investigations of castellated structures for iter: the effect of castellation shaping

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A. Litnovsky et al., Third rehearsals on PSI-18, Jülich, Monday, May 19, 2008

Investigations of castellated structures for ITER: the effect of castellation shaping

and alignment on fuel retention and impurity deposition in gaps

A. Litnovsky

P. Wienhold, V. Philipps, K. Krieger, A. Kirschner, D. Matveev, D. Borodin, G. Sergienko, O. Schmitz, A. Kreter, A. Pospieszczyk, U. Samm,

S. Richter, U. Breuer, J. P. Gunn, M. Komm, Z. Pekarek and TEXTOR Team

of 13A. Litnovsky et al., PSI-18, Toledo, Spain

Monday, May 26, 2008

Motivation

Investigations at TEXTOR

Castellated structures had rectangular and shaped cells

Castellated vertical target of ITER

divertor

Shaping made to minimize deposition in gaps

Radioactive tritium (the fusion fuel) can be accumulated in the gaps in between the cells

The divertor and the first wall of ITER will be

castellated by splitting it into small-size cells to

insure thermo-mechanical durability of ITER

Limiter with castellated structures exposed in the SOL of TEXTOR



Exposure in TEXTOR

Metallically shiny plasma-facing surfaces: exposure under erosion-dominated

conditions

16 discharges, 112 s.

Te~20 eV

Ne~6*1012cm-3

Averaged fluence 2.2×1020 D/cm2

Bulk temperature 200oC-250oC

Local heating of plasma closest edge up to 1500oC

Details of exposure:

20o

Toroidal gap

Poloidal gap

Po

loid

al d

irec

tio

n Toroidal direction

Bt, Ip

SOL Plasma

20o

Bt

Shaped cells Rectangular cells

We discriminate:

Poloidal and toroidal gaps

Shaped and non-shaped cells

Cell dimensions: 10x10x12/15 mm, gap width: 0.5 mm

W castellated limiter



Analyses after exposure

EPMAscan

NRAscan

SIMS (calibrated

with Dektak)

Accuracy of C, D quantification is ~ 20%.

Secondary-Ion Mass spectrometry SIMS (FZJ)

Quantification of C and D in the deposits

Stylus profiling with DEKTAK (FZJ),

Metal mixing in the deposit and its quantification

Nuclear Reaction Analysis, NRA (IPP Garching)

Electron Probe MicroAnalysis, EPMA (RWTH Aachen)

Trapping ratio of carbon and deuterium in gaps

Depth distributions of elements

TasksTypical scheme of investigations



43,7%

69.6

2.3

36

2.5

41.6

3.0

27.3

3.5Observations:

Strong W intermixing on the plasma-closest edges of gaps

(up to 70 at. % of W);

W-fraction decreases rapidly with the depth of the gap:

λW<λc

W intermixing in the deposits: typical examples

Metal mixing in the deposits will make cleaning of the gaps

difficult in ITER

at. %at. %

at. %



Typical profiles of C and D deposition in poloidal gaps

0 5 10 15 20 25 300.00

2.50x1017

5.00x1017

2.0x1018

2.5x1018

0 5 10 15 20 25 300.00

2.50x1017

5.00x1017

2.0x1018

2.5x1018

0

C, D (x10), at./cm2

Distance along the gap, mm

C rect.

C shaped.

D rect.

D shaped.

Less peaked profiles in shaped gaps

Bottom of the gap (later in this

presentation)

NRA data

Erosionzone

Maximum deposition on the shadowed side

Plasma

Plasma

open

sidePlasma

shadowed

side

030



D=3.03C=20.4

shadowed

Summary:

Comparable C amount

Less D in the gaps of shaped cells

Low D/C ratio: D/C<5%

Poloidal gaps: results

D= 0.54C=11.2

open

D=0.96 C=13.9

open

D=9.69C=21

shadowed

D, [1015 at]C, [1016 at]

Rectangular cellsShaped cells

Further shape optimization required

Up to 70 at. % W in the deposit

e.g. by making the roofs less steep



Summary:

Toroidal gaps: results

Bt

Left of field line direction

Right of field line direction

C deposition: 13-25×1016 at.D accumulation: 1-10×1015 at.

W fraction: 0-4 at. %

C deposition: 28-45×1016 at.D accumulation: 1-15×1015 at.

W fraction: 5-10 at. %

Less deposition on the left side

W intermixing is relatively low

Significant C deposition and D accumulation detected

Shape optimization is necessary



Results: comparison of poloidal and toroidal gaps

Total integrated amount

Shaped Rectangular Type of gaps

D, (1015) at. 3.57 10.7 Poloidal gaps

2.05 6.55 Toroidal gaps, row 1


C, (1016) at. 31.6 34.9 Poloidal gaps



For parallel plasma fluence: 6.6*1020 D/cm2

with 3% C in the plasma

Material trapped in the gaps

Comparable amounts of C and D in toroidal and poloidal gaps

Toroidal gaps cannot be excluded from analyses of fuel retention and impurity deposition

Dtrap<1.2*10-4 (<0.01% of impinging D flux)

Ctrap<0.1 (<10% of impinging C flux)



Preliminary results:

No metal intermixing in the deposits;

No clear poloidal / toroidal difference in the deposition:

deposition via neutrals?

Deposition at the bottom of a castellation: first investigations

80-200 nm

Up to 200 nm thick deposits;

Same behavior observed in the long-term experiment with ALT tile [1].

[1] A. Kreter et al., Meeting of the ITPA TG on DSOL, Avila, Spain, 2008.

Bottom surfaces contain about 14% of C amount deposited in poloidal gaps

Deposition at the bottom of gaps must be taken into account and understood



Modeling of deposition in the gaps

Model

Reflection or neutral collisions alone cannot explain observed deposition

profiles

Partial qualitative agreement with experiments (with chemical erosion)

Current status

Modeling algorithms further to be improved

Transport along straight lines, reflection at the walls

Neutral collisions included

Chemical erosion

Homogeneous mixing model (HMM)

1016, at/cm2

0

10

20

30

0 5 10 15 20 25 30

02

46

810

-15 -10 -5 0 5 10 15SURFACE: (Stuck) t=0.000s

x (toroidal direction) [mm]

y (poloidal direction) [mm]

3dgap total=1.63e+005 average=1.34e+002

200 400 600 800 1000120014001600


Gap width 0.5mm

Carbon



Summary

At least two times less D in the poloidal gaps of shaped cells. Less than 30%

difference in C deposition. Geometry has to be further optimized;

Significant amount of W found intermixed into deposit. This will provide

difficulties in cleaning deposits in the gaps;

Modeling of C deposition reproduced only partly the experimental

deposition patterns. Further improvement of algorithms is needed.

Toroidal gaps contain comparable amount C and D as poloidal ones and

cannot be excluded from analyses of carbon transport and fuel accumulation;

The limiter was exposed in the erosion-dominated conditions. Nevertheless,

there are deposition-dominated conditions in the gaps.

About 10% of impinging C and less than 0.01% of impinging D fluxes was

trapped in the gaps;

Deposition at gap’s bottom cannot be described by the simple particle

reflection and calls for the further clarification of deposition mechanisms;



Thank you



Further steps

Better characterization of the deposits at the bottom of the gaps:

NRA in IPP Garching.

New exposure of castellated limiter: cells with conventional and

optimized shaping at three different angles to magnetic field.

Plasma background in the gaps: Ne, Ni, Te, Ti, vpar (J. Gunn, CEA) to be

introduced in the modeling codes

Gap

Additional slide 1



C and D Trapping ratio in the gaps

Additional slide 2

To be compared with:

Maximum D content in the poloidal gaps: 1.1*1016 D

Maximum D content in the toroidal gaps: 2.7*1016 D

Maximum C content in the poloidal gaps: 3.5*1017 C

Maximum C content in the toroidal gaps: 6.5*1017 C

Parallel plasma fluence: 6.6*1020 D/cm2

to e-side: 2.2*1020 D/cm2

to i-side: 4.2*1020 D/cm2

assuming 3% of C in plasma:

to e-side: 6.6*1018 C/cm2

to i-side: 12.6*1018 C/cm2

Which yields to:

Dtrap<1.2*10-4 (<0.01% of impinging D flux)

Ctrap<0.1 (<10% of impinging C flux)



Toroidal gaps: quantification of deposits

Additional slide 3

2D deposition patterns were selected, based on the color of a deposit

For each pattern the thickness was taken based on NRA/EPMA/SIMS results

This thickness was cross-calibrated with colorimetrical tables

Thickness was then re-calculated to the total amount of atoms and multiplied to the area of the deposition patterns

plasma flow

6153

53

190

61136

190

90

90

225

136

53

61

136

190190225

190

61

53

136

190

A2A3

A1

A4

A2

A3

A1

A4

A6

A7

A5

A8A6A5

P. Wienhold



Toroidal gaps: the nature of deposition

Additional slide 4

Bt

Left of field line direction

Right of field line direction

plasma viewing side

plasma viewing side

plasma shadowed side

plasma shadowed side

Plasma

Plasma Bt

Similar deposition patterns on toroidal gaps independently on plasma-shadowing

Cannot be described based on misalignment of magnetic field

May be explained by gyro-motion of particlesGap width might be decisive

Bt

Gap width 0.5 mm

RL(D+) ~ 1.6 mm

RL(C4+) ~ 1 mm

D+,Cx+



0 2 4 6 8 10 12 145.0x1016

1.0x1017

1.5x1017

2.0x1017

2.5x1017

3.0x1017

3.5x1017


C, at/cm2

Plasma flow

AB

A: Plasma facing side of the gap

SIMS (calibrated

with Dektak)

EPMAscan

~ 1 mmEPMA value

(without W mixing)~ 25 nm

NRA value~ 42 nm

SIMS/DEKTAK value

~ 50 nm

EPMA modeled value (41 at. % W

in the deposit)

EPMA, NRA, SIMS/DEKTAK

NRAscan

W intermixing in the deposit

Additional slide 5



Plasma-open sides

Plasma-shadowed sides

Additional slide 6

investigations of castellated structures for iter: the effect of castellation shaping

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