the spectral band 700 nm – 170 nm we used a high stability broad-band light source and an optical spectrum analyzer. Before any irradiation, all the passive components were also tested to determine the change of polarization for single wavelength laser radiation, as the polarization state of the input optical radiation is scrambled for almost all polarization states. The laser radiation from the tunable laser source is coupled at the input of a polarization controller, while the polarization controller output radiation is connected to the passive optical element under test (splitter or fix attenuator). The output radiation from this device is measured by a polarization analyzer. For all the performed measurements we used several laser wavelengths from 1510 nm to 1620 nm, in 10 nm increments. At each wavelength we set the initial polarization state at point “H” on the Poincaré sphere, and the azimuth and the ellipticity for this starting point were recorded. The measurement was run by arbitrarily scanning of the polarization states, over the Poincaré sphere in 5O increments.

The changes in the azimuth, ellipticity of the exiting optical radiation, as well as the three normalized Stokes parameters were simultaneously recorded.

For the evaluation of the irradiation induced changes in the TEC module a compact set-up was designed, which includes: a constant current, variable source and a thermocouple connected for data acquisition to a National Instruments USB-controlled module. We changed manually the electrical power applied to the TEC and we measured the temperature changes and stability, in both operating modes (heating and cooling facet).

In the case of the circuit board laser drivers the set-up included: a laser diode emitting at λ = 670 nm; a calibrated laser power meter; a regulated voltage supply (5V); a digital voltmeter and a digital multimeter; and a Si detector embedded into an integrating sphere, operating in the visible – near-IR range. During the measurements, the background optical radiation was about 0.2 μW. The measurements were performed on the following quantities: emitted optical power; the laser diode driving current; the embedded monitoring photodiode current. Data acquisition lasted for about 30 min in each case, and the sampling interval was of 2.5 min. The variable resistor on the circuit board was set to provide a 19 – 40 mA direct current and its setting was kept the same during all the experiment.


2.3 Irradiation conditions Table 1 details the total doses for the gamma-ray irradiation. In Table 2 is given the same

information for the neutron irradiation, while the doses for the electron beam irradiation are presented in Table 3.

Table 1. The total irradiation dose per irradiation step, for the gamma-ray irradiation

Irradiated part/ irradiation step

1 2 3 4

OFS-1 100 Gy 1 kGy 10 kGy 100 kGy OFA-1 100 Gy 1 kGy 10 kGy 100 kGy LDC-g 100 Gy 200 Gy 1 kGy - TEC-g 100 Gy 1 kGy 20 kGy -

Table 2. The fluence per irradiation step, for the neutron irradiation Irradiated part/ irradiation step

1 2

OFS-2 9 x 1012 n/cm2 1,4 x 1013 n/cm2

OFA-2 9 x 1012 n/cm2 1,4 x 1013 n/cm2

LDC-n 9 x 1012 n/cm2 1,4 x 1013 n/cm2

TEC-n 9 x 1012 n/cm2 1,4 x 1013 n/cm2

Table 3. The total irradiation dose per irradiation step, for the electron beam irradiation Irradiated part/ irradiation step

1 2 3 4

LDC-e 50 Gy 500 Gy 5 kGy 20 kGy TEC-e 50 Gy 500 Gy 10 kGy -

3. Results

The degradation of the circuit board parameters subjected to gamma-ray irradiation made impossible the proper operation of the optical power control loop as the driving current increased dramatically and produced a thermal stress in the laser diode, hence its destruction. Following this situation, the driving current delivered by the circuit was set to a low value and the laser diode was replaced with a new one. After the irradiation a decrease of the driver capability to maintain a constant value of the output optical power was observed, as it is indicated by the higher values of the standard deviation for the diode current and the monitoring photodiode current.

In the case of the thermoelectric cooler irradiated by an electron beam (for the total irradiation dose of 10.550 kGy) there is an increase, as compared to the same device before any irradiation, of about 10 OC of the highest temperature level reached by the device for the same input electrical power level. At the same time the electron beam irradiation produced an increase by about 8 OC of the temperature differences existing between the two facets of the device (the parameter ΔTmax). A similar module irradiated by gamma-rays exhibits an increase of about 6 OC of the highest temperature level as compared with the non-irradiated device, and a slight decrease of the difference between the two facets (about 2 OC), at a total irradiation dose of 21.1 kGy. The measurements were done at the same electrical power levels injected to the

2007 Annual Report of the EURATOM-MEC Association 51

device. In the case of neutron irradiation, the lower temperature level increased by 11 OC for a fluence of 9 x 1012 n/cm2.

For the gamma and neutron irradiations carried out on the passive optical components for optical fiber systems we did not notice a significant change for the optical transmission in the 700 nm – 1700 nm spectral range, for both the attenuators and the optical fiber splitters. In the case of the optical fiber splitters and attenuators irradiated with gamma-rays a perturbation of the polarization state of the transmitted optical radiation was observed, as the input optical radiation polarization state is scanned over the entire Poincaré sphere. In Figures 1-3 can be noticed this phenomenon, as it appears from the investigated parameters (i.e. the azimuth, the ellipticity or the Stokes parameters). Changes appear also in the variation of the transmitted optical power (Figures 4 and 5).

July-17-2007 15h03m48s M00215134 PAX5710 Polarization Analyzing System July-04-2008 M00218806 PAN5710IR3 (1350 - 1700nm) July-05-2007 1.51E-03 6.67E+01 Auto 0.00E+00 2 1 1 3.00E-02 1024 1

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November-01-2007 11h55m32s M00215134 PAX5710 Polarization Analyzing System July-04-2008 M00218806 PAN5710IR3 (1350 - 1700nm) July-05-2007 1.51E-03 6.67E+01 Auto 0.00E+00 2 1 1 3.00E-02 1024 1

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a b Figure 1. The change of the first Stokes’ parameter, in the case of the optical fiber attenuator OFA-1: a – before the irradiation; b – after the gamma-ray irradiation at a total dose of 10 kGy, at λ = 1510 nm.


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a b Figure 2. The change of the azimuth, in the case of the optical fiber attenuator OFA-1: a – before the irradiation; b – after the gamma-ray irradiation at a total dose of 10 kGy, at λ = 1550 nm.

August-03-2007 13h10m47s M00215134 PAX5710 Polarization Analyzing System July-04-2008 M00218806 PAN5710IR3 (1350 - 1700nm) July-05-2007 1.55E-03 6.67E+01 Auto 0.00E+00 2 1 1 3.00E-02 1024 1

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Figure 3. The change of the ellipticity, in the case of the optical fiber attenuator OFA-1: a – after the gamma-ray irradiation at a total dose of 100 Gy; b – after the gamma-ray irradiation at a total dose of 10 kGy, at λ = 1550 nm.



3.07E+01 0 0.00E+00 Auto Splitter 1.2; 1610 nm a

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Figure 4. The change of the transmitted optical power, in the case of the optical fiber splitter OFS-1.2: a – before the irradiation; b – after the gamma-ray irradiation at a total dose of 10 kGy, at λ = 1610 nm.


3.07E+01 0 0.00E+00 Auto Atenuatorr 1; 1510 nm a

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a b

Figure 5. The change of the transmitted optical power, in the case of the optical fiber attenuator OFA-1.2: a – before the irradiation; b – after the gamma-ray irradiation at a total dose of 10 kGy, at λ = 1610 nm. Conference papers

[1] Dan Sporea, Adelina Sporea, Ion Vata, “Comparative study of gamma-ray and neutron irradiated laser diodes”, Proceedings of Photonics North, Ottawa, Canada, June 2007.

[2] Dan Sporea, Adelina Sporea, Constantin Oproiu, Ion Vata, “Evaluation of irradiation effects on semiconductor lasers subjected to gamma-ray, electron beam and neutron irradiation”, International workshop on ITER-LMJ-NIF components in harsh environments, Cadarache, France, June 2007.

[3] Dan Sporea, Adelina Sporea, Benoit Brichard, “Irradiation-induced UV optical attenuation in optical fibers for plasma diagnostics” International workshop on ITER-LMJ_NIF components in harsh environments, Cadarache, France, June 2007.

[4] Dan Sporea, Adelina Sporea, Constantin Oproiu, “On-line evaluation of gamma-ray irradiated large diameter optical fibers for plasma diagnostics”, Proceedings of ICONE 15, 15th International Conference on Nuclear Engineering, Nagoya, Japan, April 2007.

[5] Dan Sporea, “Update on the radiation effects in optical fibers and optoelectronic components for fusion installations”, 4 th Association Days Meeting, Ramnicu-Valcea, Romania, October 2007.

Published papers

[1] Dan Sporea, Adelina Sporea, “Radiation effects in sapphire optical fibers”, Physica Status Solidi (c) 4, No.3 (2007) Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim.

[2] Dan Sporea, Adelina Sporea, “Dynamics of the radiation induced color centers in optical fibers for plasma diagnostics”, Fusion Engineering and Design, vol 82, issues 5-14, 2007, 1372-1378, DOI information: 10.1016/j.fusengdes.2007.05.053


THE DEVELOPMENT OF MOD-TFA PRECURSORS FOR THE DEPOSITION OF THICK YBCO FILMS ON METALLIC SUBSTRATES FOR THE SUPERCONDUCTING COATED CONDUCTORS

T. Petrisor, Lelia Ciontea

Technical University of Cluj-Napoca, Cluj-Napoca

1. 1. Introduction

In order to scale-up the YBa2Cu3O7-x (YBCO) coated conductors technology, the development of a low cost method for the deposition of epitaxial YBCO thick film is essential. The chemical solution deposition (CSD) is promising since this technique fulfils the requirements of the fabrication of coated conductors at industrial level (versatility, low vacuum, inexpensive, high deposition rate, easy control of the stoichiometry, etc.). Until now, the trifluoroacetates metal organic deposition (TFA-MOD) has been the most used CSD method for the YBCO superconducting film deposition. Using this method several groups [1,2,3,4] have obtained high performance YBCO superconducting films with a critical current density (Jc) in the range of J, approximately 2-3 MA at 77 K and zero magnetic field. Nevertheless, the major drawback of the TFA-MOD method consists in the evolvement of hydrofluoric acid, during the heat treatment of the precursor film, which limits the thickness of the film and enhances the pyrolysis time.

A modified TFA-MOD method, using only barium trifluoroacetate, is presented. The yttrium and copper triflouroacetates were replaced by the alcoholic solutions of Cu and Y acetates dispersed in propionic acid. The characterization of the epitaxially grown YBCO films, on both (001) SrTiO3 and CeO2/YSZ/CeO2/Pd buffered Ni-5at.%W substrates, using the modified TFA-MOD method is also presented.

2. 2. Experimental

The coating solution was prepared starting from yttrium acetate Y(CH3COO)3.4H2O, barium trifluoroacetate Ba(CF3COO)2 and copper acetate Cu(CH3COO)2.H2O corresponding to the 1:2:3 stoichiometry. While the barium trifluoroacetate was dissolved in methanol, the Y and Cu acetates were separately dispersed in methanol, treated with an excess of propionic acid C2H5COOH, then treated with NH4OH until the solutions became clear. The three solutions were mixed together under stirring and concentrated by the removal of solvents under vacuum. The resulting solution was spin coated both on (001) SrTiO3 and CeO2/YSZ/CeO2/Pd buffered Ni-5at.%W substrates at a spinning rate of 4000 RPM for 60 seconds.

The dried (600C) precursors were investigated by FT-IR (Fourier Transformed Infrared Spectroscopy), QMS (Quadrupole Mass Spectroscopy), and thermal analyses (DTA and TG). The thermal analysis (TG-DTA) of the YBCO gel powder was performed under dynamic nitrogen atmosphere in the temperature range 20-700oC, at a rate of 10oC/min. The TG-DTA thermal analyses have been coupled with a quadrupole mass spectrometer using an atmospheric sampling residual gas analyzer (200 QMS Stanford Research System). Scanning Electro Microscopy (SEM) and X-Ray Diffraction (XRD) were used for the investigation of the film morphology and crystalline structure, respectively. The temperature dependence of the electrical resistance was determined by a four point method. The YBCO film thickness was measured directly by means of a profilometer on a step patterned film.


3.Results and discussions

3.1Precursor chemistry

The TG-DTA analysis (Figure 1) have revealed that the decomposition of the YBCO precursor takes place in four successive stages. The first weight loss occurs in the temperature range 60-135oC and corresponds to a loss of 1.4 % from the initial weight, evidenced by the endothermic effect at 118oC in the DTA plot. As demonstrated by the QMS measurements (Figure 2), this weight loss can be attributed to the evaporation of the residual water from the dried gel. The second stage between 135-250oC is accompanied by the evolvement of CO2 (m/z=44), acidic rests (m/z=43) and methanol (m/z=31), as well as by an endothermic peak at 239oC in the DTA plot. The main weight loss (43%) takes place between 250 and 320oC. The gaseous products evolved in this temperature range are CO2 (m/z=44), acidic rests (m/z=43) and water (m/z=18) due to the combustion of organic moieties, evidenced by a broad exothermic band with three peaks at 321, 330, 353oC respectively, each one representing a decomposition reaction, consistent with the decomposition of the individual metallic precursors. At the same time, complex competitive oxidation processes due to the burning of the organic moieties take place between 250-450oC. CO (m/z=28) evolvement could not be registered due to the overlapping with the nitrogen molecule. Nevertheless, it is presumed to convert to CO2 during the combustion process. From room temperature to 320oC the total weight loss is about 60 %. Between 460 and 800oC a slight weight gain is registered, associated with the probable oxidation of Cu+ to Cu2+.

Figure 1. DTA and TG curve for YBCO dried Figure 2. TG-MS results on YBCO dried gel. The FTIR spectrum of the YBCO powder is shown in Figure 3. A broad band between 3750 and 3100 cm-1 is due to the O-H stretching vibrations from the residual alcohol, water and Metal O-H bonds. The peaks at ~3000 cm-1 and the peaks in the region between 1500 and 700 cm-1 are due to the alkyl groups (C-H), while those corresponding to 800 and 726 cm-1 are attributed to the C-F bonds due to barium trifluoroacetate. The sample presents the carboxylate (COO-) characteristic vibration frequencies at 1681 cm-1, as well as the skeletal vibration of propionates at 1076 cm-1 found for the yttrium and copper individual precursors only. The peak at 1302 cm-1 corresponds to the CH2 symmetric vibration associated with propionates. The peaks below 660 cm-1 are attributed to the Me-O bond stretching vibrations.


Figure 3. FTIR spectra of the YBCO dried gel at 110 0C. The sample presents the COO-characteristic vibration frequencies at 1681 cm-1, as well as the skeletal vibration of propionates at 1076 cm-1. 3.2 YBCO film growth and characterization

To obtain epitaxial YBCO superconducting films the precursor films undergo a two step heat treatment. First, they were heat treated up to 400oC under humidified oxygen (about 17 Torr H2O and balance oxygen) as follows: up to 80oC with 10oC/min, up to 200oC with 2oC/min, up to 250oC with 0.5oC/min and finally to 400oC with 10oC/min, and then cooled to room temperature in the same nominal gas environment (Figure 4). The high temperature thermal treatment (Figure 5) has been performed at 800oC and 850oC, at a rate of 10oC/min, for about two and one hour, respectively in an environment of humid oxygen and nitrogen (about 17 Torr H2O, oxygen pressure 70 miliTorr and balance nitrogen) and another 10 minutes in the dry mixture of oxygen and nitrogen (70 miliTorr oxygen and balance nitrogen). The film was cooled to about 450oC in the same nominal gas environment at a rate of 10oC/min, held at this temperature for 1 hour in oxygen and subsequently cooled to room temperature.

Figure 4. Pyrolysis heat treatment. Figure 5. Crystallisation heat treatment.

Figure 6. X-ray diffraction pattern of YBCO film on (100) SrTiO3 substrate.

Figure 7. X-ray diffraction pattern of YBCO on CeO2/YSZ/CeO2/Pd buffered Ni-5at.%W substrate.


The X-ray θ–2θ scans (Figures 6 and 7) both for the YBCO/STO and YBCO/ CeO2/YSZ/CeO2/Pd/Ni-W films present only (00l) YBCO peaks indicating that the films have a high degree of epitaxy with the c-axis perpendicular to the substrate. As it can be seen from figure 7, besides the peaks corresponding to the YBCO/ CeO2/YSZ/CeO2/Pd/Ni-W structure, the X-ray diffraction pattern exihibits also the peaks corresponding to NiO, NiWO4 and BaCeO3. The nickel oxide and the NiWO4 compound are mainly formed during the deposition by PLD of the first CeO2 layer [5], while the BaCeO3 is formed at the interface between the CeO2 cap layer and the YBCO film during the growth thermal treatment at 8500C. The peaks corresponding to Pd are not observed, indicating that Pd is completely diffused into the Ni-W substrate during the deposition at high temperature, both of the CeO2 cap layer and the YBCO film. The formation of a superficial layer of Ni-W-Pd solid solution is confirmed by the peaks very close to the Ni-W (200) reflection. The ω-scan of the (005) peak for the YBCO/STO film has a full-width at half-maximum (FWHM) of 0.24 °, close to that observed in the YBCO films grown by PLD. The rocking curve (Figure 8) through the (002)Ni-W, (002)CeO2, (002)YSZ peaks along the transverse direction (TD) and (005)YBCO peak along the rolling direction (RD) have an out-of-plane FWHM of 8.2°, 4.2°, 4° and 1.9°, respectively. The improvement of the CeO2 texture is an effect of the Pd film, which has a sharper texture with respect to the Ni-W substrate [5]. On the other hand, the lattice mismatch between CeO2 and Pd is 1.6%, much smaller than that between CeO2 and Ni of 8.4%. The YBCO film grown at 8500C on the CeO2/YSZ/CeO2/Pd/Ni-W architecture has an excellent out-of-plain texture with Δω=1.90 for the (005) reflection.

Figure 8. ϕ -scan through the (002)Ni-W, (002)YSZ, (002)CeO2 and (005)YBCO peaks.

The YBCO film deposited by spin coating under the conditions described above has the thickness of about 0.6 μm. The surface morphology of the YBCO film grown on the CeO2/YSZ/CeO2/Pd/Ni-W architecture is shown in Figure 9. A surface free of cracks, but with some holes can be seen. In spite of the voids, the c-axis oriented grains are well connected over the pores, as it can be seen from the high magnification SEM image (Figure 9 right). The spherical particulates are nanocrystallites of CuO [3]. It is to be noted that no needle-like particulates representing a or b-axis oriented YBCO grains were observed. The high magnification SEM image reveals that the voids are at the surface of the YBCO film and, therefore, the film can be considered electrically continuous. The films grown on STO have the same morphology. At the same time, the morphology of the YBCO film is similar with that of the film grown by the trifluoroacetate precursors (TFA-MOD) [6].


Figure 9. SEM image of YBCO film grown on CeO2/YSZ/CeO2/Pd buffered Ni-W substrate at 8500C at two different magnifications (left 20.00 K X and right 135.66 K X ).

The electrical resistance versus temperature for the YBCO films grown at different

temperatures is presented in Figure 10. The YBCO film grown on STO at 8000C exhibits a lower critical temperature (Tc(R=0)=88K) and a nonlinear behavior in the normal state with the ratio R(300)/R(100)=2. On the contrary, the films grown at 8500C on both STO and NiW exhibit a linear behavior of the electrical resistance in the normal state with the ratio R(300)/R(100) of 3 and 3.3, respectively, close to the intrinsic value of the optimally doped YBCO. The zero resistance critical temperature (Tc(R=0)) is 91.3 K and 91.6 K, respectively. Nevertheless, the films grown at 8500C have a two step transition, indicating a multiphase system. This behavior is more evident for the YBCO film on NiW, where two sharp transitions (ΔT=1 K) can be observed, one with the Tconset=94.7 K and another one with the Tconset=92.5 K. The volume fraction of the two phases, estimated from the resistance change, is of about 50%. The presence in the film of a high amount of phase with high transition temperature (Tc(R=0)=93 K) should improve the superconducting carrying capacity. The critical current density versus magnetic field at 40K for the YBCO film grown on (100) SrTiO3 at 850 0C is presented in Figure 11. The critical current density was derived from the M(B) curves at different magnetic field using the Beam critical state model.

Figure 10. Electrical resistance versus temperature for the YBCO film grown on (100) SrTiO3 at 800 0C (green) and 8500C (red) and for the YBCO film grown at 8500C on CeO2/YSZ/CeO2/Pd buffered Ni-5at.%W substrate (blue).

Figura 11. Critical current density vs magnetic field at 40K for the YBCO film grown on (100) SrTiO3 at 850 0C.


3. Conclusions

A modified TFA-MOD method for the deposition of YBCO films was studied. With respect to the conventional TFA-MOD, the yttrium and copper trifluoroacetates were replaced by the alcoholic solution of Cu and Y acetates dispersed in propionic acid. The method presents several advantages such as: eliminates the evolvement of hydrofluoric acid and, as a result, permits the deposition of thicker YBCO films, overcomes the problems related to the sublimation of copper trifluoroacetate and shortens the conversion time of the precursor film. The YBCO film grown both on (100) SrTiO3 and CeO2/YSZ/CeO2/Pd buffered Ni-5at.%W substrate exhibits good morphological, structural and superconducting properties with Tc(R=0) greater than 91K and an out-of-plain texture with of 0.240 and 0.190 , respectively.

References

[1] P. C. McIntrye, M. J. Cima and A. Roshko, “Epitaxial nucleation and growth of chemically derived Ba2YCu3O7-x thin films on (001) SrTiO3”, J. Appl. Phys. 77 (1995) 5263-72.

[2] J. A. Smith, M. J. Cima and N. Sonnenberg, “High critical current thick MOD-derived YBCO films”, IEEE Transactions on Applied Superconductivity 9 (1999) 1531-34.

[3] T. Araki, I. Hirabayashi, “A chemical approach to YBa2Cu3O7-x-coated Sauperconductors-metalorganic deposition using trifluoroacetates topical review”, Supercond. Sci. Technol. 16 (2003) R71-94.

[4] N. Roma, S. Morlens, S. Ricart, K. Zalamanova, J. M. Moreto, A. Pomar, T. Puig and X. Obradors, “Acid anhydrides: a simple route to highly pure organometallic solutions for superconducting films”, Supercond.Sci.Technol 19 (2006) 521-27.

[5] G. Celentano, V. Galluzzi, A. Mancini, A. Rufoloni, A. Vannozzi, A. Augieri, T. Petrisor, L. Ciontea and U. Gambardella, “YBCO coated conductors on highly textured Pd-buffered Ni-W tape”, Journal of Physics: Conference Series 43 (2006) 158–61.

[6] A. Rufoloni, A. Augieri, G. Celentano, V. Galluzzi, A. Mancini, A. Vannozzi, T. Petrisor, L. Ciontea, V. Boffa and U. Gambardella, “YBa Cu O films prepared by TFA-MOD method for coated conductor application”,

2 3 7-x

Journal of Physics: Conference Series 43 (2006) 199-202.

Publised papers

1. A. Augieri, G. Celentano, U. Gambardella, J. Halbritter, T. Petrisor, “Analysis of angular dependence of pinning mechanisms on Ca-substituted YBa2Cu3O7-δ epitaxial thin films”, Superconductor Science and Technology 20 (4), art. no. 013, pp. 381-385 (2007).

2. V. Galluzzi, A. Augieri, L. Ciontea, G. Celentano, F. Fabbri, U. Gambardella, A. Mancini, T. Petrisor, N. Pompeo, A. Rufolonu, E. Silva, A. Vannozzi, “YBa2Cu3O7-δ films with BaZrO 3 inclusions for strong-pinning in superconducting films on single crystal substrate”, IEEE Transactions on Applied Superconductivity 17 (2), pp. 3628-3631 (2007).

3. A. Vannozzi, A. Augieri, G. Celentano, L. Ciontea, F. Fabbri, V. Galluzzi, U. Gambardella, A. Mancini, T. Petrisor, A. Rufoloni, “Cube textured substrates for YBCO coated conductors: Influence of initial grain size and strain conditions during tape rolling”, IEEE Transactions on Applied Superconductivity 17 (2), pp. 3436-3439 (2007).

http://www.scopus.com/scopus/search/submit/author.url?author=Augieri%2c+A.&origin=resultslist&authorId=8602108400&src=s


V. Avrigeanu, M. Avrigeanu, and F.L. Roman

DEVELOPMENT OF CALCULATION TOOLS: CALCULATION OF CROSS SECTIONS FOR 50,52CR UP TO 60 MEV TW6-TTMN-001B (EFDA/O7-1627)

“Horia Hulubei” National Institute of R&D for Physics and Nuclear Engineering, Magurele

The activation cross sections of the isotopes 50,52Cr in the excitation-energy range up to 60 MeV have been analyzed as part of a general investigation [1-5] of the reaction mechanisms of fast neutrons at low and medium energies. This analysis results also enabled a stringent test of models for the above-mentioned nuclear processes, although our primary aim was to comply with the needs of a sound, complete and reliable neutron-induced cross section data library to address safety and environmental issues of the fusion programme [6]. Thus, in order to gain insight into this problem, we have analyzed these activation cross sections using the parameter databases obtained previously by global optimization within the computer codes TALYS [7] and EMPIRE-II [8], as well as a local parameter set within the STAPRE-H code [9] using the pre-equilibrium emission (PE) model Geometry-Dependent Hybrid (GDH). No fine tuning was done to optimize the description of the nucleon emission for all the cases, but for STAPRE-H a consistent set of local parameters has previously been established or validated on the basis of independent experimental information of, e.g., neutron total cross sections, proton reaction cross sections, low-lying level and resonance data, and gamma-ray strength functions based on neutron-capture data. The comparison of various calculations, including their sensitivity to model approaches and parameters, has concerned all the activation channels for which there are measured data. It was thus avoided the use of model parameters which have been improperly adjusted to take into account properties peculiar to specific nuclei in the decay cascade. This work has had to be done for the four isotopes since the consistency of model calculations can be proved only by consideration of at least all stable isotopes of an element. This point is of particular importance for the Cr isotopes due to a known model inconsistency found by EC/JRC scientists within the trial [10] to describe unitary the (n,p) and (n,2n) reaction cross sections up to the incident energy of 20 MeV. This lack of consistency remains also within the more recent work of Han [11] which stops at the incident energy of 20 MeV while more data are known up to 40 MeV. Actually the complexity and difficultness of this analysis for the Cr isotopes have been proved even as an unsolved sample case within the EMPIRE-II manual (Ref. [8], pp. 171-172).

The global predictions provided by TALYS-1.0 and EMPIRE-II v.2.19 up to 60 MeV for 50,52Cr are shown in Figs. 2-4, in comparison with the experimental data available at lower energies. One may note that the above-mentioned model inconsistency [10] is still surrounded by these calculated results, the (n,p) and (n,2n) reactions being suitable described by using the TALYS code, deeply involved in the EAF/EFF development, only in the case of the isotope 52Cr. The very few data sets available above 20 MeV but below 40 MeV neutrons on 50,52Cr are also not described except four 52Cr(n,2n)51Cr reaction cross section data. On the other hand the EMPIRE-II results decrease systematically faster with energy than the TALYS ones. Under these conditions the correctness of model predictions for the incident energies up to 60 MeV is rather questionable. Thus, the overview of the calculated activation cross sections shows that an analysis based on a consistent local parameter set is necessary in order to explain the differences between the experimental, calculated and evaluated cross sections.

59


0.1 1 100

2

4

6

8Newson (s) (1961)Spencer+(s) (1972)Dyumin+ (1972)Abfalterer+(2001): natCr

n + 50Cr

σ t (b)

Koning+ (global, 2003): S0=2.24, S

1=.73

Koning+ (local, 2003): S0=2.19, S

1=.72

-mod.(E<4): S0=2.12, S

1=.62

RIPL2: S0=2.40(40) (E=500keV)

S1=0.71(16) (E=450keV)

0.1 1 10

2

4

6

8 Thibault+ (1967) Foster Jr+ (1971) Spencer+ (1972) Dyumin+ (1972) Carlton+ (2000) Abfalterer+ (2001): natCr

n + 52Cr

Koning+ (global, 2003): S0=2.24, S1=.70 Koning+ (local, 2003): S0=2.24, S1=.69 - mod.(E<4MeV): S0=2.22, S1=.59

[ aV=.587+.02E, rV=1.21-.005E ]

RIPL2: E=500keV: S0=2.85(45)

E=450keV: S1=0.30(5)

0.1 1 10

2

4

6

8

Foster Jr+ (1971) Dyumin+ (1972) Abfalterer+(2001): natCr

n + 53Cr

σ t (b)

En (MeV)

RIPL2: S0=4.60(100) (E=100keV)

Koning+ (global, 2003): S0=4.04, S1=.73 Koning+ (local, 2003): S

0=4.04, S

1=.72

-mod. (E<4): S0=3.77, S

1=.61

0.1 1 100

5

10

15

20 Farrell+ (s) (1966) Dyumin+ (1972) Agrawal+ (s) (1984) Abfalterer+(2001): natCr

n + 54Cr

En (MeV)

RIPL2: S0=2.30(100) (E=500keV) S1=0.67(11) (E=450keV)

Koning+ (global, 2003): S0=2.23, S1=.66 Koning+ (local, 2003): S0=2.23, S1=.66 - mod.(E<4): S0=2.20, S1=.58

Figure 1. Comparison of calculated and measured neutron total cross sections for all stable Cr isotopes

15 20 25 30 35 400.00

0.05

0.10

0.15

0.20

Qaim+ (1972)Araminowicz+(1973) Valkonen (1976)Sigg (1976)Sailer+ (1977)Berrada (1984)

50Cr(n,2n)49Cr

Dighe+ (1991)Ercan+ (1991)Uwamino+(1992)Pansare+ (1993)

TALYS-1 EMPIRE-2.19 Degas EMPIRE-2.19 HMS STAPRE-H

Bahal+ (1984)Ribansky+ (1985)Pepelnik+ (1985)Zhou M+ (1987)Ghorai+ (1987)Ykeda+ (1988)

continuum

25 30 35 400.000

0.005

0.010

Uwamino+ (1992)

50Cr(n,3n)48Cr


5 10 15 200.00

0.05

0.10

0.15

Dendorfer+ (1981)

50Cr(n,α)47Ti

σ (b)


10 15 201E-3

0.01

0.1

Grimes+ (1979)

50Cr(n,xd)

TALYS-1 STAPRE-H

En (MeV)

10 15 200.0

0.3

0.6

0.9

Klochkova+ (1992)Fessler+ (1999)

50Cr(n,np+pn+d)49V

En (MeV)


(n,pn) (n,np) (n,d)

5 10 15 200.0

0.1

0.2

0.3

0.4

0.5


50Cr(n,p)50V Klochkova+ (1992)

0 5 10 15 200.0

0.3

0.6

0.9

Grimes+ (1979)

50Cr(n,xp)

STAPRE-H (n,pn) (n,np)

TALYS-1 EMPIRE-2.19 Degas EMPIRE-2.19 HMS

5 10 15 200.00

0.05

0.10

0.15

Dolya+ (1973)Grimes+ (1979)Matsuyama+ (1993)

50Cr(n,xα)


(n,αn) (n,nα)

Figure 2. Comparison of calculated and measured activation cross sections for 50Cr isotope.

The neutron optical potential was the first subject of the local parameter analysis. A basic point in this respect is the highlighting that the global potential [12], used by default in both TALYS

60


and EMPIRE codes, does not reproduce the minimum around the neutron energy of 1-2 MeV for the total neutron cross sections of the mass A~60 nuclei. Following also their comment of the constant geometry parameters which may be responsible for this aspect, we have applied the SPRT method by using the RIPL-2 data [13] for the low-energy neutron scattering properties (S0, S1, R’) and the available measured neutron total cross sections [14] of the neutron total cross sections for 50,52,53,54Cr stable isotopes beyond the neutron energy of even 60 MeV (Fig. 1). A decrease of approx. 7% for this cross section has thus resulted around the incident energy of 1 MeV, corresponding to the average energy of the statistically emitted neutrons. Next, this potential has been involved in the calculation of the corresponding collective inelastic scattering cross sections by means of the direct-interaction distorted-wave Born approximation (DWBA) method, with the local version of the computer code DWUCK4 [15]. The collective state parameters of Han [11] were used in this respect, fractions of the direct inelastic scattering to compound nucleus cross section being obtained as large as approx.11% for 50Cr and approx.7% for 52Cr and decreasing with the energy by approx.50% up to 60 MeV.

Figure 3. Comparison of calculated and measured activation cross sections for 52Cr isotope.

The proton optical potential [12] was also modified on the basis of a comparison of calculated and measured cross sections of (p,n) reaction on 51V target nucleus as well as the corresponding (p,γ) reaction data. At the same time, the electric dipole γ-ray strength functions fE1(Eγ) which are used for the calculation of the γ-ray transmission coefficients, have been obtained by means of a modified energy-dependent Breit-Wigner (EDBW) model [16]. Moreover, systematic EDBW correction factors FSR were obtained by using the experimental average radiative widths Γγ0

exp of the s-wave neutron resonances, and assuming that FSR=Γγ0exp/Γγ0

EDBW. Next, the fE1(Eγ) thus obtained were checked by calculation of capture data.

61

15 20 25 30 35 400.0

0.2

0.4

0.6

Sailer+ (1977)Berrada (1983)Ribansky+ (1985)Ghorai+ (1987)

52Cr(n,2n)51Cr

TALYS-1 EMPIRE-II Degas EMPIRE-II HMS STAPRE-H

Lychagin+ (1988)Ykeda+ (1988)Wagner+(1989)Liskien+ (1989)Ercan+ (1991)Uwamino+ (1992)Molla+ (1997)Fessler+ (1998)

12 14 16 18 200.00

0.05

0.10

0.15

TALYS-1 EMPIRE-II D. EMPIRE-II HMS STAPRE-H

52Cr(n,2n)51mCr

Deak+(1974)

[3/2-, 3.3ns]

5 10 15 200.00

0.05

0.10

0.15Prasad+ (1971)Desler+ (1973)Aleksandrov+(1975)Valkonen (1976)Qaim (1977)Artem'ev+ (1980)Smith+ (1980) Viennot+ (1982)Ribansky+(1985)Gupta+ (1985)Luc+ (1986)Ghorai+ (1987)Ikeda+ (1987)Kawade+(1990)

52Cr(n,p)52V

Viennot+(1991)Ercan+ (1991)Osman+ (1996)Kasugai+(1998)Fessler+ (1998)Thiep+ (2003)

TALYS-1 EMPIRE-II Degas EMPIRE-II HMS STAPRE-H

σ (b

)

5 10 15 200.0

0.1

0.2

0.3

TALYS-1 EMPIRE-II Degas EMPIRE-II HMS

52Cr(n,xp)Grimes+ (1979)

STAPRE-H (n,pn) (n,np)

5 10 15 201E-5

1E-4

1E-3

0.01

0.1

Dolya+ (1973)Grimes+ (1979)Grimes+ (1979): natCrPaulsen+ (1981): natCrWattecamps+(1983): natCrKneff+ (1986): natCr

52Cr(n,xα)

TALYS-1 EMPIRE-II Degas EMPIRE-II HMS STAPRE-H STAPRE-H: natCr

12 16 201E-4

1E-3

0.01

En (MeV)

Grimes+ (1979)

52Cr(n,xd)

TALYS-1 STAPRE-H

0 5 10 15 200.0

0.5

1.0

1.5

2.0

En (MeV)

52Cr(n,n')

IRMM (2007) IRMM (2007) -av. σR

σ(n,n)+(n,n') [+DI]

σ(n,n') [+DI]


Finally a suitable description has been obtained, in the limits of the more recent data, of all activation cross sections for the Cr stable isotopes (Figs. 2-4). The good agreement of the calculated cross sections with the more recent data between 14 and 21 MeV solved thus the model inconsistency pointed out previously. The basic point in this respect has been the use within the PE model of an advanced particle-hole state density [17]. On the other hand, the predictive power of the local-approach model calculations has been just proved by the overestimation of measured cross sections for 50Cr(n,2n)49Cr reaction above 15 MeV (Fig. 2) in quite close agreement with results [18] of the most recent integral data analysis. The present consistent model calculations have also pointed out the questionable saturation of the excitation functions of (n,n’p+d) reaction on 53,54Cr isotopes, showing the need of additional measurements.

Figure 4. Comparison of calculated and measured activation cross sections for Cr isotopes. 53,54

10 15 200.00

0.01

0.02Husain+ (1967)Qaim (1974)Valkonen (1976)Sailer+ (1977)Artem'ev+ (1980)Ribansky+ (1985)Luc+ (1986)Dighe+ (1991)Ercan+ (1991)Kawade+ (1992)Grallert+ (1993)Osman+ (1996)Kasugai+ (1998)Fessler+ (1998)Thiep+ (2003)

54Cr(n,α)51Ti

En (MeV)


14 16 18 201E-4

1E-3

0.01

0.1

Ribansky+(1985)Luc+ (1986)Fessler+ (1998) Sakane+(2002) Thiep+(2003)

54Cr(n,np+pn+d)53V


(n,pn) (n,np) (n,d)

10 15 200.00

0.02

0.04

Husain+ (1967)Valkonen (1976)Qaim+ (1977)Artem'ev+ (1980)Bahal+ (1984)Ribansky+ (1985)Luc+ (1986)Kawade+ (1992)Kasugai+ (1998)Fessler+ (1998)Thiep+ (2003)

54Cr(n,p)54V

σ (b

)


TH - (n,2np)

5 10 15 200.00

0.02

0.04

Dolya+ (1973)

TALYS-1 EMPIRE-2.19 Degas EMPIRE-2.19 HMS

54Cr(n,xα)

En

(MeV)

STAPRE-H (n,αn) (n,nα)

20 25 301E-3

0.01

0.1


53Cr(n,3n)51Cr

σ (b

)

E (M V)

Qaim+ (1980)

14 16 18 20 221E-3

0.01

0.1Ribansky+ (1985)Luc+ (1986)Fessler+ (1998) Sakane+ (2002) Thiep+ (2003)

53Cr(n,np+pn+d)52V


(n,pn) (n,np) (n,d)

TH - (n,2np)

5 10 15 200.00

0.02

0.04

0.06

Qaim (1977)Artem'ev+ (1980)Smith+ (1980) Viennot+ (1982)+Bahal+ (1984)+Ribansky+ (1985)Pepelnik+ (1985)Luc+ (1986)Kawade+ (1990)

53Cr(n,p)53V

Ercan+ (1991)Viennot+ (1991)Osman+ (1996)Kasugai+(1998)Fessler+ (1998)Thiep+ (2003)


5 10 15 200.00

0.05

0.10

TALYS-1 EMPIRE-2.19 Degas EMPIRE-2.19 HMS STAPRE-H (n,αn) (n,nα)

53Cr(n,xα)Dolya+ (1973)

62


Since no free parameter was involved in the GDH calculations while the same common parame

eferences

ters of OMP and nuclear level density were used in the DWBA, GDH and HF model calculations, the proper description of a large body of data without free parameters validated both the adopted nuclear model assumptions and parameter set. Actually it resulted to be of rather equal importance the agreement with both the data for, e.g., the (n,p) and (n,2n) reactions around the neutron energy of 14 MeV, and the (n,xn) data above 20 MeV. The former is critical for the check of partial level densities which trigger the PE calculated data, while the latter makes possible the analysis of basic assumptions of PE model within an enlarged energy range.

R

P., Avrigeanu V.[1] Reimer , Plompen A.J.M., Qaim S.M., “Reaction mechanisms of fast neutrons on 51V below 21 MeV”, Phys. Rev. C 65 (2001) 014604.

[2] Semkova V., Avrigeanu V., Glodariu T., Koning A.J., Plompen A.J.M., Smith D.L., Sudar S., “A systematic investigation of reaction cross sections and isomer ratios for neutrons up to 20 MeV on Ni-isotopes and 59Co”, Nucl. Phys. A 730 (2004) 255.

[3] Reimer P., Avrigeanu V., Chuvaev S., Filatenkov A.A., Glodariu T., Koning A.J., Plompen A.J.M., Qaim S.M., Smith D.L., Weigmann H., “Reaction mechanisms of fast neutrons on stable Mo isotopes below 21 MeV”, Phys. Rev. C 71 (2005) 044617.

[4] Avrigeanu V., Chuvaev S.V.,. Eichin R, Filatenkov A.A., Forrest R.A., Freiesleben H., Herman M., Koning A.J., Seidel K., “Pre-equilibrium reactions on the stable tungsten isotopes at low energy”, Nucl. Phys. A 765 (2006) 1;

Avrigeanu M., von Oertzen W., Avrigeanu V., “On temperature dependence of the optical potential for alpha-particles at low energies”, Nucl. Phys. A764 (2006) 246.

[5] Avrigeanu M., Chuvaev S.V., Filatenkov A.A., Forrest R.A., Herman M., Koning A.J., Plompen A.J.M., Roman F.L., Avrigeanu V., “Fast-neutron induced pre-equilibrium reactions on 55Mn and 63,65Cu at energies up to 40 MeV”, Nucl. Phys. A 806 (2008) 15.

[6] Forrest R.A., Kopecky J., Fus. Eng. Design 82 (2007) 73.

[7] Koning A.J., Hilaire S., Duijvestijn M.C., in: R.C. Haight et al. (Eds.), Proc. Int. Conf. on

in M., Trkov A., Ventura A., Zerkin V., in:

Nuclear Data for Science and Technology, 2004, Santa Fe, AIP Conf. Proc. No. 769 (American Institute of Physics, New York, 2005), p. 1154.

[8] Herman M., Oblozinsky P., Capote R., SR.C. Haight, M.B. Chadwick, T. Kawano and P. Talou (Eds.), Proc. Int. Conf. on Nuclear Data for Science and Technology (ND2004), 26 Sept. - 1 Oct. 2004, Santa Fe, New Mexico (AIP, New York, 2005), p. 1184; EMPIRE-II v.2.19, http://www-nds.iaea.org/empire/.

[9] Avrigeanu M., Avrigeanu V., “STAPRE-H95 Computer Code”, IPNE Report NP-86-1995,

6.

ys. A 713 (2003) 231.

s/

Bucharest, 1995, and references therein; News OECD/NEA Data Bank 17 (1995) 22.

[10] Fessler A., Wattecamps E., Smith D.L., Qaim S.M., Phys. Rev. C 58 (1998) 99

[11] Han Y., Nucl. Phys. A 748 (2005) 75.

[12] Koning A.J., Delaroche, J.P., Nucl. Ph

[13] Ignatyuk A.V., at http://www-nds.iaea.or.at/RIPL-2/resonance .

[14] EXFOR Nuclear reaction data, http://www-nds.iaea.or.at/exfor/ .

63

http://tandem.nipne.ro/%7Evavrig/publications/2003/papni_01.ps

http://www-nds.iaea.or.at/RIPL-2/resonances/


[15] Avrigeanu M., Avrigeanu V., “Computer code system DWUCK4 (INPE-Bucharest local version)”, NEA-DB Index: NESC9872/09; News NEA DB 17, 22 (1995), OECD/NEA, Paris.

[16] Avrigeanu M., Avrigeanu V., Cata G., Ivascu M., „EDBW model for the E1 gamma-ray strength function in the mass range 50<A<90“, Rev. Roum. Phys. 32 (1987) 837.

[17] Avrigeanu M., Avrigeanu V., Comp. Phys. Comm. 112 (1998) 191; Harangozo A., Stetcu I., Avrigeanu M., Avrigeanu V., Phys. Rev. C 58 (1998) 295.

[18] R.A. Forrest, EASY-2007 Validation, Report EFFDOC-1036, EFF/EAF Meeting, Aix-en-Provence, 21-23 May 2008.

64


ITER-LIKE WALL PROJECT AT JET: OPTIMIZATION AND MANUFACTURING AND TESTING OF 10 µm W -COATINGS FOR CFC TILES TO BE INSTALLED IN JET JW6-TA-EP2-ILC-01 (EFDA/06-1507)

Cristian Ruset, Eduard Grigore, Ion Munteanu, Nicolae Budica, Daniela Nendrean

National Institute for Laser, Plasma and Radiation Physics, Magurele

1. Introduction

Currently, the primary ITER materials choice is a full beryllium main wall with CFC (carbon fiber composite) at the strike points and tungsten at divertor baffles and dome. Since this combination has never been tested in a tokamak, ITER-like Wall project has been launched at JET (Joint European Torus) with the aim to replace the actual CFC first wall with a new one, comprising the same materials choice as it planned for ITER operating with the same materials planned for ITER. In the R&D phase of this project (EFDA Task Agreement Code: JW5-TA-EP-BEW-02), seven PVD and CVD technologies have been developed for 10 μm W coating of CFC material by EURATOM Associations from Germany, France, Finland and Romania in cooperation with European coating Companies. As a result of High Heat Flux (HHF) tests carried out in the hydrogen beam facility GLADIS at IPP Garching, Combined Magnetron Sputtering and Ion Implantation (CMSII) technology, proposed by MEdC, was selected for 10 μm W coating of about 1,000 tiles, of different size and dimensions, under industrial conditions

2. Objectives

The main objective of the present project is the coating with 10 µm W approximately 1,000 CFC tiles for installation in JET. About 10% of the coated tiles will be tested at IPP Garching in GLADIS facility in order to check the reproducibility of the deposition process.

In order to achieve this objective, the following deliverables and milestones have been defined for 2007:

- D2/M2 Optimization of the coating method, as applied during the R&D phase, in order to apply it to the JET relevant larger scale production

- D3/M3 Designing, building, commissioning and operating the new W-coatings facilities required for the JET relevant larger scale production and quality control.

3. Results and discussions

The CMSII technique involves simultaneous magnetron sputtering and high energy ion bombardment. In the deposition process, three low pressure electrical discharges (magnetron discharge, DC bias discharge and high voltage pulse discharge) are superposed. Typical parameters for the high voltage pulse discharge are: U = 30 – 50 kV, τ = 20 μs, f = 25 Hz. The DC bias up to – 900 V is applied between pulses.

The plasma ions from the magnetron discharge are accelerated during the high voltage pulses and strike initially the substrate and then the coating itself during its growing with


energies of tens of keV. As a result of the periodical ion bombardment the following effects occur:

- An increase of the surface mobility of the deposited atoms which leads to a high densification of the layer.

- An extremely dense, pore free nano-structure is produced. TEM analyses have shown crystallites with a size of less than 10 nm [1].

- A stress relief at the interface and within the layer. Due to this effect, coatings with a thickness of 10 – 30 μm can be produced. In comparison, the maximum W coating thickness which can be deposited on fine grain graphite substrate is 3 μm.

The deposition rate for CMSII technology is in the range of 4 – 8 μm/h depending on the coating to be deposited.

3.1. Optimization of the coating method

The tiles tested in the R&D phase of the project were coated in a relative small experimental unit (Φ300 x 420 mm) with only one magnetron, using CMSII technique.

The transfer of this technique from this small unit to an industrial unit with 24 magnetrons was an important and difficult task of this project. Many scientific and technological aspects, particularly the plasma interaction from two or more magnetrons were unknown. In order to get information about these aspects an existing vacuum chamber (Φ500 x 450 mm) was transformed into a CMSII deposition chamber with 4 magnetrons.

The experiments carried out with this chamber were focused on the following targets:

- Control of the CMSII discharge produced by two and four magnetrons running simultaneously

- Coating uniformity obtained with two magnetrons

- Optimization of the processing parameters in order to obtain W coatings with a thickness of 9 – 12 μm and the same properties like those produced with the small experimental unit.

The main results of these experiments can be summarized as follows:

a) A four channel power supply has been designed, manufactured and successfully tested as a preliminary version of the power supply for the industrial coating unit. The main characteristics of this power supply were: voltage range: 200 – 1,000 V, current intensity: 0.1 – 2.0 A, current stability: > 98%, number of channels: 4, independent control of each channel, protection to transients to arc discharge.

b) The processing parameters have been optimized and a coating area of approx. 200 x 100 mm where the thickness non-uniformity is in the range of ± 10% was established.

c) The HHF tests carried out at IPP Garching in September 2007 with a lot of CFC samples coated with the intermediary chamber were successfully. The tiles coated perpendicular to fiber planes have been subjected initially to a thermal screening with increasing power densities from 10 MW/m2 up to 23 MW/m2 for 1.5 s. This test was followed by a cycling loading of 100 pulses at 16.5 MW/m2 for 2 s. Microscopic examination of the tested samples reveals no hints of buckling or delamination. Only small tensile cracks have been detected (Fig.1).


Figure 1. SEM images of the W coatings after HHF tests

3.2. Design and manufacturing of the New Coating Facility (NCF)

Schematic representation and the general view of the coating equipment are shown in Fig.2. The deposition chamber, made of stainless steel with double wall for water cooling, has an inner diameter of 800 mm and a height of 750 mm. Twenty-four magnetrons are positioned inside the chamber in 8 columns of 3 magnetrons each. By this way a usable volume Φ 420 x 360 mm is provided. On the top lid of the chamber there is a ceramic insulator designed to sustain 100 kV. On this insulator a double axes rotating load support is installed. Four widows allow the visual inspection of the load during the coating process. Mass flow controllers (MFC) are used to introduce argon and reactive gases (if necessary) into the deposition chamber. A turbomolecular pump (TMP) of 1,000 l/s ensures the evacuation of the chamber. A gas analyzer with a quadrupole mass spectrometer is connected to the chamber in order to monitor the composition of the deposition atmosphere. The magnetrons are energized by an 18 channel DC power supply with a total power of 25 kW, which was designed and manufactured in the framework of this project. The current intensity for each channel is stabilized (±1%) and can be adjusted independently in the range of 0.3 – 2.5 A. Arc suppression devices are installed to each channel.

(a)

(b)

Figure 2. CMSII coating equipment: schematic representation (a) and general view (b) The High Voltage Pulse Generator provides pulses up to 50 kV with a width of ~ 20 μs, a frequency of 12.5 - 50 Hz and a maximum current intensity of ~ 40 A.


The coating facility was commissioned by 31.12.2008.

Figure 3. W coated tiles during the HHF test

The first lot of samples coated with the NCF was successfully tested at IPP Garching in hydrogen beam GLADIS machine at power densities up to 23.5 MW/m2 for 1.5 s and cycling loading at 16.5 MW/m2 for 2s. No blisters or delamination have been detected after HHF tests. The surface of the W coating looked like that shown in Fig.1. A picture of the tiles during the thermal screening is shown in Fig. 3.

3.3. Qualification of the GDOES method for quality control of the W coatings.

Glow Discharge Optical Emission Spectrometry (GDOES) is currently used for measurement of the coating thickness and impurities as a quality control technique for industrial production. The equipment used for this purpose is GDA – 750 HP machine supplied by SPECTRUMA GmbH, Germany (Fig.4). In each coating run, four witness titanium samples are introduced at different locations including areas where the coating thickness exhibits maximum and minimum values. This solution has been chosen because CFC is porous and it can not be used directly for GDOES analysis. After coating, Ti witness samples are analyzed. Typical depth profiles of W, Mo, C, O and Ti are shown in Fig.5. As it can be seen the C and O concentrations within the W coating are negligible. The thickness measured by GDOES was compared with the thickness measured on the same sample by optical microscopy with a precision of 0.2 μm. The values correspond within an error range of ± 1 μm.

Figure 4. GDOES equipment Figure 5. Typical GDOES depth profiles of W, Mo, C and O for a W coating deposited on Ti substrate

3.4. Development of a suitable technique for removal of W coating from CFC tiles

The CFC tiles are very expensive. In order to recover these tiles in case of failure of the coating process, a device and a method were developed for removing the W coating from the CFC surface. The method involves mechanical removal with a diamond tool (Φ15 x 15 mm). A small electric motor rotates a cylindrical diamond tool which easily removes the W coating. By means of a micrometric screw, the depth of the removed layer can be adjusted in the range of 50 – 500 μm. After W removal, the tile is U/S cleaned and dried.


4. Conclusions

All the tasks of the MEdC Association for 2007, in the framework of the Task Agreement JW6-TA-EP2-ILC-01 were fulfilled. Industrial coating equipment based on CMSII deposition method was designed, manufactured and commissioned. The HHF tests carried out at IPP Garching in GLADIS machine proved that the CMSII method can be successfully applied at industrial scale.


A close cooperation with IPP Garching and JET is established. In April 2007 two specialists from MEdC (C.Ruset and E.Grigore) participated in HHF tests at IPP Garching. Technical aspects concerning the HHF test parameters and their correlation with the W coating characteristics have been discussed. C.Ruset attended the two Project Board and the associated technical meetings at JET.

6. Activities to be performed in 2008

In 2008 the CMSII technology for W coating of CFC substrates will be qualified and CFC tiles from JET first wall will be coated. Quality control documents will be issued for each coating run.

7. Publications

[1] C. Ruset, E. Grigore, H. Maier, R. Neu, X. Li, H. Dong, R. Mitteau, X. Courtois and JET EFDA contributors, “W Coatings Deposited on CFC Tiles by Combined Magnetron Sputtering and Ion Implantation Technique”, Phys. Scr. T128, 2007, 171 – 174.

[2] H. Maier, R. Neu, H. Greuner, Ch. Hopf, G.F. Matthews, G. Piazza, T. Hirai, G. Counsell, X. Courtois, R. Mitteau, E. Gauthier, J. Likonen, G. Maddaluno, V. Philipps, B. Riccardi, C. Ruset, EFDA-JET Team, “Tungsten Coatings for the JET ITER-like Wall Project”, Journal of Nuclear Materials, Vol. 363-365, 2007, p 1246-1250.

[3] H. Maier, T. Hirai, M. Rubel, R. Neu, Ph. Mertens, H. Greuner, Ch. Hopf, G. F. Matthews, O. Neubauer, G. Piazza, E. Gauthier, J. Likonen, R. Mitteau, G. Maddaluno, B. Riccardi, V. Philipps, C. Ruset, C.P. Lungu, I. Uytdenhouwen and JET EFDA contributors, “Tungsten and Beryllium Armour Development for the JET ITER-like Wall Project”, Nucl. Fusion, Vol. 47, 2007, pp. 222-227.

[4] R.Neu, H. Maier, E. Gauthier, H. Greuner, T. Hirai, Ch. Hopf, J. Likonen, G. Maddaluno, G. F. Matthews, R. Mitteau, V. Philipps, G. Piazza, C. Ruset, JET EFDA contributors, “Investigation of Tungsten Coatings on Graphite and CFC”, Phys. Scr. T128, 2007, 150 – 156.

[5] G. F. Matthews, P. Edwards, T.Hirai, M. Kear, A. Lioure, P. Lomas, A. Loving, C. Lungu, H. Maier, P. Martens, D. Neilson, R. Neu, J. Pamela, V. Philipps, G. Piazza, V. Riccardo, M. Rubel, C. Ruset, E. Villedieu and M. Way, “Overview of the ITER-like wall project”, Phys. Scr. T128, 2007, 137 – 143.

[6] T. Hirai, H. Maier, M. Rubel, Ph. Mertens, R. Neu, E. Gauthier, J. Likonen, C. Lungu, G. Maddaluno, G. F. Matthews, R. Mitteau, O. Neubauer, G. Piazza, V.Philipps, B. Riccardi, C. Ruset, I. Uytdenhouwen and JET EFDA contributors, "R&D on full tungsten

http://www.sciencedirect.com/science/journal/00223115

http://www.sciencedirect.com/science?_ob=PublicationURL&_tockey=%23TOC%235595%232007%23996369999%23655376%23FLA%23&_cdi=5595&_pubType=J&view=c&_auth=y&_acct=C000058829&_version=1&_urlVersion=0&_userid=2886418&md5=c877cbb30dbadebb9c135d8e77b5f161


divertor and beryllium wall for JET ITER-like Wall Project", Fusion Engineering and Design, Vol. 82, Issues 15-24, Oct. 2007, pp 1839-1845.

[7] R.Mitteau, JM. Missiaen, P. Brustolin, O. Ozer, A. Durocher, C. Ruset, C.P. Lungu, X. Courtois, C. Dominicy, H. Maier, C. Grisolia, G. Piazza, P Chappuis, “Recent Developments Toward the Use of Tungsten as Armour Material in Plasma Facing Components”, Fusion Engineering and Design, Vol. 82, Issues 15-24 Oct. 2007, pp 1700-1705.


TW6-TTFD-TPI-55 (EFDA/06-1511) UPDATE OF ITER ISS-WDS PROCESS DESIGN – 2

A. Lazar*, S. Brad*, N. Sofalca*, M. Vijulie* I.Cristescu**,L. Dör**, W. Wurster**

*National Institute for Cryogenics and Isotope Technologies (ICIT), Rm. Valcea **Forshungszentrum Karlsruhe – Tritium Laboratory, Germany

1. Introduction

A Task Force Fuel Cycle has been established at FZK aiming to design the deuterium/tritium system of ITER. The MEdC Association provides support on activities at TLK dedicated to the design of the Tritium Plant Systems.

The ITER Isotope Separation System (ISS) and Water Detritiation System (WDS) are to be integrated in order to reduce potential chronic tritium emissions from the ISS. This is achieved by routing the top (protium) product from the ISS to a feed point near the bottom end of the WDS Liquid Phase Catalytic Exchange (LPCE) Column. This provides an additional barrier against ISS emissions and should mitigate the memory effects due to process parameter fluctuations in the ISS.

During operation of ITER, tritiated water will be produced in various systems. The expected sources are [1], [2], [3]:

• condensate generated from the normal operation of various atmosphere detritiation systems and HVACs.

• tritium process component maintenance,

• condensate from the air coolers in the containment volume (designed to limit overpressures from an ex-vessel coolant leak),

• air detritiation dryers in the containment volume,

• the tokamak cooling water system maintenance drain and the tokamak cooling water system vent gas condensate,

• in-vessel component maintenance drain collected in the hot cell

• condensate from the standby vent detritiation system and the standby atmosphere detritiation system operated during tritium contamination accidents.

The objective of this task was to update the designs of the ITER ISS and WDS as documented in the 2001 FDR (Final Design Report) taken into account the result and the recommendation of the FMEA report [3] and experimental results from ongoing R&D tasks. During the preparation of the Design Description Document package for the final report of ITER 2001 [1], [2] a number of trades off between the Tritium Plant subsystems have been already identified. The design of the strongly interlinked inner deuterium-tritium fuel cycle of ITER need to balance requirements between all subsystems and shall be based upon experimentally proven technologies.


The CATIA V5 software was chosen to create, using the PRM (Project Resource Management) [8], the 3D layouts of plant sites by defining the buildings, the major areas, all the way down in the plant area, the path to the equipment and so on. Subareas for facilities and technological equipments have been created within the plant. The system allows a hierarchical approach including true partition of space with shared boundaries, areas with multi patches, and so on.

2. Process Description

The Combined Electrolysis Catalytic Exchange (CECE) method in combination with Cryogenic Distillation (CD) was chosen for tritium recovery from tritiated water which will be produced during ITER operation. A potential combination of the ITER WDS and of the ITER ISS is shown in Figure 1.

Figure 1. Block diagram of ISS-WDS

The ITER water detritiation plant comprises two subsystems to process and temporary

store the quantities of tritiated water: the water detritiation system (WDS) and the tritiated water holding tank system [2]. The tritiated water holding tank system stores the various tritiated water streams according to the tritium concentrations prior to processing. Tritiated water is fed into the LPCE column at heights given by the tritium concentration. At the top of LPCE column natural water is added. Enriched tritiated water is collected in the column boiler from where is sent through a purification unit to the electrolyzer unit. An advanced electrolyzer based on solid polymer electrodes (SPE) was proposed for the ITER WDS to avoid the production of a mixed liquid waste containing high-level tritium concentration (mixture of tritiated water and alkaline solution). The electrolyzer decomposes tritium-enriched water into the tritium-rich H2 gas and O2 gas streams. The mixture is sent to a moister separator and to a membrane permeator for chemical purification and the purified permeate is fed into the ISS column 1 (CD1). The O2 gas stream, which contains a small fraction of HT gas and tritiated moisture (HTO), is sent for purification to the Oxygen Striping Column. Tritiated water with high tritium concentration will be primarily processed within a VPCE column.


Part of the tritium-rich H2 gas stream is sent to the hydrogen isotope separation system (ISS) for tritium recovery, and the rest of tritium-rich H2 gas stream is fed to the catalytic isotope exchange tower. The ISS utilizes cryogenic distillation to separate hydrogen isotope mixtures [1]. The ISS of ITER is considered to be the major source for stack releases and in this case it will be in the elemental form (HT). The ITER ISS columns consist of four distillation columns (CD1, CD2, CD3 and CD4). The four cryogenic distillation columns are directly connected and equilibrators (Eq) are employed to split the mixed hydrogen isotopes into H2, D2 and T2, respectively.

Thereby the WDS is employed for detritiation and the ISS is employed for tritium recovery.

3. Plant Layout Design

A 3D layout of the WDS and ISS systems in the building has been developed based on the FDR 2001 report and the recommendation from the reports presented at Tritium Plant Project Team (TPPT) Meeting Cadarache, 8-10 October 2007 [4],[5]. Figure 2 shows vertical views of the major equipment of the ISS and WDS within the building.

Equipment WDS

Equipment ISS

Equipment arrangement in the TP building

WDS-ISS section of TP building

Figure 2. WDS-ISS section of Tritium Plant Building The designing work involved spaces reservation for the major equipment of ITER ISS-

WDS system, analysis of the area/volume allocations and optimization of the general 3D layout of plants and equipment. The arrangement of the constituent process systems has been optimized in terms of minimizing the length of interconnections, and has taken into account provision of adequate space for operation and maintenance, separation of the building areas into zones. The 3D layouts are organized in structures called product tree (skeleton) comprising assemblies, sub-assemblies and parts [8].

Numbering, naming and symbols of components and equipment were chosen according to: ITER-FEAT Tritium Plant Numbering System (Doc. No. 32 OD 0010) [6],


ITER_Plant_Identifica_ITER_D_27KSGF_v10 [7], CATIA_V5_E&S_ITER_D_25DD33_v1.0 [8].

The equipment placement into the WDS-ISS layout was done taking into consideration the building constraints [6], [5]:

WDS system:

• B2 (basement level 2) - emergency tanks, L level holding tanks, LL level holding tanks, H level holding, WDS process drain tank.

• B1 (basement level 1) - Tritium monitors for tritiated water, 2 electrolyzer, 2 electrical cabinet-electrolyzer, space reservation for piping connections.

• L1 (Level 1) – 6 electrolyzer, 6 electrical cabinet-electrolyzer, space reservation for collecting vessels.

• L2 (Level 2) - space reservation for LPCE columns & O2 stripping column, Space reservation for VPCE column.

• L3 (level 3) - space reservation for LPCE columns & O2 stripping column, Electrical cabinet for CECE process, hydrogen purification unit.

• L4 (level 4) - space reservation for LPCE columns & O2 stripping column, heating and cooling systems for LPCE columns, H2 discharge.

ISS system:

• L2 (level 2) - helium buffer tank, H2 expansion tank, space for temporary storage of lower CD cold-box, helium purification unit, helium compressor.

• L3 (level 3) - CD lower cold-box, refrigeration unit, H2 expansion tank, helium purification & gas impurity measurement.

• L4 (level 4) - CD cold-box, ISS Hard shell confinement, Vacuum system, Electrical cabinet for CD system, H2 discharge.

From the 3D WDS and ISS layouts, plot plans were generated with equipment arrangement for each floor. The plot plans has be send for review to the EFDA CSU Garching /ITER.

4. Collaborative work

Work related to these topics belongs to the task TW6-TTFD-TPI- 55-2 (Art.5.1b) from the EFDA Technology Work program 2006 and was done in collaboration with FZK Association team during the period January 2007 - December 2007.

Part of this work has been performed during the two-month Mobility Secondment of A. Lazar at Forshungszentrum Karlsruhe – Tritium Laboratory, Germany.

5. Conclusions

Development of a Water Detritiation System (WDS) and an Isotope Separation System (ISS), configuration and design of critical components are essential for ITER. ITER needs the WDS&ISS systems to process tritiated water, which was accumulated from operation and also the tritiated water which will be generated during decommissioning. The WDS is based on the Combined Electrolysis Catalytic Exchange (CECE) process and is envisaged to work in


combination with the ISS with the aim to recover tritium contained in the processed tritiated water.

The Block diagrams were developed in the Piping and Instrumentation Diagram application from the CATIA V5 software, having as reference the process diagrams from the DDD _32_E report [2], DDD_32_B report [1] and FMEA report [3].

A 3D layout of the WDS and ISS systems arrangement into the building has been developed and plot plans were generated with equipment arrangement for each floor. The plot plans have been sent for review to the EFDA CSU Garching / ITER.

The ITER inputs will be used in a near future for the process design update of the WDS-ISS systems, if required.

References[1] Kveton O. K., “Tritium Plant and Detritiation Systems - Hydrogen Isotope Separation System (WBS 3.2B)”.

[2] Yoshida H., “Tritium Plant and Detritiation Systems Water Detritiation System and Tritiated Water Holding Tank System (WBS 3.2.E)”.

[3] Rizzello C., Pinna T., “Failure Mode and Effect Analysis for Water Detritiation System of ITER”.

[4] Cristescu I., “WDS-ISS space allocation”, presented at Tritium Plant Project Team (TPPT) Meeting Cadarache, 8-10 October 2007.

[5] Beloglazov S., “TP_Layout_ITER_D_28YUVW_v1.0[1]”, presented at Tritium Plant Project Team (TPPT) Meeting Cadarache, 8-10 October 2007.

[6] Gugla M., Yoshida H., “ITER-FEAT Tritium Plant Numbering System (Doc. No. 32 OD 0010)”

[7] Belogazov S., Chiocchio S.,“ITER_Plant_Identifica_ITER_D_27KSGF_v1_0”

[8] Belogazov S., Caldwell C., Glugla M., Lazar A., Lux M., Wagner R., Weber V.,“PRM and Standard Parts Catalogues in CATIA V5 for Tritium containing Systems and Components”

[9] Standards,“EN ISO 10628”, “EN ISO 3511”, “DIN 28401”, “ASME section VIII, div.1 – Rules for construction of pressure vessels”.


PRODUCTION OF BERYLLIUM COATINGS FOR INCONEL CLADDING AND BERYLLIUM TILE MARKERS FOR THE ITER-LIKE WALL PROJECT-1 JW6-TA-EP2-ILB-01 (EFDA/06-1526)

C. P. Lungu, I. Mustata, A. Anghel, A. M. Lungu, P. Chiru, C. C. Surdu Bob, O. Pompilian

National Institute for Laser, Plasma and Radiation Physics, Magurele 1. Objectives

Manufacturing of 8 μm Be-coatings of Inconel Cladding and Beryllium tile Markers for installation in JET

2. 8 μm Be-coatings of inconel cladding tile production preparation

In order to prepare the documents for the qualification of the thermal evaporation method for beryllium coating of the inconel cladding tiles, the specialists from the Nuclear Fuel Plant (NFP), Pitesti and National Institute for Laser, Plasma and Radiation Physics, Magurele, Bucharest, Romania prepared the complete set of the documents that were approved by JET, Culham:

• Manufacture Engineering Procedure (MEP-JET-01)

• Manufacture Operating Procedure (MOP-JET-01)

• QUALITY PROGRAMME FOR BE-JET PROJECT (QPP-JET-01)

• QUALITY INSPECTION OF BERYLLIUM COATING (QVI-JET 01)

• QUALITY PLAN PC_JET

• SRP-JET-01- Reconstruction of Be coat thickness

• SRP-JET-01- Reconstruction of INCONEL pieces Be coated

To improve the thermal evaporation deposition method, the specialists from NFP Pitesti tested the evaporation system in order to simulate the complicated tile surfaces. There have been designed and manufactured policarbonate moc-ups at 1:1 scale in order to simulate the deposition on real tiles. The design of the rotating cupola support of the tiles during evaporation has been also optimized. The cupola and jigging devices of the inconel tiles are shown in Figures 1 and 2. The design of the cupola and jigging devices was performed using the AUTOCAD software.


The preliminary tests for the qualification of the Beryllium films deposition method on inconel tiles have been started using the special designed cupola at NFP Pitesti

2.1 Thickness tests for the beryllium layers

There were coated with Be and measured 9 witness samples positioned at lower, middle and upper part of the new cupola. The MICRODERM backscattering based apparatus of the NFP Pitesti was used to perform the measurements. The measured values were within the thickness margins imposed by the project (Be films of 7 – 9 μm thickness):

Average thickness: 8,41 μm

Minimum thickness : 7,65 μm

Maximal thickness : 9 μm

2.2 Adherence tests:

Using a hard steel device sharpened at a 30o angle, parallel grooves, at 2 mm distance each to other, were scratched on the beryllium coated surface, so that the Beryllium film to be fully penetrated. The adherence was good if the film between the grooves was not completely removed after the passage of the sharpened steel device.

As presented in Figures 3-6, the adherence of the beryllium layers was very good.

Figure 3. Coating of beryllium on inconel in the

left-up side Figure 4. Coating of beryllium on inconel in

the right-up side.

Figure 1. The design of the new cupola for

supporting inconel tiles Figure 2. The design of the inconel tiles

jigging device


Figure 5. Coating of beryllium on inconel in

the left-down. Figure 6. Coating of beryllium on inconel in

the right-down side. 3. Beryllium tile markers production preparation

For the deposition process, using thermionic vacuum arc (TVA) method [1] it was designed and manufactured a new evaporation system and a new sample heating rotative system to improve the Ni and Be deposition system and to ensure increased adhesion and uniform depositions. The technical drafts were previously sent for approval to JET.

As a result of JET experts discussions and amendments there have been finalized the following documents for thermionic vacuum arc coating method:

o Marker coating process procedure

o Inspection procedure for marker coated tiles)

o Quality plan for markers production

o Quality program for markers production

o Handling, packing and transport specification for markers

During October-December 2007, have been performed activities for the qualification of the deposition method for the Be-Ni test samples, which involved the deposition on parallelepiped shape stainless steel blocks coated with a Ni layer of 2±0.5μm and then a Be layer of 7-9 μm.

For this purpose, it was designed a detailed program for Ni/Be films coatings, with a view of improving their operating parameters and also the substrates temperature during samples sputtering cleaning in Argon plasma glow discharge. After the experimental preliminary depositions, it was decided that this temperature to be of 300oC±20oC; accordingly, adequate modification of the conditions included in the software that controls the system parameters during the deposition was made.

For monitoring the substrate temperature during Ni and Be coatings, it was designed a Germanium windows viewing system allowing the monitoring of infrared radiations in the range 8-14 μm using a thermovision sensor device.

Figure 2 shows the temperature distribution on the test sample surface before starting the sputtering cleaning treatment in Argon glow discharge.


50 100 150 200 250 300

30

40

50

60

70

80

90

100

Ther

mov

isio

n te

mpe

ratu

re, o C

Thermocouple temperature, oC

Figure 7. The calibration curve of the

thermovision device. Figure 8. Temperature distribution on the test sample

surface (94±4oC thermovision device values, conforming with the real values of 300oC ±20oC )

The prepared films, coated on the test surface, were analyzed using a GDOES device (glow

discharge optical emission spectroscopy) in support of Horiba-Jobin Yvon Company. Figure 9 shows the depth profile of the coated layers composition, including their interface and the base material. There have not been identified any carbon impurities in the beryllium layer. In the nickel layer, carbon concentration was below the allowed limits: <5%.

Figure 9. Ni-Be film compositional profile, coated on stainless steel substrate.

4. High-heat flux tests.

As a collaborative action a series of marker samples produced at NILPRP were analysed using several material analysis techniques [2-6] before and after high-heat flux (HHF) testing with an electron beam in the JUDITH facility at FZJ Juelich, Germany. HHF screening tests allowed the determination of power and energy density limits deposited onto the surface until the damage to a marker occurred. A cyclic test served to assess the thermal fatigue under repetitive power loads. The major results may be summarised as follows: (i) the markers survived without noticeable damage power loads of 4.5 MW m-2 for 10 s (energy density 45 MJ m-2) and fifty repetitive pulses performed at 3.5 MW m-2 each lasting 10 s, i.e. corresponding to the total energy deposition of 1750 MJ m-2; (ii) in both cases the surface temperature measured with an infrared camera was around 600 oC; (iii) the damage to the Be coating occurred at power loads of 5 MW m-2 for 10 s.

Plots in the Figure 10 show depth profiles obtained by secondary ion mass spectrometry (SIMS) for two marker coupons: (a) unexposed to heat loads and (b) after HHF test carried out


for 10 s at power density of 4 MW m-2, i.e. total energy density of 40 MJ m-2. Both profiles are quite similar (Be coating thickness approximately 9.5 μm) thus indicating that the applied power loads neither damage the coating nor cause intermixing of Be and Ni. There are some impurity species (Al, Si, Fe) but their content is below 1 % as determined by ion beam analysis, energy and wavelength dispersive X-ray spectroscopy. Figure 11 shows a metallographic cross-section of the HHF tested coupon. A clear separation of beryllium and nickel proves the durability of the coatings.

Depth (μm)

0 2 4 6 8 10 12 14

Inte

nsity

(s-1

)

100

101

102

103

104

105 BeCAlSiCaFeNi

Depth (μm)

0 2 4 6 8 10 12 14

Inte

nsity

(s-1

)

100

101

102

103

104

105 BeCAlSiCaFeNi

a b

Figure 10. SIMS depth profiles for two markers: a) “as produced”; b) HHF tested at 40 MJ m-2.

Ni

Be

OCNi

Be

OCNi

Be

OC

Epoxy resin

Be coating

Bulk Be

Ni interlayer

Figure 11. Metallographic cross-section of a marker sample after cyclic test (50 pulses) at 3.5 MW m-2.

To check the adherence and thermo-mechanical properties of the Be layers deposited on inconel by thermal evaporation method, a number of test samples were exposed to high power loads in JUDITH [6]. The screening test was carried out in the range from 0.4 to 2.6 MW m-2 in pulses lasting up to 11 s. In the cyclic test fifty consecutive 10 s pulses were performed at the power of 1 MW m-2, i.e.10 MJ m-2 per pulse. Figure 5 shows the layer structure before (a) and after the test at the power load of 1.8 MW m-2 for 11 s corresponding to the energy load of 20 MJ m-2 (b). In both cases the coating topography is nearly identical. It proves that no damage (e.g. melting or exfoliation) is caused by energy loads exceeding at least three times the level characteristic for a regular plasma operation. As assessed, the coating on inconel would melt at energy loads exceeding 30 MJ m-2 [6].


a b

50 μm 50 μm

Figure 12. Beryllium coatings on inconel: (a) “as produced”; (b) after HHF test of 20 MJ m-2.

5. Conclusions

The required documentations for the production of the beryllium coatings on the inconel tiles and markers to be installed on the first wall of JET, were elaborated. A new evaporation system, a new cupola and inconel jigging devices were designed and manufactured. Witness samples for uniformity and adherence tests were also produced.

Samples were tested at high-heat flux using JUDITH facility at FZJ Juelich, Germany. The results of samples analysis before and after HHF testing indicate that the beryllium coatings on inconel and marker limiters should withstand conditions of the regular JET operation without melting, exfoliation or phase transformation. This is particularly important in the case of the marker tiles for long-term Be erosion studies in the main chamber. However, local melting of Be tiles (with and without markers) cannot be excluded in the case of excessive power loads experiments. In this case the extent of erosion will be assessed by mechanical methods. The scientific and technical program has led to the selection of methods for a large-scale manufacturing of protective coatings on the inner wall cladding and marker tiles. The thickness markers, prior to their installation in JET, will be determined by means of ion beam analysis methods.

References

[1] C. P. Lungu, I. Mustata, V. Zaroschi, A. M. Lungu, A. Anghel, P. Chiru, M. Rubel, P. Coad G. F. Matthews and JET-EFDA contributors, “Beryllium Coatings on Metals: Development of Process and Characterizations of Layers”, Phys. Scr. T128 (2007) 157–161.

[2] M.J. Rubel, V. Bailescu, J.P. Coad, T. Hirai, J. Likonen, J. Linke, C.P. Lungu, G.F. Matthews, L. Pedrick, V. Riccardo, P. Sundelin, E. Villedieu and JET-EFDA Contributors, “Beryllium plasma-facing components for the ITER-Like Wall Project at JET”, 17th International Vacuum Congress (IVC-17), July 2-6, 2007, Stockholm, Sweden.

[3] G. F. Matthews, P. Edwards, T. Hirai, M. Kear, A. Lioure, P. Lomas, A. Loving, C. P. Lungu, H Maier, P. Mertens, D. Neilson, R. Neu, J. Pamela, V. Philipps, G. Piazza, V. Riccardo, M Rubel, C Ruset, E. Villedieu and M. Way on behalf of the ITER-likeWall Project Team 1−11, “Overview of the ITER-like wall project”, Phys. Scr. T128 (2007) 137–143.

[4] H. Maier, T. Hirai, M. Rubel, R. Neu , Ph. Mertens, H. Greuner, Ch. Hopf, G. F. Matthews, O. Neubauer, G. Piazza, E. Gauthier, J. Likonen, R. Mitteau, G. Maddaluno, B. Riccardi, V. Philipps, C. Ruset, C. P. Lungu, I. Uytdenhouwen and JET EFDA contributors, "Tungsten and Beryllium Armour Development for the JET ITER-like Wall


Project", Nucl. Fusion 47 (2007) 222–227.

[5] G. F. Matthews, P. Edwards, T.Hirai, M. Kear, A. Lioure, P. Lomas, A. Loving, C. Lungu, H. Maier, P. Martens, D. Neilson, R. Neu, J. Pamela, V. Philipps, G. Piazza, V. Riccardo, M. Rubel, C. Ruset, E. Villedieu and M. Way, “Overview of the ITER-like wall project”, Phys. Scr. T128, 2007, 137 – 143.

[6] T. Hirai, J. Linke, P. Sundelin, M. Rubel, W. Kühnlein, E. Wessel, J. P. Coad, C. P. Lungu, G. F. Matthews, “Characterization and heat flux testing of beryllium coatings on Inconel for JET ITER-like wall project”, Phys. Scr. T128 (2007) 166–170.


CHARACTERIZATION OF FUEL RETENTION IN ITER RELEVANT MIXED MATERIALS TW6-TPP-RETMIX


National Institute for Laser, Plasma and Radiation Physics, Magurele Objective: Provision of Be-coated W, Graphite and CFC specimens for implantation/retention studies

One of the major issues related to the operation of ITER concerns the issue of tritium retention, which is linked directly to the material choice for ITER plasma facing components. Most studies of fuel retention (and removal) have been focused, so far, on the pure materials to be used in ITER, namely C, W and Be.

The experiments carried out within this task characterized the fuel retention properties of the foreseen mixes of materials to be formed in ITER and as far as possible, in conditions close to those expected in ITER: (i) the mixed-materials have included possible combinations of C, W and Be; (ii) deuterium (to simulate the behavior of tritium) was introduced in the mixed material layers by ion implantation at low energy (~200 eV) similar to the energy range expected at the ITER divertor.

Following implantation, the deuterium content and depth profile of the deuterium layer was determined and was compared to the depth profile and chemical composition of the mixed-material layer by surface analysis techniques. Other techniques, such as Thermal Desorption Spectroscopy (TDS), were applied to determine quantitatively the absolute deuterium retention in the samples and, from this technique, the baking temperature which would be required in ITER in order to remove the retained fuel in these mixed-material layers by heat treatment.[1]

The production of Be coated C and W samples for studies of hydrogen retention was performed using the original technology of Thermionic Vacuum Arc (TVA) developed at NILPRP, Magurele-Bucharest, Romania.[2]

1. Preparation of Be-coated W, Graphite and CFC specimens for implantation/ retention studies

The Be-coated W, Graphite and CFC specimens for implantation/retention studies were made in four series (one series for each substrate temperature of 200, 300, 400 and 500oC). For each series 8 specimens of each substrate (polished CFC and mirror polished W and graphite of 15mm x 15 mm x 1 mm) were used. Also, for each deposition series, witness samples made of the same materials were used for comparing the morphological and compositional studies like AFM, SEM and EDS.

The substrates delivered to EURATOM MEdC Association by IPP Garching, were coated with Be films of 20 -100 nm and sent to IPP for fuel retention tests.

The impurities concentrations of the witness samples Be/CFC and Be/graphite deposited at 200 0C were extracted from Electron Dispersive X-ray Spectroscopy (EDXS)


measurements. More impurities were found on Be/CFC specimens (2.17 at%O, 0.13at%Mg, 0.2 at% Al and 0.11 at%Si) compared with those of the Be/graphite specimens (2.47% O) prepared at 200oC substrate temperature.

At the request of the IPP Garching, W films deposited on Be substrates (15mm x 15 mm x 1 mm) at RT and 450oC temperature were prepared for D retention studies. Tungsten films were prepared using the following parameters: the filament current (If = 60 A), the discharge voltage: 1200V ± 100V, the current of the discharge: 1800mA ± 200 mA.

1.1. Influence of the applied bias on the substrate during the coating process

In order to study if the morphology of beryllium films is influenced by applying a bias on the substrate during the coating process, films having hundred nanometers thickness were prepared using the Thermionic Vacuum Arc method. The substrates coated with beryllium consisted in tiles made of materials relevant to ITER technology like graphite, CFC and tungsten. The coated samples were exposed to deuterium ion beam in order to study the deuterium retention.

The films used in this study were coated by the standard TVA deposition procedure using a circular tungsten cathode using beryllium plasma parameters: base chamber pressure of 1.2 x 10-5 Torr, Ifilament= 45A, Udischarge= 300 V and Idischarge= 1.5 A. We applied on the substrates during deposition potentials between -200 V and +700 V. These potentials were applied using a special voltage source with respect to the ground potential. Taking into account that the TVA plasma potential is of some hundreds volts above the ground namely of +200 V, it results that when applying -200 V from the source, the potential difference between the plasma and the substrate connected to a negative potential is in fact of 400 V and this makes that the energy of the ions arriving at the substrate to be of 400 eV. On the contrary, when applying +700 V from the source, the potential difference between the plasma and substrate is in fact only +500V.

The samples were analyzed by means of AFM and SEM for morphological characterization, EDXS (Electron Dispersive X-ray Spectroscopy) and RBS (Rutherford Backscattering Spectroscopy) for composition, NRA (Nuclear Reaction Analysis) and TDS (Thermal Desorption Spectroscopy) for deuterium retention.

Be coatings on polished silicon, substrates were also prepared as witness samples in order to study the bias voltage influence on surface morphology. After the film deposition, the thickness and composition of each film were measured by RBS. The surface morphology was observed by SEM.

Figure 1. Depth profile composition of the Be film coated

on graphite (bias: -200V)


on graphite (bias: -100V)


on graphite (bias: +700V)

0 2000 4000 6000 8000 10000 12000 140000

20

40

60

80

100

Con

cent

ratio

n (%

)

Depth (1E15 atoms/cm2)

Be C O Si

Be/C @ - 200V bias on substrate

0 2000 4000 6000 8000 10000 12000 140000

20

40

60

80

100

Con

cent

ratio

n (%

)

Depth (x 1E15 at/cm2)

Be O C Si

Be/C @ -100V bias on substrate

0 2000 4000 6000 8000 10000 120000

20

40

60

80

100

Con

cent

ratio

n (%

)

Depth (1E15 at/cm2)

Be/C @ +700V bias on substrate before TDS

Be O C Si


Figures 1-3 show examples of each film composition obtained by RBS for the case of Be coated on graphite using -200 V, -100V and +700 V bias applied from an external voltage source on substrates. The thickness of coated films was basically situated around 700 nm, values in good agreement with the ones obtained from the “in situ” measurements by a QBM. The RBS measurements were performed using 2.6 MeV He4 ion beam.

In order to obtain the film composition from RBS experimental data, the SIMNRA code developed at IPP Garching was used. The only impurities found were oxygen from the BeO layer formed on the film’s surface due to air exposure. Figure 4 shows an example of RBS spectrum and fitting procedure.

As a first result, it was observed that the films deposited using negative bias were more adherent to the substrates than the ones grown using positive bias. This behavior was expected since by applying a negative bias on the substrate the positive beryllium ions generated in the TVA plasma were accelerated to the substrate, while applying a positive bias, the ions were decelerated and even rejected depending on the bias value.

0 200 4000

200

400

600

800

C substrate

experimental data simulation using SIMNRA code

Cou

nts

(a.u

.)

Channel

O from

surface

Be film

Figure 4. RBS spectrum of the Be film coated on graphite substrate.

The smoothness and the compactness of the beryllium films grown using negative bias was higher that of the films grown using positive bias, as shown in Fig. 5 from the SEM and in Fig. 6 from the AFM images. This is the result of the bombardment of the substrate surface by energetic ions.

a) b) Figure 5. SEM images of Be films deposited on graphite using a negative bias (a) and a positive

bias (b) applied during the deposition


a)

b)

Figure 6. 3D AFM for negatively biased (a) and positively biased (b) beryllium films deposited on Si substrate

2. Deuterium implantation and NRA analysis

Deuterium implantation was performed at IPP Garching, Germany, during a collaborative action, in the High Current Ion Source. The energy of D ion beam was 600 eV D3

+ (meaning 200 eV/D) at an incident direction normal to the target surface at room temperature. The fluence was 5 x 1022 D/m2 for each sample. After the implantation, the amount of D retained in the films was determined by Nuclear Reaction Analysis (NRA) using 3He ion beam. The concentration of D in the films was measured by means of D (3He, α)p reaction with the 3He energy of 0.69 MeV. The α particles generated by the nuclear reaction were energy analyzed with a small angle surface barrier detector at laboratory angle of 102o. The obtained α particle spectrum was converted to D depth profile using SIMNRA code.

Figure 7 presents the Deuterium distribution in the Be films prepared using: a) -200V bias on substrate, b) -100 V bias on substrate and c) +700 V bias on substrate.

0 2000 4000 6000 8000 10000

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

3.25

3.50

3.75

D c

once

ntra

tion

(%)

Depth (1E15 at/cm2)

@ -200V @ -100V @+700V

D concentration vs. Depth

Figure 7. Deuterium distribution in the Be films prepared using: a) -200V b) -100 V and c) +700 V bias on substrates.

3. Thermal desorption analysis

Thermal desorption characteristics were obtained by placing samples in a quartz tube attached to a vacuum chamber pumped by a turbo-molecular pump. A programmable infrared furnace encloses the quartz tube to heat the sample uniformly. The temperature of the sample was monitored using a thermocouple in contact with the quartz tube. The partial pressures of the


evolving gas species were measured with a quadrupole mass spectrometer (QMS). The base pressure in the vacuum chamber was kept below 10-8 torr.

Once the samples have been inserted into the bake-out chamber and the base pressure has fallen below 10-8 torr, the infrared furnace was programmed to linearly increase the sample temperature at a rate of 15 K per minute up to 1033 K. Following the temperature ramp, the sample temperature was maintained at 1033 K for an additional 5 min and then cooled to room temperature over the following 60 min.

From RBS measurements done after the TDS was observed that the pure Be films was transformed into a mixture of Be, BeO and Be2C after heating the sample. This behavior was expected since it is known that Be2C phase appears at 500oC.

100 200 300 400 500 600 700

1E14

1E15

1E16

1E14

1E15

1E16

D2 e

volu

tion

(mol

ecul

es)

Sample temperature (oC)

Be/C @ +700V bias on substrate

Figure 8. D2 molecules desorbed evolution during heating the sample obtained from TDS measurements

Fig. 8 represents the TDS spectrum of the D2 molecules desorbed from the sample. As

can be observed, three deuterium molecules peaks were observed at sample temperatures of 270oC, 475oC and 585oC. The first peak, being also the highest one, corresponds to the deuterium desorbed from the area near the surface while the other two come from the deuterium trapped in deeper areas during implantation. Calculating the amount of deuterium desorbed from the sample, we found out that all the implanted deuterium was desorbed during the TDS measurements. This result was confirmed by NRA analysis after TDS by means that no α peak was observed.

4. Conclusions

Using thermionic vacuum arc method, were prepared high quality beryllium films on tungsten, graphite and CFC substrates. The prepared specimens were sent to IPP Garching for evaluation of the film quality and deuterium retention. During a collaborative action the specimens prepared using bias voltages were analyzed too.

In the case of C or Be coating, D retention property was predominantly determined by the film, but was observed almost no influence from the substrate. This was caused that D diffusion in C or Be at a room temperature could be almost negligible, i.e. most of D was retained around implantation range. Eventually, D retention in film was saturated at a certain amount. In both C and D films, the D saturation level for 200eV D implantation was ~ 7.0 x 1020 D/m2.


The D retention in W coated samples did not reach the saturation in this experimental fluence range. On the other hand, D bulk accumulation was inhibited because D diffusion in the substrate (Be or C) is much lower than that in W at a room temperature, hence total retention decreases compared to poly-crystalline bulk tungsten.

References

[1] K. Sugiyama, K. Krieger, C.P. Lungu, J. Roth Hydrogen retention in ITER relevant mixed material layers, sent to: Fusion Engineering and Design, 2008

[2] C. P. Lungu, I. Mustata, V. Zaroschi, A. M. Lungu, A. Anghel, P. Chiru, M. Rubel, P. Coad G. F. Matthews and JET-EFDA contributors, Beryllium Coatings on Metals: Development of Process and Characterizations of Layers, Physica Scripta T128 (2007) 157–161.


G. Dinescu, B. Mitu, E. R. Ionita, C. Stancu

National Institute of Laser, Plasma and Radiation Physics, Magurele

1. Introduction

Fuel accumulation in the wall, critical for long term operation of a fusion machine, is even more prominent for surfaces with grooves and voids, due to increase of surface area and co-deposition in the gaps. The inter-tile spaces have dimensions in the 3-6 mm range, but the slit widths in the castellated tiles are much smaller. For example, Be tiles from JET have castellated blocks of 6 x 6 mm with a groove deepness of 6 mm and 0.6 mm slit width. In the case of ITER, castellation in cells of 10 x 10 x 10 mm with slits of 0.5 mm was proposed for the vertical target of the divertor. Various cleaning techniques already approached, like laser, flash-lamp, oxidation and glow discharge have the drawback of limited access in narrow spaces. The adequacy of sub-atmospheric radiofrequency discharges for discharge cleaning of inter-tile spaces and gaps in castellated tiles, based on the capability of low pressure radiofrequency discharges to spread and burn in narrow spaces, is reported.

The project aims at the elaboration of a laboratory scale Inside-Gap Plasma Generator (IGPG) device, compatible with scanning operations, and the assessment of its cleaning capabilities. For the reported period the objectives were related to the design and building up of an IGPG tool at laboratory scale, supporting scanning of castellated surfaces with its plasma column and the assessment of the cleaning potential of IGPG device in the case of planar and inside gap surfaces.

2. Results and discussion

2.1. Feasibility of plasma generation inside gaps. Operation domains

By assuming that the cleaning process is supported by the presence of plasma in the proximity of the co-deposited wall, the problem which has to be solved is the plasma sustaining inside the small gaps. A critical condition for plasma existence is that the given volume offers enough room for sheath development. Namely, the minimal condition to be fulfilled in order to allow the discharge penetration inside the small gaps is D (size-width of gap) ≥ Lsh (plasma sheath thickness). The problem of plasma development inside the gap is then translated into finding solutions to handle the sheath thickness for becoming smaller than the gap width. Roughly, the order of magnitude of the sheath thickness is given by the Debye length, which scales with plasma parameters as λd~(Te/ne)1/2 . They (Te, ne) can be handled through the injected RF power and pressure because, in the given ranges of those parameters, the increase of power increases the electron density (via increase of the ionization rate) and the increase of pressure decreases the electronic temperature (via thermalization of electrons by collisions with the cold gas).

DEVELOPMENT OF AN INSIDE-GAP PLASMA GENERATOR FOR WALL CLEANING APPLICATIONS JW6-FT-JET-A


A dedicated set-up for investigating the conditions in which the discharge is operating inside small gaps was built-up. From the examination of the power and pressure values for which the discharge penetrates into the gaps of various widths size (see Figure 1) the operation domains in the pressure- power coordinates were obtained. The domains are defined by the areas enclosed inside by the polygonal figures connecting the points corresponding to appearance and disappearance of plasma in the gaps. The graphs in the Figure 2 show, that going towards narrow spaces and larger distances the domains are less extended. Such as, the discharge in gaps of 0.6-0.8 mm was obtained for a pressure range of 25-80 mbar, but only at power values larger than 150 W. Also, Figure 2 allows determining the widths for which the discharge simultaneously coexists in more than one grove. For example, the point characterized by a pressure of 120 mbar and a power of 50 W is situated inside the domains corresponding to d=0.9, 1.0, 1.7, 2.0 mm, but the plasma does not penetrate inside the gaps of 0.8 and 0.7 mm (the point in discussion is outside the existence domains corresponding to those sizes). Similar experiments performed with nitrogen discharges show that inside gap operation in nitrogen is more restrictive than in argon: no discharge was obtained in gaps narrower than 0.9 mm, and the pressure window is in the zone 1-14 mbar.

In addition to these studies, electrodes from other materials, like iron were used, but not noticeable differences were obtained compared to the aluminum electrodes.

2.2 Inside-Gap Plasma Generator (IGPG) tool at laboratory scale

2.2.1 The design of IGPG tool

The IGPG consists of a water cooled movable electrode, mounted in a vacuum chamber, which can be placed at various distances from the castellated surface. The electrode design is shown in Figure 3. It is made as a brass cylindrical chamber, water cooled inside, with a radius of 20 mm. On one side the electrode is covered by an insulating Teflon jacket which prevents the discharge spreading on its backside. The electrode is supported by a long arm, which is used to perform the translation movement along the castellated surface. The arm is made from more tubings, enclosed each into the other having various roles: i) the smallest contains the water cooling pipes and is connected to the RF supply; ii) the next one as diameter is a glass tubing having insulating role; iii) the next one is a stainless steel grounded tubing having the role of screening, so the RF field does not escape outside, preventing the discharge burning around the arm.; iv) the last one is glass made and is greased allowing the low friction translation movement.

Figure 1. Image of discharge penetration inside grooves having

different widths

0 20 40 60 80 100 120 140 1600

20

40

60

80

100

d=2 mm

d=1.7 mm

d=1.4 mm

d=1 mm

d=0.9 mm

d=0.6 mmd=0.8 mm

Pre

ssur

e [m

bar]

Fwd. Power [W]

Figure 2. The operation domains in power pressure space

(argon, various gap widths, distance 60 mm)


2.2.2 The IGPG system and the block diagram

The block diagram of the setup and the parameter ranges are presented in Figure 4.

A schematic view and an image of the IGPG setup are presented in Figure 5. In Figure 6 are presented images of the plasma operation with the electrode placed in the front of a castellated surface. It should be noticed the formation of the plasma column in front of the electrode and the penetration of the discharge inside the gaps. During the experiments it was checked out that this plasma column moves together with the electrode, while keeping the discharge inside the gaps, which was one of the main objectives of the works for the above mentioned period. The translation speed was in the range 0-16 cm/min.

RF electrode, brass, 8mm diameter

Insulating jacket (teflon)

690mm

R=20mm

Cooling pipes

Insulating glass tubing , 10mm diameter

Stainless steel tubing (for screening),12mm dimeter

Glass external envelope, 18mm diameter

Cooled electrode, brass, R=20mm

Figure 3. The design of the IGPG electrode

Figure 4. The Block diagram of the IGPG system; The parameter ranges: RF power: 10 -200 W, 13.56 MHz Gas: Argon Mass flow rate: 1-7000 sccm Pressure range: 1-400 mbar Water flow rate (cooling rate): 0.3 l/min Translation speed: 0- 16 cm/min Distance IGPG-surface: 18-45 mm

Vacuum chamber

Vacuum pump RF Generator

Cooling

Motion control

Power supply

Flow and pressure control (PID)

Pressure monitoring unit

Gas

IGPG


2.2.3 Heating and sputtering

The actual IGPG device has a cooling circuit with water flow rate of 0.3 l/min which allows continuous work with maxim power of 200 W. In order to use higher power better cooling will be necessary. Although sputtering was not investigated in detail, since no changes of the polishing degree of the electrode were visually observed after hours of operation, an indication of low sputtering degree was obtained.

2.3 Assessment of the potential of the realized tool in cleaning of planar and inside gap surfaces

2.3.1 The removal rates for graphite made castellated surfaces

The removal rates were determined by gravimetry. A castellated surface was machined in a graphite bloc and submitted to IGPG treatment (Figure 7). The mass was measured after various treatment times. From the mass decrease the removal rate was calculated. The obtained value was 2.24 10-4g/min. Nevertheless, this experiment does not provide distinctive values for the removal rate on the flat upper surface and removal rate inside the gap.

2.3.2 Assessment of inside gap cleaning of castellated surfaces

Castellated pieces which can be mounted and dismounted from separate parts were designed and machined. The parts consisted of optical polished aluminum cubes (Figure 8) and were vacuum coated with amorphous hydrogenated carbon by Plasma Assisted Chemical Vapor Deposition Technique (PACVD). The coating was realized on all cube’s sides in a PACVD reactor, with argon plasma injected with acetylene. An image of

Vacuum pump

Motor control

Cooling Gas RF Movable IGPG

electrode

Castellated surface

Vacuum chamber

Figure 5. The schematic of the IGPG setup ( top) and its image ( bottom) Figure 6. Images of the IGPG tool in

operation

-40 -20 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280

36.13036.13536.14036.14536.15036.15536.16036.16536.17036.17536.18036.18536.19036.19536.20036.205

Mas

s [g]

Time treatment [min.]

δm/δ t≅ 2.23667E-4 g/min

exposed surface: S≅6969,3 mm2

density: ρ=2700 Kg/m3

erosion rate: δd/δt≅12 nm/min

Figure 7. Determination of removal rates of castellated graphite material ( parameters: pressure 37 mbar, distance 50 mm, RF power 30 W, argon gas

flow 600 sccm); inset: image during treatment

Figure 8. The assembling of castellated surfaces from parts


the castellated substrate during the deposition process, taken through the reactor window is shown in Figure 9a. An image of the coated castellated substrate is presented in Figure 9b.

The coating thickness, as measured by Atomic Force Microscopy and ellipsometry, was 1.2 μm. The coated assembled castellated specimen was submitted to cleaning by IGPG in conditions of discharge generation inside the gaps. The cleaning process is exemplified by the image in Figure 10a,

and the image of the resulted specimen after a determined cleaning time is shown in Figure 10b, from which it may be noticed the cleaning effect of the discharge.

The quantitative measurement of the material removal was performed with the ellipsometric technique. In this technique the change in polarization (given by the polarization angles Ψ and Δ) of an incident linearly polarized light beam, after the reflection on a layer coated substrate, is converted into layer thickness (d). In order to obtain time dependent removal rates, after cleaning at various times, the castellated piece was disassembled (see Figure 10c) in cubes. A separate cube (Figure 10d) was chosen for quantitative measurement of the cleaning process. The obtained dependence of the thickness of the un-removed layer upon the position (i.e. upon gap deepness) for different times is shown in Figure 11a, while the dependence of thickness upon time for the front side is shown in Figure 11b. It is seen that in less than 5 minutes the layer, initially 1.2 µm thick, is completely removed up to 3 mm deepness. In this portion the removal rate is about 0.24 µm /min. As concerning the front side the removal rates varies between 0.01 and 0.04 µm /min.

Thus, the potential of the IGPG tool for cleaning surfaces inside gaps was demonstrated. The highest removal rates are obtained at the gaps upper margins, which is very convenient because the co-deposited layers on the real castellated tiles are thicker in these regions.

Figure 10. Image of the cleaning process and image of partial cleaned assembled and disassembled castellated piece

-5 0 5 10 15 20 25 30 35 40

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Thic

knes

s (μ

m)

Time (min.)

front

1 2 3 4 5 6

0.0

0.2

0.4

0.6

0.8

1.0

1.2

t=35 mint=30 min

t=25 min

t=20 min

t=15 min

t=10 min

t=5 min

Thic

knes

s (μ

m)

Position (mm)

t=0 min

Figures 15a) Dependence of layer thickness upon deepness, on the lateral side and for different treatment times; b) Dependence of layer thickness, on the front side, upon the time

Figure 9. a) Al substrate during the deposition process on one side of the cubes; b) Overall coated assembled castellated specimen

a) b) c) d)


2.5 The phenomenon of discharge self-structuring (discharge segmentation)

During the experiment an unexpected difficulty appeared, which can be a drawback of using IGPG tool for tiles cleaning. The difficulty is related to the phenomenon of discharge structuring. Shortly, it consists of the breaking of discharge uniformity inside the gas and the appearance of a series of small discharges spatially separated. This phenomenon is described by the images in Figure16a. The origin of this phenomenon is not known, and can be a source of intensive research in the gas discharge physics. Nevertheless this is an undesired effect and its prevention is of paramount importance. As the position of the separated small discharges is power dependent, experiments have been performed with power modulated discharges. Indeed by pulsing the RF discharge the inside gap discharge uniformity is improved, as observed in the image in figure 16b.

3. Conclusions

A laboratory scale Inside-Gap Plasma Generator (IGPG) setup was designed and built-up. It consists of a water cooled movable electrode, mounted in a vacuum chamber. The setup operation is assisted by a motion control system, RF power supplying system, pressure control system, cooling system. The simultaneous movement of the plasma column and inside gap plasma generation was demonstrated. Contact areas of plasma column with the castellated electrode of 10 cm2 were obtained, simultaneously with discharge maintaining in gaps of 0.8 mm width.

Experiments were performed in order to assess at laboratory scale the effectiveness of the IGPG tool in modification/removal of carbon from surfaces. In a set of experiments the removal of carbon from graphite surfaces with a rate of 2.2 x 10-4 g/min (equivalent of thickness 0.012 µm /min) was obtained in non-optimized conditions. Another set of experiments was performed on a castellated assemble (1.5 mm gap width), vacuum coated with amorphous hydrogenated carbon. The thickness profile inside the gaps, at different treatment times, was measured by ellipsometry. It is proved that the cleaning process is effective, the removal rate depending on deepness: the cleaning proceeds faster at the upper gap margins (about 0.24 µm /min for the first 2 mm), where in fact the co-deposited layers on real tokamak tiles are thicker.

A phenomenon of plasma structuring (discharge segmentation) inside gaps was observed, and experiments proved that by discharge power modulation it can be avoided.

Suggested further work

The IGPG cleaning is very promising, as it is effective both on large area and in localized places, as gaps. Further work is necessary to optimize the removal rate, for example by changing gases and using gas mixtures, including oxygen and nitrogen, and using higher power variants (with more intensive cooling). Moreover, the study of the processes responsible for the observed cleaning effect is necessary. The IGPG can also act on carbon containing powders existing in gaps.

Figure 16. Image of inside argon gap discharge working at 70 W, 34 mbar,

600 sccm: a) continuous

power,; b) same parameter values but pulsed operation, duty cycle= 31.6%, Ton=11.8 ms, Toff=25.5 ms


Publications

1. C. Stancu, I. Luciu, R.E. Ionita, B. Mitu, G. Dinescu , “Operation Domains of an Inside-Gap RF Discharge”, Proceedings of the 28th International Conference on Phenomena in Ionized Gases, July 15-20, 2007, Prague, Czech Republic

2. C. Stancu, M. Teodorescu, E. R. Ionita, T. Acsente, G. Dinescu, “Inside-Gap RF Discharge Generator for Cleaning Applications”, Book of Abstracts, 14th International Conference on Plasma Physics and Applications, September 14-18, 2007, Brasov, Romania, poster presentation.


Be COATINGS OF CFC AND W EU TARGETS FOR EXPOSURE TO ITER-RELEVANT TYPE I ELM, AND MITIGATED DISRUPTION LOADS IN PLASMA-GUN FACILITIES TW6-TPP-BECOAT


National Institute of Laser, Plasma and Radiation Physics, Magurele

1. Objectives

Production of Be coatings for the 4 W and 4 CFC EU targets to be exposed to ITER-like transient loads on the Russian plasma-gun facilities with the following specifications:

(i) The coating thicknesses to be in the range of 0.05 to 50 μm. This range covers the two extreme cases for Be coverage of the target in ITER. Some elements of the target (~10 mm x 10 mm) may be left uncoated so that they can be used as reference in the experiments.

(ii) The Be coatings of the targets have to be carried out at temperatures in the range of 200-500oC to explore the effect of surface temperature on mixed material formation and its subsequent influence on the behaviour of the coatings under transient loads.

2. Beryllium coatings

Based on the results obtained on the preliminary coatings of thick Be film performed during 2006 at the National Institute for Lasers, Plasma and Radiation Physics (NILPRP) using the existent TVA technology and taking count of the fact that until now the largest thickness of the Be film was 10 μm, it was decided to improve the coating device and the process. In order to assess the main issues of the project’s tasks a new evaporation system, suitable for deposition thicknesses in the range of 50 – 100 μm, was designed. A new rotating system for heating the substrates, using molybdenum active elements, to a maximum temperature of 500 ˚C was designed and realized in order to improve the uniformity of the coatings. An automatic control panel of the deposition system was installed in collaboration with experts from NFP.

Be film Thick-ness

200oC

500oC

SEM XPS SEM XPS 15nm

System Name: XI ASCIIPass Energy: 100.00 eVCharge Bias: 0.0 eV1 Jun 1907

LIN 1Be1s

Counts

Binding Energy, eV122 118 114 110 106

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

BeBeO

Composition Table35.5% Be 1s (A)64.5% Be 1s (B)

A

B

A 111.59 eV 1.40 eV 99.2149 cts B 114.17 eV 2.22 eV 179.844 cts

Baseline: 119.79 to 107.24 eVChi square: 5.15608

Be

1s

System Name: XI ASCIIPass Energy: 100.00 eVCharge Bias: 0.0 eV1 Jun 1907Counts


50

150

250

350

450

550

650

750

850

A

A 114.00 eV 2.36 eV 305.107 ctsBaseline: 118.84 to 109.66 eVChi square: 3.01703 B

e 1s

54nm


3Be1sLIN


LIN 4Be1s

Counts


50

150

250

350

450

550

650

BeO

A A 113.82 eV 2.57 eV 269.83 ctsBaseline: 119.01 to 108.65 eVChi square: 2.91546 B

e 1s

Counts


100

300

500

700

900

1100

1300

1500

Be

BeO

Composition Table38.7% Be 1s (A)61.3% Be 1s (B)

B

ABe

1s

A 111.47 eV 1.65 eV 458.545 cts B 114.16 eV 2.25 eV 724.447 cts

Baseline: 118.53 to 108.57 eVChi square: 2.90445

Figure 1. SEM and XPS analysis of the beryllium films deposited on stainless steel substrates at 200O C and 500oC, respectively.


According to the time schedule of the project, preliminary Be depositions of 15 nm, 50 nm, 25 and 50μm thickness were prepared on stainless steel and glass substrates at the minimum and maximum temperatures of the 200-500oC interval.

Figure 1 show the Scanning Electron Microscope (SEM) images and Be1s X-ray Photoelectron Spectra (XPS) of the beryllium films deposited at 200oC and 500oC, respectively.

The SEM images showed no significant differences of the surface morphologies of the films prepared at different temperatures. The XPS spectra showed drastic change of the Be1s peak from two Be1s peaks to a single one. Connected with the fact that the film adherence, evaluated by a pulling-test and after discussion with the FZJ Juelich collaborators, it was decided that all beryllium film coatings to be performed at 500oC, the maximum temperature provided by the heating element.

The stainless steel substrates were smooth (2 samples - grinded with #1000 grit paper) and rough (2 samples - sandblasted with ZrO2 powder of 15-20 μm) and the glass substrates were smooth glass of 0.5 mm in thickness.

Figure 2. AFM image of Be film deposited on sand blasted stainless steel: Rrms = 255,7 nm, Rabs=200,4nm

In order to study the morphology of the 25 μm deposited films, AFM measurements were carried out. These measurements revealed the importance of the substrate’s smoothness as shown in Fig.2 and Fig. 3.

Figure 3. AFM image of Be film deposited on smooth stainless steel: Rrms=19,18nm, Rabs=12,7nm A comparison of the roughness of Be deposited films obtained by AFM measurements, between the smooth stainless steel substrate and the rough one reveals that even at such thicknesses (25 μm) the substrate roughness influences the roughness of the deposited films.


The adherence of Be films were tested by adhesive tape and pulling test. The stainless steel substrates coated with Be passed the test (Detaching load: 10 N; no delamination under adhesive tape test).

The film thickness was measured by a Mitutoyo stylus profiler and confirmed the measurements performed using an “in situ” quartz balance: Be coatings were in the range of 25+/- 2 μm and 50+/-5 μm.

Disc substrates made of tungsten (30 mm in diameter, 3 mm thickness) were received from FZJ Juelich, Germany and sandblasted at NFP Pitesti. Small test W and CFC samples (30-50) with typical surface dimensions of (10-20) mm x (10-20) mm were coated with Be spanning the range of conditions above. These samples were characterized at FZJ Julich by e-beam loads and will be the basis of the final specifications for the coatings of the EU W and CFC targets.

W discs of 30mm in diameter and 5 mm in thickness (received from FZJ Juelich) were used as substrates for thick Be coatings (50-100 μm). In Fig.4 the heating element and the W substrate holder are presented.

Figure 4. Set-up of the W discs substrate holder and the heating element. The evaporation of beryllium, using thermionic vacuum arc deposition equipment was

performed using the following parameters:

The intensity of the heated cathode filament: 60 A; the potential applied on the anode: 1200 ±50 V; the intensity of the discharge current: 1500 ± 100 mA; Thick Be films were deposited on the W discs, as shown in Fig. 5. The thick Be film deposited on sandblasted W disc presented higher adherence compared with polished W discs.

Figure 5. W discs coated with Beryllium.


After discussion with the specialists of the Close Support Unit-Garching, it was decided to perform Be coatings of 10 μm and 100 μm on W and CFC substrates sent by FZJ specialists as follows:

2 Be-coated (10 μm & 100 μm) CFC TARCAR target plate parts (Fig. 6)

2 Be-coated (10 μm & 100 μm) W TARCAR target plate parts (Fig.7)

2 Be-coated (10 μm) CFC coupons

2 Be-coated (100 μm) CFC coupons

2 Be-coated (100 μm) W disk samples

4 Be-coated (10 μm) W disk samples

Fig. 6. Be film (100 μm) deposited on CFC TARCAR target plate parts

Figure 7. Be film (100 μm) deposited on W TARCAR target plate parts

The 100 μm thickness Be films and the bulk Be used as evaporation target were

analyzed by XRD (X-ray diffraction). Fig. 7 shows the X-ray diffraction spectra of the bulk Be target used as evaporation material (black curve) and the spectrum of the Be film prepared by TVA method. The XRD spectrum of the bulk Be material shows the characteristic peaks of Be but these peaks are broadened suggesting a disordered structure; On contrary, XRD spectrum of the Be film shows sharp peaks, suggesting a preferred orientation on the Be (002) crystallographic plane. Some peaks of the second phase of BeO also were identified on the bulk Be target spectrum.


Figure 8. XRD patterns of the thick Be films prepared by TVA methos (red curve) and Bulk Be target used for evaporation (black curve)

Finally the CFC and W TARCAR substrates, CFC coupons and W discs coated with 10 μm and 100 μm Be films were thermally sealed into double polyethylene bags and sent to FZJ Juelich for thermal load tests.

Collaborative actions:

During the project a close cooperation was established between NILPR and FZJ Juelich (Dr. J. Linke and Dr T. Hirai)

3. Conclusions

Based on the results obtained on the beryllium film preparation in the range of 200nm to 8 μm using thermionic vacuum arc method, the coating system was improved by designing and realization of a new evaporation system, suitable for deposition thicknesses in the range of 50 – 100 μm. A new rotating system for heating the substrates to a maximum temperature of 500 ˚C was designed and realized in order to improve the uniformity of the coatings.

Be coatings of 10 μm & 100 μm were performed on: CFC and W TARCAR target plate parts, CFC coupons and W disk samples. The samples were sent to FZJ Juelich for termal load tests.

40 50 60 70 80

Be

(100

)

Be (1

01)

Be (0

02)

Cou

nts

[a.u

.]

2θ

(Be bulk) (Be film)


TW6-TTFD-TR 64 ENDURANCE TESTS FOR WDS COPONENTS

M. Vladu*, S. Brad*, F. Vasut*, M. Vijulie*, M.Constantin*

C.Postolache**,L. Matei**

*National Institute for Cryogenics and Isotopes Technologies (ICIT), Ramnicu Valcea **”Horia Hulubei” National Institute of Physics and Nuclear Engineering(IFIN - HH), Magurele 1. Introduction

The Water Detritiation System (WDS) of ITER is a safety related component since is the final barrier against tritium discharge into the environment. Therefore, its subcomponents have to be qualified and prediction on the time evolution of performances to be made.

Endurance tests at tritium low concentrations on Pt/PTFE catalyst have been performed by different research institutes. In this respect, ICIT extended the endurance tests with tritiated water of 100 Ci/kg for an LPCE column. With the experimental rig manufactured, ICIT investigated the time behavior of the catalyst. For ITER relevant operation condition, this experimental rig had a small LPCE column filled with the same type of catalyst as in the TLK LPCE column, and operated for one year with tritiated water of 100 Cikg-1.

The quality of the tritiated water was measured on a regularly basis at 1, 3, 6, 9 and 12 months and the time evolution of the fluoride content in the tritiated water was determinated.

2. Experimental rig setup

The main parts of the endurance test experimental rig, shown in Figure 1 and Figure 2, are the following:

• 1 - LPCE column;

• 2 - Process pump for tritiated water- Bran+Luebbe PS 100 (P1);

• 3 - Tritiated water vessel (V1)

• 4 - Electric heat exchanger for normal water (H101);

• 5 - Heat exchanger for tritiated water (V2);

• 6 - Normal water pump (P2);

5

6

41

3

2

Figure 1. Experimental rig


All components of the rig, in contact with tritiated water, were made from polish pickling 316L stainless steel (the same material proposed to be use for the LPCE columns of WDS from ITER). All the technological pipes were acid pickling, to remove the oxides and impurities resulting from the machining and welding process. After that, the rig was subjected to pneumatic and hydraulic leakage tests at 3 bars. The LPCE column had a heating jacket with normal water, for maintaining the tritiated water at a constant temperature of 800C. Inside the column was created an overpressure (1 bar of pure hydrogen). In order to ensure an equal distribution on the catalyst, the inlet of the tritiated water was made by a spreading system. The operating duty tests with distillated water, were performed to calibrate the temperature of the feedback control loop of the tritiated water from the LPCE column. In the H101, inside of three concentric 316L pipes are installed electric resistances of 700W power each one. One of these electric resistances worked permanently, one was used for regulating the water temperature and one was for back-up. Inside the V2 vessel there was a heating coil through which the process pump P1 circulated the tritiated water. In this way, the pre-heated tritiated water was introduced to regulate the system temperature at a specific value. The required flow rate of the tritiated water 5 l/h was regulated from the pump (P1) and flow rate of the heating water (P2) was calculated in order to obtain the necessary heat power for the electric resistances. All the experimental parameters were acquired with a Field-Point acquisition system using Lab-View software.

For this experiment, ICIT manufactured the necessary hydrophobic catalyst, of the same type as the one used in the TLK LPCE column. The tritiated water of 100 Ci/l was obtained into a reactor by burning an HT mixture on Al/Pd catalyst. In order to comply with the radioactive safety procedures, the experimental rig was installed and all the experiments were carry out in a tritium monitored glove box, at IFIN HH Bucuresti.

3. Experiments inputs

• For the LPCE column we manufactured and used 150 g Pt/C/PTFE catalyst;

• Vessel V1 was filled with 900 ml of HTO with a radioactive concentration of 3,28 TBq (88,5 Ci)/l;

• Vessel V2 was filled with approximately two liters of distillated water;

• Tritiated water flow rate in the LPCE column – 5 l/h;

• Temperature of the tritiated water in the LPCE column – 800 C;

• Pressure inside of the LPCE column - 1 bar (pure H2)

4. Experiments determinations

Catalyst and HTO samples were investigate at 1, 3, 6, 9, 12 months.

• Analysis of the pH and fluoride ion total concentration for the tritiated water;

• Determination of radiation-induced modifications in the catalyst exposed

The stability of the isotopic exchange catalyst was analysed by:

• evaluating the absorbed dosed in tritiated water solutions;

• simulating by quantum chemical methods the behaviour of the isotopic exchange catalysts which were exposed to tritiated water;


• stimulating radiolytic processes by exposing the catalyst immersed into water to a gamma radiation field;

• mechanical behaviour of the isotopic exchange catalyst in the presence of tritiated water.

5. Process description

Figure 2. Block diagram of the experimental rig for catalyst endurance test

The normal water enters through the bottom of the column jacket, get out from the upper part and filled the V2 vessel. Pump P2 circulates the normal water from V2 vessel through the H101 heater to the column jacket, and the cycle is repeated. From V1 vessel, the tritiated water is circulated by the P1 pump, through the heating coil from V2 vessel into the LPCE column. The tritiated water temperature is measured in three points, at the inlet, middle and outlet of the column. If the average temperature obtained from these three measurements, is different from 80oC, the feedback control loop decrease/increase the power supply to the regulating resistance from H101.

6. Catalyst manufacture process

The endurance tests were performed with the same type of catalyst as in the TLK LPCE column. The catalyst was produced by ICIT through platinum deposition on an activated charcoal support, before mixing it with PTFE (polytetrafluoroethylene) used as hydrophobic material. The catalyst molded into the cylindrical shape (Φ = 2, 5 mm, L = 15 mm) was baked into a controlled atmosphere glass column at 355-360oC in order to improve its the mechanical stability.

Figure 3. Pt/C/PTFE catalyst


7. Results

1. The calculus of the absorbed doses when isotopic exchange catalysts were exposed to tritiated water

1.1 The calculus of the absorbed doses in tritiated water solution

Doses and doses flow absorbed in tritiated water solution through auto-irradiation were calculated as follows:

D Edm

Edm

ee

Edm

e

ebloc total

t

t

tT

tT

= ⋅ ⋅ = ⋅ ⋅ ⋅ − = ⋅ ⋅ ⋅ −−

−

− ⋅

− ⋅

1100

1100

1 1100

10 0

0 693

0 693

1 2

1 2

Φ Λ Λ( ) ( ),

,

/

/

λ

λ

where Dbloc = the absorbed dose in tritiated block water expressed in rad (10-2 Gy);

Λ = the activity expressed in Bq;

m = sample mass expressed in g;

Ed = β radiation average energy of tritium expressed in erg;

t = exposure time expressed in s;

l = tritiated water thickness layer expressed in μm;

φtotal = total β radiation flow emitted during the exposure;

Λ0 = initial activity expressed in Bq;

λ = tritium disintegration constant;

T1/2 = tritium reducing time to one half.

1.2 The calculus of absorbed doses at the interface of isotopic exchange catalysts which were exposed to tritiated water

The catalyst was exposed to ionizing-radiations field which were emitted by the disintegration of tritium atoms from the HTO. Due to the very low β radiation emitted by tritium, the direct measurements of the dose absorbed by the isotopic exchange catalysts using classical techniques are actually impossible. In order to determine the absorbed dose at the isotopic exchange catalyst interface, a calculus model was developed.

The absorbed dose flow was calculated using the formula:

∑∑=

=

=

=

⋅⋅=Φ=Φ60

0

60

0

i

iiii

i

ii yαφ

Where:

iΦ =radiation flow emitted by tritium atoms from the tritiated water volume delimited by the

surfaces of two semispheres;

iα = emission coefficient in the direction of the catalyst on a semispherical calotte surface;


iy =attenuation coefficient of the β-radiation, which is emitted by tritium, from the water volume

delimited by the surfaces of two semispheres, absorbed in the volume given by the interior semisphere surface in the tritiated water layer.

Based on the data which were previously presented, dose flow value was calculated using the formula:

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛

+−⋅⋅++⋅⋅⋅⋅= −

=

=

−

∑1

11333

1037131,92

77078.5/60

0

222

iieii

mnd i

i

i

π

In this case the absorbed dose flow can be calculated by taking into account the radioactive concentration of the tritiated water (n) expressed in GBq/l.

sGynsmradnd /9.331/71,89 μ⋅=⋅=

In Table 1, the absorbed doses at the interface of the isotopic exchange catalyst are presented.

Table 1. The values of the doses absorbed into the isotopic exchange catalyst superficial layer.

Calculated dose [kGy] Radioactive concentration [GBq/L]

d [mGy/s] 1 day

[kGy] 1 month [MGy]

3 months [MGy]

6 months [MGy]

12 months [MGy]

370 122 10.6 0.3 1.0 1.9 3.8 740 246 21.2 0.6 1.9 3.8 7.6 1110 368 31.8 1.0 2.9 5.7 11.5 1480 491 42.5 1.3 3.8 7.6 15.3 1850 614 53.1 1.6 4.8 9.6 19.1 3700 1228 106.1 3.2 9.6 19.1 38.2 37000 12284 1061.3 31.8 95.5 191.0 382.1

2. Simulation by quantum chemical methods of the behaviour of the isotopic exchange catalysts which were exposed to tritiated water

Radiochemical yields (G) were associated to the possibility of homolytic break up action. Thus, G represents the amount (Gi) of homolytic break up of each chemical bound from LUMO’s peripheral orbital coverage area. Gi is directly proportional with LUMO orbital coverage per chemical bound and inversely proportional with the chemical bound strength.

∑∑ ⋅==i

LUMOii BE

AaGG

Where: a – represent a parameterization constant deduced from experimental values, ALUMOi represents the value of the LUMO orbital coverage degree per chemical bound, and EBi represents the bound energies for each evaluated chemical.

Based on the obtained results, we have determined the following values for polymeric bound break up: G= 3.033, G (F direct emission)= 0,214 and G (F total emission)= 6.214.

Table 2. Bound energies values, LUMO orbital coverage, and individual G values

Chemical bound BE ALUMO Gi Chemical bound BE ALUMO Gi

C1-F 110.5 0.005 0.0045 C1-C2 64.18 0.02 0.0312C2-F 110.7 0.01 0.0090 C2-C3 53.42 0.03 0.0562


C3-F 124.3 0.02 0.0161 C3-C4 60.18 0.08 0.1329C4-F 120.2 0.03 0.0250 C4-C5 5.83 0.15 2.5729C5-F 120.2 0.03 0.0250 C5-C6 60.21 0.08 0.1329C6-F 124.3 0.02 0.0161 C6-C7 53.45 0.03 0.0561C7-F 110.7 0.01 0.0090 C7-C8 64.23 0.02 0.0311C8-F 110.4 0.005 0.0045 Due to the solid structure of PTFE, the resulted F atoms from the primary process migrated heavily in the material. This process facilitated the recombining processes, namely the addition of F atoms to the the free radicals which were formed, respectively to vinylic groupings induced in the break up action. This is why the G value (-F total) was diminished and the estimated value was of approx. 3, similar to G break up value.

Because the irradiation was performed in water vapours medium, F emission was associated to HF formation:

2F⋅ + 2H2O → 2HF + 2HO⋅ → 2HF + H2O + 1/2O2

F⋅ + F⋅ → F2 (+HOH) → HF + 1/2O2

3. Radiolytic processes simulation by exposing the catalyst immersed into water to a gamma radiation field

The stimulation of the isotopic exchange catalyst behaviour in the presence of HTO was performed by exposing the catalyst immersed into ultrapure water to a gamma radiation field emitted from a source of Co-60. The samples were introduced into HDPE (NALGEN) vials and exposed to a dose flow of 1.58 kGy/h. The absorbed doses were of 50, 100, 150, 200, 440 and 2000 kGy.

Radiation-induced modifications were carried out by FTIR spectrometry using ATR TENSOR 27 (Bruker) spectrometer.

HF emission was analyzed by determining the pH and fluoride ion from the immersion water. The determinations were performed using a pH meter-ionometer WTW INOLAB MULTI 720 with pH microelectrode and fluoride ion selective electrode in a range 1-10-6 mol/l.

The identification of radiation-induced modifications using IR spectrometry

In order to validate the suggested model by quantum chemical simulation, the IR analysis of the irradiated samples was carried out with a view to identify the specific radiation-induced grouping lines COOH, OH and C-H. The obtained spectra were intercompared with the results obtained for CF3COOH and Cl3CH.

Spectral analysis confirmed the presence of COOH, C(=O)-H or CF2H groupings. Thus, the specific unassociated carboxylic OH strip (3470 cm-1), as well as the trifluoride acetic acid characteristic strips (2600 cm-1, 1783 cm-1, 1447 cm-1, 813 cm-1) were identified. The drops from the fields 2930 cm-1 and 2850 cm-1 which were presented in the control sample, grown in intensity and can be attributed to C-H aldehydic groupings or more probably to F2C-H.

Fluoride ion emission analysis

The results obtained from the spectral analysis are also confirmed by the HF emission analysis using pH-analysis and fluoride ion concentration in watery solutions (Table 1.).


Table 3. F radiation-induced emission into the isotopic exchange catalyst.

Sample Dose [kGy] pH F- conc [mol/l] Nr. F mol/g G(F-)

PTFE : H2O 50 3.3 1.90E-03 4.37E-05 3.28 PTFE : H2O 100 3.5 1.66E-03 8.30E-07 3.12 PTFE : H2O 150 3.4 5.32E-03 2.32E-06 3.06 PTFE : H2O 200 3.1 6.35E-03 1.59E-06 2.99 PTFE : H2O 440 2.8 5.56E-03 1.86E-06 2.37 PTFE : H2O 2000 1.8 1.52E-02 5.06E-06 1.42 The study of the isotopic exchange catalyst behaviour in the presence of tritiated water

150 g of catalyst and 900 ml of HTO with a radioactive concentration of 3,28 TBq (88,5 Ci)/l were introduced in the installation in order to execute endurance tests.

Catalyst and HTO samples were subject to the following exposure periods: 1, 3, 6, 9, 12 months.

Tritiated water was analyzed by determining pH and fluoride ion total concentration.

The catalyst was decontaminated by multiple washings using deionised water, methylic alcohol and finally dried for 24 hours, stocked in a vacuum exicator.

The radiation-induced modifications were analyzed by IR spectrometry.

Tritiated water characterization

Tritiated water was characterized by determining pH and fluoride ions.

In order to determine pH and the F- content from the tritiated water samples we used a pH-meter/Inolab potentiometer pH 720, manufactured by WTW GmbH Company-Germany, to which we added a selective ion electrode, with a solid membrane for F- DC219-F, manufactured by Mettler Toledo Company.

Selective ion electrode calibration

In a 150ml high density polyethylene vessel we measured 100 ml of the lowest concentrated F-

standard solution, on which we added 2 mL electrolyte solution (ISA). The solution was homogenized by magnetic stirring and after approximately 1 minute the potential value was read. After the stabilization, in order to obtain a new concentration, the solution was adjusted to an exact volume.

After each measurement, the electrode was washed with distilled water and gently wiped using a filter paper.

Through successive dilutions from the 1000 ppm standard solution we prepared 2 solutions having the concentration close to the one of the samples.

Based on the performed measurements, the calibration curve presented in Figure 4 was drawn.

The analysis of the tritiated water radioactive samples

From the testing installation regarding the resilience of the catalyst which was exposed to tritiated water in a dynamic system, tritiated water samples with volumes of 2 mL were extracted.

For the radioactive samples, the measurements were carried out after 1, 3, 6, 9, 12 months from the catalyst exposure time in tritiated water with a radioactive concentration of 90 Ci/l.


Sample’s pH was determined using InLab 423pH microelectrode.

In order to determine F- ion content, the samples were prepared as follows:

2 mL of tritiated water were diluted into 8 mL of deionized H2O to which 0.200 mL of electrolyte is added as to not modify the system operation parameters.

Concentration value was calculated with a 2.5 correction factor.

The obtained results are presented in Figure 5 and Table 4.

y = 0.0203x2 - 2.2727x + 32.65R 2 = 0.9997

-40

-30

-20

-10

0

10

20

30

40

0 10 20 30 40 50 60

Concentration F-

Pote

ntia

l (m

V)

Figure 4 Calibration curve for determining F- from the H2O samples

y = -1.0172x 2 + 28.288xR2 = 0.9846

0

50

100

150

200

250

0 2 4 6 8 10 12 14Exposure period [months]

F io

n co

ncen

trat

ion

[mg/

900

ml]

Figure 5. Variation of fluoride ions concentration on HTO in terms of exposure period Table 4. Determination of the radiation-induced effects in PTFE by HTO exposure to 3,28 TBq (88.5 Ci)/l concentration


Exp. period. [months] pH

F ion concentration in

HTO [ppm]

F total emission [mg]

Relative loss referred to PTFE

F loss relative percentage [% F emitted/F total

PTFE]

0 5.6 0 0 0 0.001 5.26 29.44 26.5 0.0002 0.0173 5.7 90.67 81.6 0.0007 0.0526 5.9 126.80 118.2 0.0010 0.0759 5.7 207.22 186.5 0.0016 0.118

12 5.6 209.22 188.3 0.0016 0.119 8. Collaborative work

Activities related to this task were performed in collaboration with the “Horia Hulubei” National Institute of Physics and Nuclear Engineering (IFIN – HH) starting with May 2007. Part of this work was performed during the two-month Mobility Secondment of M. Vladu at Forshungszentrum Karlsruhe – Tritium Laboratory, Germany.

9. Conclusions

The results obtained by mathematical modelling, show an exposure to very high doses of the superficial layer of the isotopic exchange catalyst for contact periods exceeding 30 days. The determined absorbed doses are much higher than the stability limit of the PTFE polymers (0,3 MGy).

The analysis of the primary and secondary radiolytic processes by quantum chemical simulations indicates different degradation mechanisms, associated with similar effects, respectively:

- the break up of the main polymeric bound and

- the HF significant emission

The simulation of radiolytical processes by exposure to gamma radiation fields, emitted by a Co-60 source, confirms the degradative mechanisms deduced by quantum chemical simulations. These effects are low (ppm) for an absorbed dose of (0,2 MGy).

The study of the isotopic exchange catalyst behaviour in the presence of tritiated water was carried out by analyzing the tritiated water and the exposed catalyst.

The analysis of the tritiated water was carried out with a view to determine the content of HF (pH determinations) and F- ion (F total)

The determined pH values were in the range of 5,5-6,0. We did not identify the presence of HF resulting from the primary or secondary radiolytic process. The presence of HP in HTO samples was contradictory to the results obtained by quantum chemical simulations and by exposure to gamma radiation fields.

The absence of HP in the samples was due to the fact that HF reacted with stainless steel, the material used in manufacturing the testing installation.

HF a highly corrosive agent (it even damages the glass) reacts with the metallic alloy forming metallic fluorides. This behaviour can be evidenced by determined F- ions concentration values and can be associated with the catalyst colour change and brownish-yellow colour of the tritiated water which was extracted from the testing installation.


F- ion concentration increased during the exposure period. The dependence on the exposure time was not linear, there was a flattening tendency of the curve to long exposure periods (absorbed doses).

Radiochemical yields (ppm F emitted/F total from PTFE) attained values between 170 ppm on 30 days exposure periods and 1190 ppm on 1 year exposure.

The catalyst exposed to HTO presented significant changes in the mechanical properties (colour) and chemical structure (determined through FTIR spectrometry).

The altering of the mechanical properties is important, starting from short exposure periods (1 month).

The chemical structure was changed by inducing some groupings COOH, CO, OH ascribed to primary and secondary radiolytical processes. The absence of modification regarding the characteristic strips of C-F bounds denotes a quite small share. The result obtained by determining F emission reflects a degradative process of approx. 0,2%, an unnoticeable change through FT IR analysis.

In conclusion, PTFE-based catalyst has a quite low stability in the presence of HTO, with a radioactive concentration of approx. 100 Ci/l. Radiolytic processes lead to HF formation and structural modifications of the hydrophobe support. Nevertheless, relatively minor structural modifications lead to significant modifications of the mechanical properties.

References

[1] I. Cristescu, U. Tamm, I.-R. Cristescu, R.-D. Penzhorn, C,J. Caldwell-Nichols

“Investigation of separation performances of various isotope exchange catalysts for the deuterium–hydrogen system”- International Conference on Tritium Science and Technology No6, Tsukuba , JAPON (12/11/2001) 2002, vol. 41 (2), no 3 (880 p.)

[2] I. Cristescu , Ioana-R. Cristescu, L. Dörr, M. Gluga, M. Murdoch, S. Welte

“Long term performances assessment of a water detritiation system components”- International Symposium on Fusion Nuclear Technology No7, Tokyo , JAPON (22/05/2005) 2006, vol. 81, no 1-7 (874 p.)

[3] Ghe. Ionita, A. Bornea, J. Braet, I. Popescu, I. Stefanescu, N. Bidica, C. Varlam, Cr. Postolache, L. Matei. “Endurance test for SCK-CEN catalytic mixed packing, proposed for water detritiation system at JET” -International Conference on Tritium Science and Technology No7, Baden-Baden, ALLEMAGNE (12/09/2004) 2005, vol. 48, no 1 (821 p.)


PUBLICATIONS IN SCIENTIFIC JOURNALS & CONTRIBUTIONS TO CONFERENCES AND WORKSHOPS

C.V. Atanasiu, S. Günter, A. Moraru, L.E. Zakharov, 34th European Physical Society Conference on Plasma Physics, Warsaw, Poland, 2-6 July 2007. C.V. Atanasiu, S. Günter, A. Moraru, L.E. Zakharov, 12th European Fusion Theory Conference, Madrid, Spain, 24-27 September 2007. R. Neu, M. Balden, R. Dux, O. Gruber, A. Herrmann, W. Suttrop, C.V. Atanasiu, et al., Plasma Physics and Controlled Fusion 49, B59 (2007).

F. Spineanu, M. Vlad, “Helicity fluctuations, generation of linking number and effects on resistivity”, International Review of Physics (IREPHYS) 1 (2007) 65. M. Vlad, F. Spineanu, S. Benkadda, “Collisions and average velocity effects on ratchet pinch”, Physics of Plasmas, accepted. F. Spineanu, M. Vlad, “The large scale two-dimensional stationary vortex in magnetized plasma” Annals of the University of Craiova, issue in honour of R. Balescu (2007). M. Vlad, F. Spineanu, ”Test particles and test modes in plasma turbulence”, Annals of the University of Craiova, issue in honour of R. Balescu (2007). F. Spineanu, M. Vlad, S. Benkadda, „The basic evolution of the angular momentum density in a field-theoretical model of vorticity transport”, 49th Annual Division of Plasma Physics Meeting, 12-16 November 2007, Orlando, Florida. M. Vlad, F. Spineanu, S. Benkadda, „Ratchet and curvature pinch in turbulent plasmas”, 49th Annual Division of Plasma Physics Meeting, 12-16 November 2007, Orlando, Florida. M. Vlad, F. Spineanu, “Trajectory trapping and structure generation in turbulent magnetized plasmas”, 49th Annual Division of Plasma Physics Meeting, 12-16 November 2007, Orlando, Florida, oral presentation.

X. Garbet, G. Steinbrecher, “Bifurcations in reduced ELM model”, Annals of University of Craiova, Physics AUC vol 17, (2007), part II, 15-25. B. Weyssow, G. Steinbrecher, “Extreme Anomalous Particle Transport in the Random Linear Amplification Model of the Edge Plasma Turbulence“, Jülich Conference on Stochasticity in Fusion Plasmas, March 05-07, 2007. Submitted to Phys Rev Letters, http:// www.fz-juelich.de/sfp/talks/2007/Poster/05_Weyssow.pdf B. Weyssow, G. Steinbrecher, “Extreme Anomalous Transport Driven by Fractional Brownian Motion”, Annals of Physics, University of Craiova, Romania, (2007), part I, 172-189

http://www.fz-juelich.de/sfp/talks/2007/Poster/05_Weyssow.pdf


G. Steinbrecher, B. Weyssow, “Stability under Perturbations of the large Time Average Motion of the Dynamical Systems with Conserved Phase-Space Volume”, arXiv: math-ph/0511o91 v1 28 Nov 05. B. Weyssow, G. Steinbrecher, “ Ergodicity and robustness ”, Annals of Physics, University of Craiova, Romania, (2007), part II, 26-30. G. Steinbrecher, W. Shaw, “Differential Equations for Quantile Functions”, http://www.mth.kcl.ac.uk/~shaww/web_page/papers/QuantileODE.pdf. N. Pometescu, B. Weyssow, “Radial and poloidal particle and energy fluxes in a turbulent non-Ohmic plasma: An ion-cyclotron resonance heating case” , Physics of Plasmas, Vol.14, 022305 (2007) N. Pometescu, “Radial Turbulent Transport of Ions in Tokamak Plasma with ICRH”, invited lecture 6-th Scool on Fusion Physics and Technology, University of Thessaly – Volos, Greece – 26-31 March 2007 N. Pometescu, “Turbulent Transport in non-Ohmic plasma : an ion-cyclotron resonance heating case”, European Fusion Theory Conference, Madrid – September 24-27, 2007 poster session. N. Pometescu, “Ion density perturbation driven by electromagnetic turbulence and ICRH”, 14-th International Conference on Plasma Physics and Applications, September 14-18 Brasov, Romania N. Pometescu, “Ion Density Perturbation in Turbulent Plasma with ICRH”, lucrare prezentata la: International Working Session on “Statistical Physics for Anomalous Transport in Plasmas”, Craiova, October 7 – 12, 2007. N. Pometescu, G. Steinbrecher, “Anomalous transport of particles in tokamak plasma”, 4th Association EUTATOM/MEdC Days Meeting, October 1st-2nd, 2007, Ramnicu Valcea I. Petrisor, M. Negrea, B.Weyssow, “Influence of magnetic stochastic drift on ion diffusion in magnetic turbulence”, Physics AUC Vol. 17 (Part I), 253-262, 2007. M. Negrea, I. Petrisor, B.Weyssow, “Diamagnetic effects on zonal flow generation in weak electrostatic turbulence”, European Fusion Theory Conference, Madrid – September 24-27, 2007 poster session. M. Negrea, I. Petrisor, B.Weyssow, “Influence of magnetic shear and stochastic electrostatic field on the electron diffusion”, 14-th International Conference on Plasma

http://www.mth.kcl.ac.uk/%7Eshaww/web_page/papers/QuantileODE.pdf


Physics and Applications, September 14-18, 2007, Brasov, Romania – poster presentation, to appear in Journal of Optoelectronics and Advanced Materials. M. Negrea, I. Petrisor, B.Weyssow, “Role of stochastic anisotropy and shear on magnetic field lines diffusion”, Plasma Physics and Controlled Fusion 49, 1767 (2007). I. Petrisor, M. Negrea, B.Weyssow, “Electron diffusion in a sheared unperturbed magnetic field and an electrostatic stochastic field”, European Fusion Theory Conference, Madrid – September 24-27, 2007 poster session. B. Weyssow, V. Remacle, B. Teaca, M. Negrea, I. Petrisor, C. Toniolo, Anomalous test particle transport in turbulent MHD magnetic fields, 14-th International Conference on Plasma Physics and Applications, September 14-18, 2007, Brasov, Romania – oral presentation, submitted to Journal of Optoelectronics and Advanced Materials. D Constantinescu, O Dumbrajs, V Igochine, B Weyssow On the accuracy of some mapping techniques used to study the magnetic field dynamics in tokamaks, accepted for publication in Nuclear Fusion, special issue: Stochasticity in Fusion Plasmas, March 2008 D. Constantinescu, On a symmetric mapping technique used for the study of the magnetic field in tokamak, presented at The International Symposium of Quantum Theory and Symmetries , Valladolid, 22-28 June 2007, submitted to Journal of Physics, Conference Series. D. Constantinescu, B. Weyssow, On guiding center map, lucrare prezentata la EFTC-12 (European Fusion Theory Conference, Madrid, 24-27 September 2007) D. Constantinescu, J. H. Misguich, J-D Reuss, B. Weyssow , The influence of the safety factor on the formation of the internal transport barriers, Physics AUC vol 17 (2007) pp 190-200 J. Adamek, M. Kocan, R. Panek, J. P.Gunn, J. Stöckel, E. Martines, R. Schrittwieser, C. Ionita, G. Popa, C. Costin, J. Brotankova, G. Van Oost, L. van de Peppel, “Comparison of Ion Temperature Measurements by Katsumata and Segmented Tunnel Probes”, 7th International Workshop on Electrical Probes in Magnetized Plasmas, Praga, Republica Ceha, 22-25 July 2007, accepted to Contributions to Plasma Physics J. Brotankova, E. Martines, J. Adamek, J. Stockel, G. Popa, C. Costin, R. Schrittwieser, C. Ionita, G. Van Oost, “Novel technique for direct measurement of the plasma diffusion coefficient in magnetised plasma”, 7th International Workshop on Electrical Probes in Magnetized Plasmas, Praga, Republica Ceha, 22-25 July 2007, accepted to Contributions to Plasma Physics


M. L. Solomon, Steluta Theodoru, G. Popa, “Secondary electron emission at Langmuir probe surface”, 14th International Conference on Plasma Physics and Applications (CPPA), Brasov, Romania, 14–18 September 2007, accepterd to JOAM C. Costin, V. Anita, R. S. Al, B. de Groot, W. Goedheer, A. W. Kleyn, W. R. Koppers, N. J. Lopes Cardozo, H. J. van der Meiden, R. J. E. van de Peppel, R. P. Prins, G. J. van Rooij, A. E. Shumack, M. L. Solomon, W. A. J. Vijvers, J. Westerhout, G. Popa, “On the power balance at the end plate of the plasma column in Pilot-PSI”, 34th EPS Conference on Plasma Physics, Varsovia, Polonia, 2-6 July 2007 M. L. Solomon, R. S. Al, V. Anita, C. Costin, B. de Groot, W. Goedheer, A. W. Kleyn, W. R. Koppers, N. J. Lopes Cardozo, H. J. van der Meiden, R. J. E. van de Peppel, R. P. Prins, G. J. van Rooij, A. E. Shumack, W. A. J. Vijvers, J. Westerhout, G. Popa, “On the self excited instabilities in the plasma column of Pilot-PSI”, 14th International Conference on Plasma Physics and Applications (CPPA), Brasov, Romania, 14–18 September 2007 M. Solomon, V. Titon, C. Andrei, G. Popa, “High density magnetised plasmas by self emissive probe”, 14th International Conference on Plasma Physics and Applications (CPPA), Brasov, Romania, 14–18 September 2007 G. Popa, “On the diagnostics methods of the rather dense and magnetized plasma”, Plasma 2007 - International Conference on Research and Applications of Plasmas, Greifswald, Germania, 16–19 October 2007 Dan Sporea, Adelina Sporea, “Radiation effects in sapphire optical fibers”, Physica Status Solidi (c) 4, No.3 (2007) Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim. Dan Sporea, Adelina Sporea, “Dynamics of the radiation induced color centers in optical fibers for plasma diagnostics”, Fusion Engineering and Design, vol. 82, issues 5-14, 2007, 1372-1378, DOI information: 10.1016/j.fusengdes.2007.05.053 Dan Sporea, Adelina Sporea, Constantin Oproiu, “On-line evaluation of gamma-ray irradiated large diameter optical fibers for plasma diagnostics”, Proceedings of ICONE 15, 15th International Conference on Nuclear Engineering, April 22-26, 2007, Nagoya, Japan. Dan Sporea, Adelina Sporea, Ion Vata, “Comparative study of gamma-ray and neutron irradiated laser diodes”, Proceedings of Photonics North, June 4-7, 2007, Ottawa, Canada. Dan Sporea, Adelina Sporea, Benoit Brichard, “Irradiation-induced UV optical attenuation in optical fibers for plasma diagnostics” International workshop on ITER-LMJ_NIF components in harsh environments, June 27-29, 2007, Cadarache, France. Dan Sporea, Adelina Sporea, Constantin Oproiu, Ion Vata, “Evaluation of irradiation effects on semiconductor lasers subjected to gamma-ray, electron beam and neutron irradiation”, International workshop on ITER-LMJ_NIF components in harsh environments, June 27-29, 2007, Cadarache, France. Dan Sporea, Adelina Sporea, S. Agnello, L. Nuccio, F.M. Gelardi, Benoît Brichard, “Evaluation of the UV optical transmission degradation of gamma-ray irradiated


optical fibers”, CLEO/ The 7th Pacific Rim Conference on Lasers and Electro-Optics, August 26-31, 2007, Seoul, Korea. Dan Sporea, “Update on the radiation effects in optical fibers and optoelectronic components for fusion installations”, 4 th Association Days Meeting, Ramnicu-Valcea, October 1 – 2, 2007. Augieri, A., Celentano, G., Gambardella, U., Halbritter, J., Petrisor, T.“Analysis of angular dependence of pinning mechanisms on Ca-substituted YBa2Cu3O7-δ epitaxial thin films” Superconductor Science and Technology 20 (4), art. no. 013, pp. 381-385 (2007) Galluzzi, V., Augieri, A., Ciontea, L., Celentano, G., Fabbri, F., Gambardella, U., Mancini, A., Petrisor,T, Pompeo,N, Rufolonu,A, Silva,E, Vannozzi, A.“YBa2Cu3O7-δ films with BaZrO 3 inclusions for strong-pinning in superconducting films on single crystal substrate”IEEE Transactions on Applied Superconductivity 17 (2), pp. 3628-3631 (2007) Vannozzi, A., Augieri, A., Celentano, G., Ciontea, L., Fabbri, F., Galluzzi, V., Gambardella, U., Mancini,A, Petrisor,T, Rufoloni, A.“Cube textured substrates for YBCO coated conductors: Influence of initial grain size and strain conditions during tape rolling” IEEE Transactions on Applied Superconductivity 17 (2), pp. 3436-3439 (2007) L Ciontea, G Celentano, A Augieri , T Ristoiu , R Suciu, M S Gabor, A Rufoloni, A Vannozzi, V Galluzzi, T Petrisor, Chemically Processed BaZrO3 Nanopowders as Artificial Pinning Centres - prezentata la 8th European Conference on Applied Superconductivity, EUCAS’07 Bruxelles 16-20 September 2007. L Ciontea, A Angrisani, G Celentano, T Petrisor jr., A Rufoloni, A Vannozzi, A Augieri, V Galuzzi, A Mancini , T Petrisor Metal Propionate Synthesis of Epitaxial YBa2Cu3O7-x Films- prezentata la 8th European Conference on Applied Superconductivity, EUCAS’07 Bruxelles 16-20 September 2007. C. Stancu, I. Luciu, R.E. Ionita, B. Mitu, G. Dinescu, Operation Domains of an Inside-Gap RF Discharge, Proceedings of the 28th ICPIG, July 15-20, 2007, Prague, Czech Republic, pag. 27-28

M. Teodorescu, E.R. Ionita, T. Acsente, G. Dinescu, Inside-Gap RF Discharge Generator for Cleaning Applications, Book of Abstracts of the 14th CPPA, September 14-18, 2007, Brasov, Romania, pag. 85 M. Avrigeanu, S.V. Chuvaev, A.A. Filatenkov, R.A. Forrest, M. Herman, A.J. Koning, A.J.M. Plompen, F.L. Roman, and V. Avrigeanu, Fast-neutron induced pre-equilibrium reactions on 55Mn and 63,65Cu at energies up to 40 MeV, E-Report arXiv:0712.0699 [ http://arxiv.org/archive/nucl-ex ]. M. Avrigeanu, R. A. Forrest, A.J. Koning, F.L. Roman and V. Avrigeanu, On the role of activation and particle-emission data for reaction model, in Int. Conf. on Nuclear Data for Science and Technology (ND-2007), Nice, France, 22-27 Apr. 2007, 4 p. (in press, http://www-dapnia.cea.fr/Sphn/nd2007/ ) .


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M. Avrigeanu, F.L. Roman, and V. Avrigeanu, On the need of microscopic models for evaluation of deuteron activation data, in Proc. 4th Workshop on Neutron Measurements, Evaluations and Application - Nuclear data needs for Generation IV and accelerator driven systems (NEMEA-4), Oct. 16-18, 2007, Prague, Czech Republic, Report EUR Technical and Scientific Research series, EC, Belgium, 2007 (in press); www.irmm.jrc.be/html/events/documents/nemea4/ProgrammeNEMEA4.pdf. V. Avrigeanu, M. Avrigeanu, and F.L. Roman, Neutron total and capture cross sections for Sn and Te isotopes, in Proc. 4th Workshop on Neutron Measurements, Evaluations and Application - Nuclear data needs for Generation IV and accelerator driven systems (NEMEA-4), Oct. 16-18, 2007, Prague, Czech Republic, Report EUR Technical and Scientific Research series, EC, Belgium, 2007 (in press). F.L. Roman, On modern computational techniques for improvement of nuclear model code performances, in Proc. 4th Workshop on Neutron Measurements, Evaluations and Application - Nuclear data needs for Generation IV and accelerator driven systems (NEMEA-4), Oct. 16-18, 2007, Prague, Czech Republic, Report EUR Technical and Scientific Research series, EC, Belgium, 2007 (in press). C. Ruset, E. Grigore, H. Maier, R. Neu, X. Li, H. Dong, R. Mitteau, X. Courtois, W coatings deposited on CFC tiles by combined magnetron sputtering and ion implantation technique, Physica Scripta T128, p.171-174, 2007. H. Maier, R. Neu, H. Greuner, Ch. Hopf, G.F. Matthews, G. Piazza, T. Hirai, G. Counsell, X. Courtois, R. Mitteau, E. Gauthier, J. Likonen, G. Maddaluno, V. Philipps, B. Riccardi, C. Ruset, EFDA-JET Team, Tungsten Coatings for the JET ITER-like Wall Project, Journal of Nuclear Materials, Vol. 363-365, 2007, p 1246-1250. H. Maier, T. Hirai, M. Rubel, R. Neu, Ph. Mertens, H. Greuner, Ch. Hopf, G. F. Matthews, O. Neubauer, G. Piazza, E. Gauthier, J. Likonen, R. Mitteau, G. Maddaluno, B. Riccardi, V. Philipps, C. Ruset, C.P. Lungu, I. Uytdenhouwen and JET EFDA contributors, Tungsten and Beryllium Armour Development for the JET ITER-like Wall Project, Nucl. Fusion, Vol. 47, 2007, pp. 222-227. R.Neu, H. Maier, E. Gauthier, H. Greuner, T. Hirai, Ch. Hopf, J. Likonen, G. Maddaluno, G. F. Matthews, R. Mitteau, V. Philipps, G. Piazza, C. Ruset, JET EFDA contributors, Investigation of Tungsten Coatings on Graphite and CFC, Phys. Scr. T128, 2007, 150 – 156. G. F. Matthews, P. Edwards, T.Hirai, M. Kear, A. Lioure, P. Lomas, A. Loving, C. Lungu, H. Maier, P. Martens, D. Neilson, R. Neu, J. Pamela, V. Philipps, G. Piazza, V. Riccardo, M. Rubel, C. Ruset, E. Villedieu and M. Way, Overview of the ITER-like wall project, Phys. Scr. T128, 2007, 137 – 143. C. P. Lungu, I. Mustata, V. Zaroschi, A. M. Lungu, P. Chiru, A. Anghel, G. Burcea, V. Bailescu, G. Dinuta, F. Din, Spectroscopic study of beryllium plasma produced by thermionic vacuum arc, Journal of Optoelectronics and Advanced Materials. 9, (2007), 884-886. C. P. Lungu, I. Mustata, A. Anghel, V. Zaroschi, A. M. Lungu, P. Chiru, O Pompilian, M. Badulescu, E. Dutu, G. Burcea, V. Bailescu, F. Miculescu, C.

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Logofatu, C. Negrila, I. Vata, E. Ivanov, D. Dudu, Comparison of beryllium film deposition by thermionic vacuum arc and thermal evaporation in vacuum methods, International Conference on Plasma Processes and Applications, September 14-18, (2007) Brasov, Romania, oral presentation C. P. Lungu, I. Mustata, V. Zaroschi, A. Anghel, A. M. Lungu, P. Chiru, O. Pompilian, C. Surdu-Bob, M. Rubel, P. Coad, G. Matthews, L. Pedrick, R. Handley, T.Hirai, J. Linke and JET-EFDA Contributors, Marker Tiles Coating by Thermionic Vacuum Arc Method, Thirtheen International Conference on Fusion Reactor Materials - ICFRM13, December 10-14, 2007, Nice, France C. P. Lungu, I. Mustata, A. Anghel, C. C. Surdu-Bob, P. Chiru, A. M. Lungu, V. Zaroschi, M. Ganciu, A. Surmeian, C. Diplasu, C. Oproiu, R. Minea, M. N. Nemtanu, G. Burcea, V. Turcanu, O. Dutulescu, F. Din, I. Vâţă, E. Ivanov, D. Dudu, M. Lazarescu, C. Logofatu, C. Negrila, F. Miculescu, M. Miculescu, V. Midoni, “Influence of ions’ bombardment on the beryllium film formation by thermionic vacuum arc”, 34th European Physical Society Conference on Plasma Physics, 2 - 6 July 2007, Warsaw, Poland, published in Europhysics Confernce Abstracts, Vol 31F, 2007, ISBN: 978-83-926290-0-9 V. Bailescu, G. Burcea, C. P. Lungu, I. Mustata, A. M. Lungu, M. Rubel, P. Coad, G. Matthews, L. Pedrick, R. Handley and JET-EFDA Contributors, “Beryllium Coating on Inconel Tiles”, 13th International Conference on Fusion Reactor Materials, ICFRM 13, December 10-14, Nice, France. C.Surdu-Bob, C.Iacob, C.Porosnicu, C.P.Lungu, O.Pompilian, “Arc plasma tailoring for the synthesis of quality W films”, 13th International Conference on Fusion Reactor Materials - ICFRM13, December 10-14, 2007, Nice, France

K. Sugiyama, K. Krieger, C.P. Lungu, J.Roth,”Hydrogen retention in ITER relevant mixed layer”, 18th International Conference on Plasma Surface Interactions, 26-30 May 2008, Spania C. P. Lungu, I. Mustata, A. Anghel, A.M. Lungu, C.C. Surdu-Bob, I. Morjan, E. Popovici, I. Voicu, I. Soare, D. Dudu, I. Ivanov, M. Lazarescu, A. Manea, C. Logofatu, C. Negrila, F. Miculescu, M. Miculescu, D. Bojin, “The Effects of CO2 Laser Beam Irradiation on Be-W Films Prepared by Thermionic Vacuum Arc Method”, EUROMAT 2007, 10-13 September, Nuremberg, Germany, poster B42-489 T. Hirai, H. Maier, M. Rubel, Ph. Mertens, R. Neu, E. Gauthier, J. Likonen, C. Lungu, G. Maddaluno, G.F. Matthews, R. Mitteau, O. Neubauer, G. Piazza, V.Philipps, B. Riccardi, C. Ruset, I. Uytdenhouwen, « R&D on full tungsten divertor and beryllium wall for JET ITER-like wall project », Fusion Engineering and Design 82 (2007) 1839–1845. Mihaela Vladu, Sebastian Brad, Mihai Vijulie, Felicia Vasut, Marin Constantin,”Endurance tests on WDS catalyst”, “Progress in Cryogenics and Isotopes Separation”, the 13th ICIT Conference, 2007. Mihaela Vladu, “Endurance tests of WDS Components. Present Status and Perspectives”, “Zilele Asociatiei EURATOM” – 2007.


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