R. A. Pitts, Univ. Basel, 07/11/20051 of 30 R. A. Pitts CRPP, Association-EURATOM Confédération Suisse, EPFL Lausanne, Switzerland Centre de Recherches en Physique des Plasmas Material erosion and migration in tokamaks with many thanks for contributions from N. Asakura 1, S. Brezinsek 2, C. Brosset 3, J. P. Coad 4, D. Coster 5, E. Dufour 3, G. Federici 6, R. Felton 4, M. E. Fenstermacher 7, R. S. Granetz 8, A. Herrmann 5, J. Horacek, A. Kirschner 2, K. Krieger 5, A. Loarte 9, J.Likonen 10, B. Lipschultz 8, A. Kukushkin 6, G. F. Matthews 4, M. Mayer 5, R. Neu 5, J. Pamela 11, B. Pégourié 3, V. Philipps 2, J. Roth 5, M. Rubel 12, L. L. Snead 13, P. C. Stangeby 14, J.D. Strachan 15, E. Tsitrone 3, W. Wampler 16, D. Whyte 17 1 JAERI, 2 FZJ-Jülich, 3 CEA Cadarache, 4 UKAEA, 5 IPP Garching, 6 ITER, 7 LLNL, 8 PSFC-MIT, 9 EFDA CSU Garching, 10 VTT-TEKES, 11 EFDA CSU Culham, 12 Alfvén Lab. RIT, 13 ORNL, 14 UTIAS, 15 PPPL, 16 SNL, 17 Univ. Wisconsin,

R. A. Pitts, Univ. Basel, 07/11/20052 of 30 Outline of the talk Introduction The components of migration Global migration accounting Material choices for the next step Conclusions

R. A. Pitts, Univ. Basel, 07/11/20053 of 30 What is migration? Not an operational issue in today’s tokamaks, but certainly will be in ITER and beyond …… Migration Transport Erosion Deposition Re-erosion =

R. A. Pitts, Univ. Basel, 07/11/20054 of 30 ●Co-deposition High erosion rates and long term migration of carbon yield high levels of Tritium retention ●Material mixing, properties Formation of compounds and alloys through the interaction of pure materials Change of material properties Migration will be important 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 Be on W forms BeW alloys already at ~800°C Surface melting point could be ~2000°C lower than for pure W

R. A. Pitts, Univ. Basel, 07/11/20055 of 30 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 Limited t = 3.0 s Diverted t = 12.0 s

R. A. Pitts, Univ. Basel, 07/11/20056 of 30 Some terminology Core plasma Divertor targets Private flux region Separatrix Scrape-off layer (SOL) Cool plasma on open field lines SOL width ~1 cm (  B) Length usually 10’s m (|| B) Poloidal cross-section InnerOuter ITER will be a divertor tokamak Divertor Plasma guided along field lines to targets remote from core plasma: low T and high n

R. A. Pitts, Univ. Basel, 07/11/20057 of 30 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 SupraFTU (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

R. A. Pitts, Univ. Basel, 07/11/20058 of 30 Parameter JET MkIIGB ( ) ITER Integral time in diverted phase14 hours0.1 hours Number of pulses57481 Energy Input220 GJ60 GJ Average power4.5 MW150 MW Divertor ion fluence1.8x10 27 *6x10 27 Upscale to ITER is a big step *code calculation 1 ITER pulse ~ 6 JET years divertor fluence 1 ITER pulse ~ 0.5 JET years energy input Matthews et al., EPS 2003

R. A. Pitts, Univ. Basel, 07/11/20059 of 30 Migration Transport Erosion Deposition Re-erosion =

R. A. Pitts, Univ. Basel, 07/11/ of 30 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

R. A. Pitts, Univ. Basel, 07/11/ of 30 Physical and chemical sputtering Chemical (carbon) 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 Roth et al., NF 44 (2004) L21 ITER divertor flux Physical D impact Eckstein et al.

R. A. Pitts, Univ. Basel, 07/11/ of 30 DD ELMs: an example of transient erosion JET #62218 t = s, ELM-freet = s, Type I ELM 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)

R. A. Pitts, Univ. Basel, 07/11/ of 30 ELMs can ablate Carbon on JET Range of energies expected per Type I ELM in ITER ~ 0.6  3.5 MJm -2 Loarte et al, Phys. Plasmas 11 (2004) MJ ELM  ~0.2 MJm -2 on the divertor target Peak T surf ~ 2500ºC ELM-free s Radiated Power 1.0 MJ ELM s

R. A. Pitts, Univ. Basel, 07/11/ of 30 ELM ablation limits ITER divertor lifetime Acceptable lifetime before target change required: 3000 full power shots  ~1 x 10 6 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

R. A. Pitts, Univ. Basel, 07/11/ of 30 A. Herrmann, AUG ELMs might also erode the main walls 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: E ion > 1 keV on ITER  erosion problem, even for high Z wall

R. A. Pitts, Univ. Basel, 07/11/ of 30 Migration Transport Erosion Deposition Re-erosion =

R. A. Pitts, Univ. Basel, 07/11/ of 30 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 EDGE2D/NIMBUS Bypass leaks Escape via divertor plasma Ionisation Gas puff CX event Transport creates & moves impurities Neutrals: From divertor plasma leakage, gas puffs, bypass leaks  low energy CX fluxes  wall sputtering Lower fluxes of energetic D 0 from deeper in the core plasma

R. A. Pitts, Univ. Basel, 07/11/ of 30 Experimentally, strong SOL flows Distance to separatrix (mm) M M M M M JT-60U (TCV) JET C-Mod BB ● N. Asakura, NF 44 (2004) 503 B. LaBombard, NF 44 (2004) 1047 S. K. Erents, PPCF 46 (2004) 1757 (Tore-Supra) D-flows: parallel Mach Number, M = v || /c s. POSITIVE towards inner target

R. A. Pitts, Univ. Basel, 07/11/ of 30 Can SOL ion flows transport material? BB E r xB,  pxB Ballooning Pfirsch- Schlüter Divertor sink ExBExB Simplified – shown in the poloidal plane only Poloidal Parallel Yes, but picture is complex – theory and experiment not yet reconciled

R. A. Pitts, Univ. Basel, 07/11/ of 30 Using tracers to study the transport JET DIII-D AUG 13 CH 4 markers are being increasingly used to get a handle on migration 2.8g 13 C, ohmic 9.3g 13 C H-mode 0.2g 13 C, L-mode 0.2g 13 C, H-mode g 13 C H-mode  gas puff just before vent and tile retrieval – pioneered on TEXTOR

R. A. Pitts, Univ. Basel, 07/11/ of 30 DIII-D End Wampler et al, JNM (2005) 134 Top injection: C13  inner target Likonen et al, Fus. Eng. Design (2003) 219 Start JET 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

R. A. Pitts, Univ. Basel, 07/11/ of 30 Migration Transport Erosion Deposition Re-erosion =

R. A. Pitts, Univ. Basel, 07/11/ of 30 Deposition sensitive to local conditions 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 Whyte et al., NF 41 (2001) 1243, NF 39 (1999) 1025 DIII-D Detached Observations consistent with a contribution to the carbon source from outside the divertor

R. A. Pitts, Univ. Basel, 07/11/ of 30 Strike point Shot number C-deposition (nm/s) Re-erosion important for C-migration Kirschner et al, JNM (2005) 17 Esser et al., JNM (2005) 84 Quartz Micro- Balance (QMB) L-mode ERO code JET Chemical erosion Reproduced by transport modelling Large increase on baseplate requires enhanced C re-erosion Migration to remote areas due to magnetic and divertor geometry

R. A. Pitts, Univ. Basel, 07/11/ of 30 Global migration accounting Transport Erosion Deposition Re-erosion =

R. A. Pitts, Univ. Basel, 07/11/ of 30 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 (2003) 311) Tore Supra Tore Supra balance: see Dufour et al, P5.002 Friday

R. A. Pitts, Univ. Basel, 07/11/ of 30 Strachan et al, NF 43 (2003) 922 JET migration accounting (I) Use spectroscopic methods + modelling to compute C sources EDGE2D/NIMBUS DIVIMP/OSM Simulation of CIII emission  intrinsic sources Divertor C-source = 5-10 x Wall source Carbon recycles 1 ton/year

R. A. Pitts, Univ. Basel, 07/11/ of 30 ~400g C JET migration accounting (II) Make balance for period with MarkII GasBox divertor: 14 hours plasma in diverted phase (50400 s, 5748 shots) 450g C (CIII) Spectroscopy + Modelling Post mortem surface analysis Deposition all at inner target Likonen et al, JNM (2005) 60, Matthews et al., EPS kg/year  strong T co-deposition Very similar result for AUG, but overall C- balance more complex Mayer et al, JNM (2005) 119 (1 year = 3.2 x 10 7 secs)

R. A. Pitts, Univ. Basel, 07/11/ of 30 W+W+ W0W0 Tungsten migration in AUG Campaign: ~1.4 hours in diverted phase (4680 s, 1205 shots) 1.3x10 18 s x10 17 s x10 17 s -1 Post mortem surface analysis: Only ~12% of inboard W source deposited in divertor ~ few % to upper divertor and other main chamber surfaces W erosion not balanced by non-local deposition – most is promptly redeposited  simpler than C picture Krieger et al, JNM (2005) 10 Larger Larmor radius helps at higher mass ~1.5 kg/year W-coated: (40% of total area)

R. A. Pitts, Univ. Basel, 07/11/ of 30 Material choices for the next step An ITER-like first wall at JET

R. A. Pitts, Univ. Basel, 07/11/ of 30 Current materials choice for ITER 350 MJ stored energy Be for the first wall Low T-retention Low Z Good oxygen getter C for the targets Low Z Does not melt W for the baffles High threshold for CX neutral sputtering W CFC Castellations for stress relief  co- deposition in gaps? Fallback option Be wall, all-W divertor Driven by the need for operational flexibility

R. A. Pitts, Univ. Basel, 07/11/ of 30 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 MW  Experiments from 2009 onwards Option 1Option 2 Be

R. A. Pitts, Univ. Basel, 07/11/ of 30Conclusions 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  Full wall materials tests in current machines Still significant uncertainties …….

R. A. Pitts, Univ. Basel, 07/11/ of 30 Reserve slides

R. A. Pitts, Univ. Basel, 07/11/ of 30 Toroidal limiters:227 Total: Neutralisers:11-2 Bumper: 1? “Obstacles”:60.5 Pump ducts: 0.02? Pumped out: Carbon balance: TEXTOR, Tore Supra Carbon Sources (g/h) Carbon Sinks (g/h) Very good balance considering the scope for error TEXTOR deposition extrapolates to ~220 kg/year of plasma Tore-Supra balance still preliminary Toroidal limiters:101 TEXTORTS 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 Low sticking – also AUG

R. A. Pitts, Univ. Basel, 07/11/ of 30 Similar observations at JET Coad et al., JNM (2003) 419 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

R. A. Pitts, Univ. Basel, 07/11/ of 30 ~400g C 22g Be JET migration accounting (II) Make balance for period with MarkII GasBox divertor: 16 hours plasma 20g Be (BeII) 450g C (CIII) Spectroscopy + Modelling Post mortem surface analysis Deposition all at inner divertor Surface layers are Be rich  C chemically eroded and migrates, Be stays put Likonen et al, JNM (2005) 60, Matthews et al., EPS kg/year  strong T co-deposition Very similar result for AUG, but overall C- balance more complex Mayer et al, JNM (2005) 119