ERO modelling of local 13 C deposition at the outer divertor of JET M. Airila, L. Aho-Mantila, S....

ERO modelling of local 13C depositionat the outer divertor of JET

M. Airila, L. Aho-Mantila, S. Brezinsek, P. Coad, A. Kirschner, J. Likonen, D. Matveev, M. Rubel,

J. Strachan, A. Widdowson, S. Wiesen

and JET EFDA Contributors

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

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Contents

• Experiment

• Geometry

• Plasma backgrounds (EDGE2D)

• Results

• Time evolution of net deposition

• Deposition patterns

• Comparison to SIMS profiles

• Spectroscopy

• Effect of ELMs

• Summary and outlook


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Overview

• ERO modelling has been carried out for JET and AUG 13C divertor injection experiments, which both are characterized by:

• injection in outer divertor SOL at the end of campaign• similar magnetic geometry

• For JET a comprehensive 2D modelling study of global migration was carried out (J. Strachan)

• Interchange of data between ERO and EDGE2D modelling• The AUG experiment series has been continued and modelling

for later injections is in progress (L. Aho-Mantila & IPP)


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Characteristics of the JET injection experiment

• Measurements of the deposition along poloidal and toroidal lines• Deposition found at different poloidal locations (i.e. also outside

outer divertor target)

• The injection is rather diffuse and toroidally distributed• Brings uncertainty to quantitative estimates• Puffing rate per injector is higher than in recent AUG

experiments

• Leakage to the top of baffle 15 to 50% of injection [J. Strachan]

• The gas enters the vessel through a shadowed area• Modelling the deposition in shadow with the 3DGap code (D.

Matveev, FZJ)


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J. Likonen


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Simulation geometry

• Simulation volume 75cm x 16cm x 16cm (t x p x r)

• Most of the volume in PFR

• Target plate approximated with an almost planar surface (realistic tile geometries will be implemented next)


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Reference case for modelling

• A set of simulation parameters was defined as the basis for all parameter variations.

• All basic features of ERO (sputtering, reflection, diffusion, thermal force etc.) switched on

• Effective sticking for hydrocarbons S = 0

• Carbon atoms and ions: TRIM reflection

• Enhanced re-erosion of deposits – factor 10 to graphite

• Shadowed area on tile surface: defined as a low-flux zone in plasma background

• Injection of CH4 at two locations, periodic boundary in toroidal direction

• Interaction depth 5 nm, time step 0.005 s

• 2cm shift in separatrix location


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EDGE2D plasma background: ne

Inter-ELM ELM-peak

”Shadow”


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EDGE2D plasma backgrounds: Te

Inter-ELM ELM-peak


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Flux profiles

• Injection relatively close to separatrix

• High relative flux variations in simulation (injection vs. background flux)

• Slows down simulation

• Separatrix position shifted by 2cm from EDGE2D solution


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Time evolution of net deposition: reference case


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Local deposition in ERO vs. experiment

SIMS: 10.9% of C-13 on tile 7 and 6.1% on tile 8 [Coad et al. JNM 363-365, 287 (2007)].

Leakage 15—50% [Strachan NF 2008]

(10.9% + 6.1%) / (0.5—0.85) = 20—34%

- Reference case assumes re-erosion of deposits 10 times enhanced compared to graphite (i.e. E = 10)

- Deposition is smaller than in experiment

- With no enhancement the deposition is higher than in experiment

- Match to experiment is achieved with E = 2.5—7 (earlier studies find match with E = 3—5)

- Other uncertainties in the experiment than the leakage have been neglected


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Deposition patterns

Reference case

No shadow


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Deposition patterns

No enh re-erosion

Sticking S = 0.7


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Comparison to SIMS (Points: SIMS, lines: ERO)Reference case No shadow

No enhanced re-erosionSticking S = 0.7


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Assumption of injection as atoms in EDGE2D

• Atomic injection

• Simulations will be run with injection as atomic carbon @ 0.05 eV and 1 eV

• It has been found that the typical reflection probability of atoms is 0.3

• For comparison a methane injection case with S = 0.7 was run

• Analysis ongoing

• Change in deposition pattern seems significant in the higher injection energy case


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ELM effects – about modelling

On top of the equilibrium obtained with the inter-ELM plasma background, successive ELM-peak and inter-ELM time steps were run (about 150 cycles)


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ELM effects – erosion and deposition

Net erosion occurs during ELM-peak, net deposition between ELMs

Time evolution towards a new surface equilibrium, which does not differ very much from the initial surface state

Initial equilibrium net deposition value = 16.9%

ELMs on => 8% (transient)

New equilibrium at 16.2%


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ELM effects – resulting deposition

Deposition profiles after ELMs Before (= reference case)


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Comparison modelling vs. spectroscopy will be done

KT3: 12 radially separated channels


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Gap model interfaced with ERO

Monte Carlo code 3DGap [FZ Jülich]:• flexible geometry, different physical models• more realistic distribution of injected methane• also traces particles provided back by ERO

• 13C deposition along surfaces inside the gap

• Work in progress • Iterations 3DGap → ERO → 3DGap → …) Source

points in ERO

continues periodically…

toro

idal


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Preliminary estimations from 3DGap

• point sources in ERO for methane injection• sticking of hydrocarbon according to

literature, no erosion• ~45-70% of injected particles return to the

gap (ERO)• ~35-75% of these particles can be trapped in

the gap (3DGap)• iterations → increase of re-deposition

inside the gap

• >15% of puffed 13C amount might be trapped inside the gap


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Summary

• Detailed modelling of 13C local deposition in 2004 JET injection experiment has been carried out

• The ERO + simple gap model reproduces measurements closely using the assumptions

• Effective sticking on hydrocarbons S = 0• Re-erosion of reposited carbon is enhanced by the factor

E ~ 2.5–7• Tile gap will be modelled in more detail with the 3DGap code

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ERO modelling of local 13 C deposition at the outer divertor of JET M. Airila, L. Aho-Mantila, S....

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