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Fusion Engineering and Design 84 (2009) 939–942

Contents lists available at ScienceDirect

Fusion Engineering and Design

journa l homepage: www.e lsev ier .com/ locate / fusengdes

evelopment of a Laser Ablation System Kit (LASK) for Tokamak in vessel tritiumnd dust inventory control

. Hernandeza,∗, H. Rochea, C. Pocheaua, C. Grisoliaa, L. Gargiuloa, A. Semerokb,. Vatryc, P. Delaportec, L. Mercadierc

Association Euratom-CEA, Cadarache, DSM/IRFM, 13108 Saint Paul lez Durance, FranceCEA Saclay, DEN/DPC/SCP/LILM, 91191 Gif sur Yvette, FranceLaboratoire Lasers, Plasmas et Procédés Photoniques, campus de Luminy, 163 av. de Luminy, 13009 Marseille, France

r t i c l e i n f o

rticle history:vailable online 20 January 2009

eywords:aser ablationIBSust recovery

ntegration

a b s t r a c t

During Tokamak operation, Plasma Facing Components (PFCs) are subjected to severe interaction withplasma. As a consequence and independently of the PFCs composition, materials eroded and then re-deposited in the form of layers on the surfaces, can flake and produce dusts. These fragile structures areable to trap part of the hydrogenated species (tritium for example) in vessel inventory. For safety reasons,it is mandatory to measure and to control vessel dust and tritium inventory.

Up to now, laser techniques are a part of the most promising methods able to solve these ITER openissues. Of special interest are laser systems loaded on a miniature tool that can be attached to a MultiPurpose Deployer (MPD) and used for laser treatments (détritiation and other), for PFCs chemical analysisas well as for micro particles recovery of dust produced during laser ablation.

Such a system (Laser Ablation System Kit: LASK) is currently under development at IRFM and thefollowing presentation will describe the current achievements of this project and the perspectives.

In this paper, we will present an innovative compact system, which, loaded on a Multi Purpose Deployer,could allow operation in a harsh environment (pressure range from atmospheric to Ultra High Vacuumand temperature up to 120 ◦C).

According to the process conditions, different treatments can be performed: at low laser fluence, PFCsthermal treatment will be expected, while at high laser fluence material will be ablated allowing Dust(and T) recovery as well as chemical analysis of material. This “in-line” chemical analysis based on LaserInduced Breakdown Spectroscopy (LIBS) enables the ablation process to be controlled and preserves thesubstrate integrity.

The paper will be focussed on the methodology followed during the LASK development and the methodused to determine a laser process window able to remove co-deposited film without damaging the bulkmaterial and taking into account external parameter variation (Multi Purpose Deployer vibrations forexample).

The first design of the system is proposed that complies with the process requirements and the externalconstraints. Special emphasis will be given on limitations, and alternatives to these limitations will be

proposed.

. Introduction

The main objective of the LASK project is the development andhe feasibility demonstration of a versatile system usable in Toka-

ak operational conditions (pressure and temperature).This system, depending on the requirements, could be used as a

leaning system: to remove co-deposited films with laser ablationr for tritium outgassing with laser heating.

∗ Corresponding author.E-mail address: caroline.hernandez@cea.fr (C. Hernandez).

920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.fusengdes.2008.12.033

© 2008 Elsevier B.V. All rights reserved.

It could also be used as a diagnostic system: to make chemicalcharacterisation (trapped T evaluation) or sampling of co-depositedfilms.

In order to demonstrate the efficiency of such a system, the finalvalidation of LASK system will be done between two plasma chocks,in Tore Supra without conditioning break (P = 10−6 Pa; T = 120 ◦C) on

the articulated arm AIA [1].

Therefore, the system has:

• to be light (weight < 10 kg) and compact (size <160 mm ×600 mm) to be plugged on the AIA;

9 eering and Design 84 (2009) 939–942

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to be airtight and cooled down by neutral gas to support very lowpressure (10−6 Pa) and high temperature (120 ◦C during operationand 200 ◦C during outgassing phase);to be able to make treatments and characterisations without bulkdamage and without introducing pollution into the vessel: laserablated matter has to be collected.

These specifications are not without impact on laser processes.In fact, laser process performances are functions of:

External parameters variation: vibrations, positioning,pressure. . .Internal parameters variation: fluence, repetition rate. . .

Thus, in order to develop a perfectly controlled laser ablationrocess, attention is focussed on the impact of these parameters on

aser processes and dust collection.

. LASK system

In order to comply with previous specifications, the first designas been proposed (Fig. 1).

To decrease collision risk during MPD displacement and toe correctly positioned above the treated area, vertical and hor-

zontal CCD cameras are loaded. Moreover, to treat a surface of00 mm × 100 mm without MPD displacement, a scanning systems included in the LASK. The laser beam is transported from the

Fig. 1. (a–c) Details of LASK Design plugged on the articulated arm AIA.

Fig. 2. Definition of a laser process window to be sure to have matter ablationwithout bulk damage.

outside of the MPD to the scanner through an optical fiber. Duringlaser ablation a plasma “plume” is generated. Radiation emitted bythis plasma is collected through the scanner and transported by adedicated optical fiber along the MPD to a spectrometer. Spectralanalysis of this radiation (LIBS) permits chemical information aboutthe ablated matter to be obtained [2]. In order to avoid vacuum ves-sel pollution (and to permit sampling), ablated matter is collectedby an adhesion system above the treated area. This system (wellknown in Microelectronic: Pulsed Laser Deposition), is composedof a simple movable Si substrate located close to the ejected matter[3].

3. Experimental results

3.1. Laser processes

According to the process conditions, different treatments can beperformed [4]: at low laser fluence (F < Fth), PFCs thermal treatmentwill be expected allowing tritium detrapping and outgassing.

At high laser fluence (F > Fth), the material will be ablated allow-ing dust (and T) recovery as well as chemical analysis. These twostates are separated by threshold fluence: Fth (J/cm2). This value isa function of the treated matter properties and laser parameters.

During laser ablation, the “in-line” analysis of the plasma light(LIBS) allows the signal from impurities contained only in the co-deposited films (Fe for example) to be followed, and the controlof ablation process to preserve the substrate integrity (End PointDetection system).

In order to develop an efficient laser ablation process withoutbulk damage and taking into account external and internal param-eters variation, a laser process window is defined (Fig. 2).

Fig. 2 presents the process window of laser ablation process asa function of the laser fluence.

This process window corresponds to an acceptable variationrange of laser fluence: compromise between high laser fluence inorder to be sure to ablate and low fluence in order to preserve thebulk material. To provide margins and to prevent inefficient laserablation process and bulk damage, this process window will bereduced.

Laser treatments are performed with an Ytterbium pulsedlaser (1064 nm, 1 mJ/pulse max, 20 W, 120 ns of pulse durationand a repetition rate between 20 and 80 Hz with a spot size of

150 �m).

It is important in the definition of this process window to defineclearly the impact of each laser beam parameter (pulse energy,repetition rate, scanning velocity, spot size. . .) on the treated mat-ter. Furthermore, a characterisation methodology has to be clearly

C. Hernandez et al. / Fusion Engineering and Design 84 (2009) 939–942 941

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feasible with Nd:Yag laser and should be difficult with an Yb laser.These preliminary results have to be confirmed.

ig. 3. (a) Laser ablation on CFC tile as a function of number of scans and Power.b) Measurements of crater depth as a function of number of scans (P = 18 W) withonfocal microscopy.

efined in order to evaluate the impact of the laser processes onhe treated area. In our case, crater depth measurements are doneith confocal microscopy with a precision of 5 �m. Fig. 3a and bresents a batch of experiments done on a CFC tile and the associ-ted measurements.

First experiments have established the threshold fluence to beth = 4.5 J/cm2 for CFC bulk.

The same experiments are under investigation to deduce Fth ofhe co-deposited films with various compositions (single elementnd multi-component layers).

.2. Laser induced breakdown spectroscopy

LIBS analyses have two functions on LASK:

End Point Detection to preserve the bulk and to increase the laserablation efficiency by following the metallic optical bands of co-deposited films (cleaning)Chemical analyses: tritium inventory for example (diagnostic)

The main challenge of the LIBS analyses in the LASK system iso use only one laser to make simultaneously laser treatments andpectroscopic analyses.

Preliminary results are not very favourable. When comparingIBS results obtained with an Nd–YAG laser (6 mJ, 5 ns, 1064 nm)

Fig. 4. Optical spectrum of CFC tile for Nd:YAG and Yb lasers.

and an Ytterbium laser (1 mJ, 120 ns, 1064 nm), it seems that, withthe Ytterbium laser (Yb), electronic temperatures are not highenough to observe the H� optical band (Fig. 4). Furthermore, dueto the fact that ablation rate is too important, the optical signalcannot be improved by increasing the number of scans. Metallicoptical bands can be seen with both lasers, so End Point Detectionis conceivable during laser ablation, whereas tritium inventory is

Fig. 5. Dust collection efficiency (by dust adhesion) as a function of external pres-sure.

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.3. Dust collection

Dust collection (mandatory to avoid in vessel pollution and toermit sampling) is performed by adhesion of ejected matter on aovable Si substrate. In order to develop an efficient dust collect-

ng system, ejected matter behavior has thus to be understood andtudied as functions of internal/external parameter variations. Inact, matter ejection is strongly dependent on the external pressure5,6]: length of ejected matter is more important when externalressure is low. Therefore an adhesion system could be efficientt very low pressure but insufficient for atmospheric pressure: thevaluation of dust collection efficiency will be done as a function ofhe external pressure, and alternatives (vacuum cleaning for exam-le) could be proposed for the high pressure range.

Fig. 5 presents a possible illustration of dust collection efficiencywith adhesion system) as a function of external pressure.

. Conclusion and perspectives

The innovative compact system proposed in this paper is aersatile system able to be used as a diagnostic system (matterharacterisation and sampling) or as a Tokamak cleaning sys-em.

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At the end of this project (2010), after a validation in Tore Supraunder real operational conditions, a special review will be done tospotlight limitations of the system (efficiency, main constraints..)and from the know-how acquired on laser treatment, LIBS, dustcollection, design and integration on a Multi Purpose Deployer, itwill be possible to propose an ITER relevant and optimised sys-tem.

In order to increase the laser ablation efficiency, if needed, amulti-scanning head system may also be proposed. In the same way,an appropriate dust collection system as a function of the expectedexternal pressure during operation will be proposed.

References

1] L. Gargiulo, J.J. Cordier, J.P. Friconneau, C. Grisolia, J.D. Palmer, Y. Perrot, et al.,Fusion Engineering and Design 82 (2007) 1996.

2] C. Grisolia, A. Semerok, J.M. Weulersse, F. Le Guern, S. Fomichev, F. Brygo, et al.,Journal of Nuclear Materials 363–365 (2007) 1138.

3] R.K. Singh, S.D. Harkness, P. Tiwari, J. Narayan, C. Jahncke, M. Paesler, PhysicalReview B 41 (13) (1990) 8843.

4] A. Semerok, S.V. Fomichev, J.-M. Weulersse, F. Brygo, P.-Y. Thro, Journal of AppliedPhysics 101 (2007), 084916.

5] T. Scharf, H.U. Krebs, Applied Physics A: Materials Sciences & Processing 75 (2002)551.

6] T.E. Itina, F. Vidal, Ph. Delaporte, M. Sentis, Applied Physics A: Materials Sciences& Processing 79 (2004) 1089.