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R. A. Pitts CRPP, Association-EURATOM Conf é d é ration Suisse, EPFL Lausanne, Switzerland

Centre de Recherches en Physique des Plasmas. Material erosion and migration in tokamaks. R. A. Pitts CRPP, Association-EURATOM Conf é d é ration Suisse, EPFL Lausanne, Switzerland. with many thanks for contributions from

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R. A. Pitts CRPP, Association-EURATOM Conf é d é ration Suisse, EPFL Lausanne, Switzerland

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  1. 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 • 0.01-0.2 g per pulse now • ITER operation suspended once 350 g T accumulated • Could be fewer than ~100 pulsesNo 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. t = 3.0 s • t = 12.0 s Limited Diverted Where do erosion and migration occur? • JET #62218: plasma visible light emission At specific structures to protect the vacuum vessel walls or isolate the plasma-surface interaction

  6. Some terminology Poloidal cross-section • Scrape-off layer (SOL) • Cool plasma on open field lines • SOL width ~1 cm ( B) • Length usually 10’s m (|| B) 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 • 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. Eckstein et al. • Roth et al., NF 44 (2004) L21 • ITER divertor flux • D impact Physical and chemical sputtering • Physical • Chemical (carbon) • No threshold • Dependent on bombarding energy, flux and surface temperature •  More optimistic prediction for ITER • Energy threshold  higher for higher Z substrate • Much higher yields for high Z projectiles

  11. ELMs: an example of transient erosion • Da • t = 19.05 s, ELM-free • t = 19.06 s, Type I ELM • JET #62218 • 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) • For more on the physics of ELMs, See Huysmans, I4.002 Thurs.

  12. ELMs can ablate Carbon on JET Radiated Power 1.0 MJ ELM 19.79 s ELM-free 19.73 s • 1 MJ ELM  ~0.2 MJm-2 on the divertor target • Peak Tsurf ~ 2500ºC • 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 Inter-ELM power: 5 MWm-2 Target thickness:CFC: 20 mmW: 10 mm No redeposition of ablated material No W melt layer loss ITER min. requirement W CFC • Minimum ITER ELM size • Federici et al, PPCF 45 (2003) 1523 Acceptable lifetime before target change required: • 3000 full power shots ~1 x 106 ELMs • 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 • Gas puff • CX event • Ionisation 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 • Escape via divertor plasma • Bypass leaks • EDGE2D/NIMBUS Ions: Cross-field transport – high ion fluxes can extend into far SOL recycled neutrals direct impurity releaseELMs ….. Eroded Impurity ions “leak” out of the divertor (T forces) SOL and divertor ion fluid flows– can entrain impurities

  16. Bj ● 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 (TCV) M N. Asakura, NF 44 (2004) 503 B. LaBombard, NF 44 (2004) 1047 S. K. Erents, PPCF 46 (2004) 1757 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 • 0.2g 13C, L-mode • 2.8g 13C, ohmic • 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 • Wampler et al, JNM 337-339 (2005) 134 • Likonen et al, Fus. Eng. Design 66-68 (2003) 219 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. 1.2 1.0 Quartz Micro-Balance (QMB) 0.8 0.6 0.4 0.2 0.0 57084 Strike point 57080 57082 57086 Shot number Re-erosion important for C-migration • Esser et al., JNM 337-339 (2005) 84 JET • L-mode C-deposition (nm/s) • ERO code • 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 337-339 (2005) 17

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

  23. Tore Supra A non-trivial task! • 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 313-316 (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/OSMSimulation 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. 450g C (CIII) ~400g C JET migration accounting (II) • Make balance for period 1999-2001 with MarkII GasBox divertor: 14 hours plasmain 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 • 215 kg/year  strong T co-deposition • (1 year = 3.2 x 107 secs) • Very similar result for AUG, but overall C-balance more complex • Mayer et al, JNM 337-339 (2005) 119 • Likonen et al, JNM 337-339 (2005) 60, Matthews et al., EPS 2003

  26. W+ W0 Tungsten migration in AUG • 2002-2003 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 • ~1.5 kg/year 0.5x1017s-1 1.1x1017s-1 • Krieger et al, JNM 337-339 (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. Be Option 1 Option 2 An ITER-like wall in JET • 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 40-45 MW •  Experiments from 2009 onwards

  30. 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 • Main chamber thermography on AUG • A. Herrmann, 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. ErxB, pxB EqxB Ballooning Pfirsch-Schlüter Divertor sink Can SOL ion flows transport material? • Yes, but picture is complex – theory and experiment not yet reconciled • Poloidal Bj • 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 313-316 (2003)Brosset et al., JNM 337-339 (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: 0.02 ? Pumped out: 1-2 0.2-2 Total: 19-20 2.7-5.5 • 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 313-316 (2003) 419

  36. 20gBe (BeII) 450gC (CIII) 22g Be ~400g C JET migration accounting (II) • Make balance for period 1999-2001 with MarkII GasBox divertor: 16 hours plasma • Spectroscopy + Modelling • Post mortem surface analysis • Deposition all at inner divertor • Surface layers are Be rich  Cchemically eroded and migrates, Be stays put • 215 kg/year  strong T co-deposition • Very similar result for AUG, but overall C-balance more complex • Mayer et al, JNM 337-339 (2005) 119 • Likonen et al, JNM 337-339 (2005) 60, Matthews et al., EPS 2003

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