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
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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