1. Material erosion and migration in tokamaks
Centre de Recherches en Physique des Plasmas
Material erosion and migration in tokamaks
R. A. Pitts
CRPP, Association-EURATOM Confédération Suisse, EPFL Lausanne, Switzerland
with many thanks for contributions from
N. Asakura1, S. Brezinsek2, C. Brosset3, J. P. Coad4, D. Coster5, E. Dufour3, G. Federici6, R. Felton4, M. E. Fenstermacher7, R. S. Granetz8, A. Herrmann5, J. Horacek, A. Kirschner2, K. Krieger5, A. Loarte9, J.Likonen10, B. Lipschultz8, A. Kukushkin6, G. F. Matthews4, M. Mayer5, R. Neu5, J. Pamela11, B. Pégourié3, V. Philipps2, J. Roth5, M. Rubel12, L. L. Snead13, P. C. Stangeby14, J.D. Strachan15, E. Tsitrone3, W. Wampler16, D. Whyte17
1JAERI, 2FZJ-Jülich, 3CEA Cadarache, 4UKAEA, 5IPP Garching, 6ITER, 7LLNL, 8PSFC-MIT, 9EFDA CSU Garching, 10VTT-TEKES, 11EFDA CSU Culham, 12Alfvén Lab. RIT, 13ORNL, 14UTIAS, 15PPPL, 16SNL,17Univ. Wisconsin,

2. Outline of the talk Introduction The components of migration
Global migration accounting
Material choices for the next step
Conclusions

3. What is migration? Migration = Erosion Transport Deposition Re-erosion
Not an operational issue in today’s tokamaks, but certainly will be in ITER and beyond ……

4. Migration will be important
Co-deposition
High erosion rates and long term migration of carbon yield high levels of Tritium retention
ITER: ~50 g T per pulse
g per pulse now
ITER operation suspended once 350 g T accumulated
Could be fewer than ~100 pulses No proven T clean-up technology
Material mixing, properties
Formation of compounds and alloys through the interaction of pure materials
Change of material properties
Be on W forms BeW alloys already at ~800°C
Surface melting point could be ~2000°C lower than for pure W

5. Where do erosion and migration occur?
JET #62218: plasma visible light emission
Limited
t = 3.0 s
Diverted
t = 12.0 s
At specific structures to protect the vacuum vessel walls or isolate the plasma-surface interaction

6. Poloidal cross-section
Some terminology
Scrape-off layer (SOL)
Cool plasma on open field lines
SOL width ~1 cm ( B)
Length usually 10’s m (|| B)
Poloidal cross-section
Core plasma
Divertor
Plasma guided along field lines to targets remote from core plasma: low T and high n
Separatrix
Private flux region
Inner
Outer
ITER will be a divertor tokamak
Divertor targets

7. Materials in today’s tokamaks
Low Z (Carbon)
High Z
Divertor
TCV, MAST, NSTX, DIII-D, JT-60U, JET
AUG (C+W)
C-Mod (Mo)
Limiter
TEXTOR, Tore Supra
FTU (Mo)
The majority of today’s medium to large size tokamaks favour Carbon  extensive operational experience
No melting / low core radiation / high edge radiation
But T-retention problem and high erosion rates of low Z mean that high Z may be the only long term solution
Living with W: see Kallenbach, I3.004, Wed.

8. Migration =
Erosion
Transport
Deposition
Re-erosion

9. Principal erosion mechanisms
Sputtering
Ions and neutrals
Physical and chemical (for carbon)
Macroscopic - transients
Melt layer losses
Evaporation, sublimation
Not generally observed in present experiments – currently the main reason for Carbon being used in the ITER divertor
Arcing, Dust (see Krasheninnikov et al, P4.019, Thurs.)

10. Physical and chemical sputtering
Chemical (carbon)
D impact
Eckstein et al.
Roth et al., NF 44 (2004) L21
ITER divertor flux
Energy threshold  higher for higher Z substrate
Much higher yields for high Z projectiles
No threshold
Dependent on bombarding energy, flux and surface temperature
 More optimistic prediction for ITER

11. ELMs: an example of transient erosionDa
H-mode  Edge MHD instabilities  Periodic bursts of particles and energy into the SOL. Type I ELMing H-mode is baseline ITER scenario
Time (s)
JET #62218
t = s, ELM-free
t = s, Type I ELM
For more on the physics of ELMs, See Huysmans, I4.002 Thurs.

12. ELMs can ablate Carbon on JET
1 MJ ELM  ~0.2 MJm-2 on the divertor target
Peak Tsurf ~ 2500ºC
ELM-free
19.73 s
Radiated Power
1.0 MJ ELM
19.79 s
Range of energies expected per Type I ELM in ITER ~ 0.6  3.5 MJm-2
Loarte et al, Phys. Plasmas 11 (2004) 2668

13. ELM ablation limits ITER divertor lifetime
Acceptable lifetime before target change required:
3000 full power shots  ~1 x 106 ELMs
Inter-ELM power: 5 MWm-2
Target thickness: CFC: 20 mm W: 10 mm
No redeposition of ablated material
No W melt layer loss
Federici et al, PPCF 45 (2003) 1523
CFC
ITER min. requirement W
Minimum ITER ELM size
Both low and high Z target materials marginal on present scalings
Significant effort in the community towards ELM mitigation

14. Migration =
Erosion
Transport
Deposition
Re-erosion

15. Transport creates & moves impurities
EDGE2D/NIMBUS
Bypass leaks
Escape via divertor plasma
Ionisation
Gas puff
CX event
Ions:
Cross-field transport – high ion fluxes can extend into far SOL  recycled neutrals  direct impurity release ELMs …..
Eroded Impurity ions “leak” out of the divertor (T forces)
SOL and divertor ion fluid flows – can entrain impurities
Neutrals:
From divertor plasma leakage, gas puffs, bypass leaks  low energy CX fluxes  wall sputtering
Lower fluxes of energetic D0 from deeper in the core plasma

16. Experimentally, strong SOL flows
D-flows: parallel Mach Number, M = v||/cs. POSITIVE towards inner target M M
JET
(Tore-Supra)
C-Mod
JT-60U Bj ●
(TCV)
N. Asakura, NF 44 (2004) 503
B. LaBombard, NF 44 (2004) 1047
S. K. Erents, PPCF 46 (2004) 1757 M
JT-60U M M
JT-60U
Distance to separatrix (mm)
Distance to separatrix (mm)
See LaBombard, I3.007 Wed., Bonnin, P2.110, today

17. Using tracers to study the transport
13CH4 markers are being increasingly used to get a handle on migration
 gas puff just before vent and tile retrieval – pioneered on TEXTOR
2.8g 13C, ohmic
0.2g 13C, L-mode
0.2g 13C, H-mode
JET
DIII-D
AUG
0.0025g 13C H-mode
9.3g 13C H-mode

18. Top injection: C13  inner target
Likonen et al, Fus. Eng. Design (2003) 219
Wampler et al, JNM (2005) 134
JET
Start
End
DIII-D
Simple conditions: ohmic, L-mode, no ELMs
DIII-D: toroidally symmetric injection, JET: toroidally localised
Data and modelling demonstrate fast flow to inner divertor
Situation more complex in H-mode and other injection points
For more on JET C13 expts. see Rubel, P2.004, today

19. Migration =
Erosion
Transport
Deposition
Re-erosion

20. Deposition sensitive to local conditions
DIII-D
Outer divertor usually hotter  favours C erosion (phys. + chem.)
Inner divertor usually colder  favours C deposition (chem. only)
C transport by SOL flows
Similar picture on most other carbon machines
Observations consistent with a contribution to the carbon source from outside the divertor
Detached
Groth et al., P4.015, Thurs.
Whyte et al., NF 41 (2001) 1243, NF 39 (1999) 1025

21. Re-erosion important for C-migration
Esser et al., JNM (2005) 84
JET
57080
57082
57084
57086
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Shot number
L-mode
Quartz Micro-Balance (QMB)
C-deposition (nm/s)
ERO code
Strike point
Reproduced by transport modelling
Large increase on baseplate requires enhanced C re-erosion
Chemical erosion
Migration to remote areas due to magnetic and divertor geometry
Kirschner et al, JNM (2005) 17

22. Global migration accounting=
Erosion
Transport
Deposition
Re-erosion

23. A non-trivial task! Tore Supra
Spectroscopic methods in plasma, post-mortem surface analysis and just plain old scraping and sweeping up  extremely rigorous balance achieved first on TEXTOR (Wienhold et al., JNM (2003) 311)
Tore Supra balance: see Dufour et al, P5.002 Friday

24. JET migration accounting (I)
Use spectroscopic methods + modelling to compute C sources
EDGE2D/NIMBUS DIVIMP/OSM Simulation of CIII emission  intrinsic sources
1 ton/year
Divertor C-source = 5-10 x Wall source
Strachan et al, NF 43 (2003) 922
Carbon recycles

25. JET migration accounting (II)
Make balance for period with MarkII GasBox divertor: 14 hours plasma in diverted phase (50400 s, 5748 shots)
Spectroscopy + Modelling
Post mortem surface analysis
Deposition all at inner target
Net erosion at main walls
No significant divertor erosion
450g C (CIII)
215 kg/year  strong T co-deposition
(1 year = 3.2 x 107 secs)
~400g C
Very similar result for AUG, but overall C-balance more complex
Mayer et al, JNM (2005) 119
Likonen et al, JNM (2005) 60, Matthews et al., EPS 2003

26. Tungsten migration in AUG
Campaign: ~1.4 hours in diverted phase (4680 s, 1205 shots)
Post mortem surface analysis:
Only ~12% of inboard W source deposited in divertor
~ few % to upper divertor and other main chamber surfaces
W-coated: (40% of total area)
1.3x1018s-1
W erosion not balanced by non-local deposition – most is promptly redeposited  simpler than C picture
Larger Larmor radius helps at higher mass
0.5x1017s-1
~1.5 kg/year W+ W0
1.1x1017s-1
Krieger et al, JNM (2005) 10

27. Material choices for the next step
An ITER-like first wall at JET

28. Current materials choice for ITER
Be for the first wall
Low T-retention
Low Z
Good oxygen getter W
C for the targets
Low Z
Does not melt
350 MJ stored energy
W for the baffles
High threshold for CX neutral sputtering
CFC
Fallback option
Be wall, all-W divertor
Castellations for stress relief  co-deposition in gaps?
Driven by the need for operational flexibility

29. An ITER-like wall in JET
Option 1
Option 2 Be
Option 1 or 2 to be chosen in 2006: Objectives
Demonstrate low T-retention
Study melt layer loss (walls and divertor)  ELMs and disruptions
Study effect of Be on W erosion
Be and W migration
Demonstrate operation without C radiation
Refine control/mitigation techniques  ELMs and disruptions
Demonstrate routine / safe operation of fully integrated ITER compatible scenarios at 3-5MA  Power upgrade to MW
 Experiments from 2009 onwards

30.  Full wall materials tests in current machines
Conclusions
Erosion and migration: Complex materials and physics
Not an operational issue now
But will be in ITER and beyond
Optimisation of core plasma performance and wall lifetime cannot be decoupled
Refine predictive capability
Still significant uncertainties …….
 Full wall materials tests in current machines

31. Reserve slides

32. ELMs might also erode the main walls
A. Herrmann, AUG
Main chamber thermography on AUG
Type I ELMs: ~25% of stored energy drop deposited on non- divertor components
ELM ion energies measured at JET walls agree with recent theory
Suggests: Eion > 1 keV on ITER  erosion problem, even for high Z wall
Herrmann et al, P1.006 Mon.

33. Can SOL ion flows transport material?
Yes, but picture is complex – theory and experiment not yet reconciled
ErxB, pxB
EqxB
Poloidal
Pfirsch-Schlüter Bj
Ballooning
Divertor sink
Parallel
Simplified – shown in the poloidal plane only

34. Carbon balance: TEXTOR, Tore Supra
Carbon Sources (g/h)
von Seggern et al, Mayer et al., Phys. Scripta T111 (2004) Wienhold et al, von. Seggern et al., JNM (2003) Brosset et al., JNM (2005) 311, E. Tsitrone et al., IAEA 2004
TEXTOR TS
Toroidal limiters: 22 7
Carbon Sinks (g/h)
Toroidal limiters: 10 1
“Obstacles”: 6 0.5
Low sticking – also AUG
Bumper: 1 ?
Neutralisers: 1 1-2
Very good balance considering the scope for error
TEXTOR deposition extrapolates to ~220 kg/year of plasma
Tore-Supra balance still preliminary
Pump ducts: ?
Pumped out:
Total:
Dufour et al, P5.002 Friday

35. Similar observations at JET
Net inner divertor deposition and little net erosion in outer divertor implies net wall source
Macroscopic flakes in regions not generally visible to plasma  migration to remote areas  high levels of T-retention
Flakes
Coad et al., JNM (2003) 419

36. JET migration accounting (II)
Make balance for period with MarkII GasBox divertor: 16 hours plasma
Spectroscopy + Modelling
Post mortem surface analysis
Deposition all at inner divertor
Surface layers are Be rich  C chemically eroded and migrates, Be stays put
20g Be (BeII)
450g C (CIII)
215 kg/year  strong T co-deposition
~400g C
22g Be
Very similar result for AUG, but overall C-balance more complex
Mayer et al, JNM (2005) 119
Likonen et al, JNM (2005) 60, Matthews et al., EPS 2003