r. a. pitts crpp, association-euratom conf é d é ration suisse, epfl lausanne, switzerland
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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 - PowerPoint PPT PresentationTRANSCRIPT
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 1 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 Material erosion and migration in tokamakstokamaks
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,
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 2 of 30
Outline of the talkOutline of the talk
• Introduction
• The components of migration
• Global migration accounting
• Material choices for the next step
• Conclusions
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 3 of 30
What is migration?What is migration?
Not an operational issue in today’s tokamaks, but certainly will be in ITER and beyond ……
Migration
TransportErosion DepositionRe-
erosion
=
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 4 of 30
● Co-depositionHigh erosion rates and long term migration of carbon yield high levels of Tritium retention
● Material mixing, propertiesFormation of compounds and alloys through the interaction of pure materials
Change of material properties
Migration will be importantMigration will be important
• 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 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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 5 of 30
Where do erosion and migration Where do erosion and migration occur?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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 6 of 30
Some terminologySome 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
Inner OuterITER 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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 7 of 30
Materials in today’s tokamaksMaterials in today’s tokamaks
Low Z (Carbon) High Z
DivertorTCV, 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
Living with W: see Kallenbach, I3.004, Wed.
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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 8 of 30
Migration
TransportErosion DepositionRe-
erosion
=
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 9 of 30
Principal erosion mechanismsPrincipal 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.)
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 10 of 30
Physical and chemical sputteringPhysical and chemical sputteringChemical (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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 11 of 30
D
ELMs: an example of transient erosionELMs: an example of transient erosion
For more on the physics of ELMs, See Huysmans, I4.002 Thurs.JET #62218
t = 19.05 s, ELM-free t = 19.06 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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 12 of 30
ELMs can ablate Carbon on JETELMs 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) 2668
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
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 13 of 30
ELM ablation limits ITER divertor ELM ablation limits ITER divertor lifetimelifetimeAcceptable lifetime before target change required:
• 3000 full power shots ~1 x 106 ELMs
Inter-ELM power: 5 MWm-2
Target thickness:CFC: 20 mmW: 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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 14 of 30
Migration
TransportErosion DepositionRe-
erosion
=
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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
EDGE2D/NIMBUS
Bypass leaks
Escape via divertor plasma
Ionisation
Gas puff
CX event
Transport creates & moves Transport creates & moves impuritiesimpurities
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
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 16 of 30
Experimentally, strong SOL flowsExperimentally, strong SOL flows
Distance to separatrix (mm)Distance to separatrix (mm)
MM
MM
M
JT-60U
JT-60U
JT-60U(TCV)
JETC-Mod
B●
N. Asakura, NF 44 (2004) 503B. LaBombard, NF 44 (2004) 1047S. K. Erents, PPCF 46 (2004) 1757
(Tore-Supra)
See LaBombard, I3.007 Wed., Bonnin, P2.110, today
D-flows: parallel Mach Number, M = v||/cs. POSITIVE towards inner target
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 17 of 30
Using tracers to study the transportUsing tracers to study the transport
JETDIII-D AUG
13CH4 markers are being increasingly used to get a handle on migration
2.8g 13C, ohmic
9.3g 13C H-mode
0.2g 13C, L-mode
0.2g 13C, H-mode
0.0025g 13C H-mode
gas puff just before vent and tile retrieval – pioneered on TEXTOR
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 18 of 30
DIII-D
End
Wampler et al, JNM 337-339 (2005) 134
Top injection: C13 Top injection: C13 inner target inner target
For more on JET C13 expts. see Rubel, P2.004, today
Likonen et al, Fus. Eng. Design 66-68 (2003) 219
StartJET
• 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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 19 of 30
Migration
TransportErosion DepositionRe-
erosion
=
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Deposition sensitive to local Deposition sensitive to local conditionsconditions
• 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
DetachedObservations consistent with a contribution to the carbon source from outside the divertor
Groth et al., P4.015, Thurs.
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 21 of 30
Strike point57080 57082 57084 570860.0
0.2
0.4
0.6
0.8
1.0
1.2
Shot number
C-d
ep
os
itio
n (
nm
/s)
Re-erosion important for C-Re-erosion important for C-migrationmigration
Kirschner et al, JNM 337-339 (2005) 17
Esser et al., JNM 337-339 (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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 22 of 30
Global migration accounting
TransportErosion DepositionRe-
erosion
=
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 23 of 30
A non-trivial task!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
Tore Supra balance: see Dufour et al, P5.002 Friday
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 24 of 30
Strachan et al, NF 43 (2003) 922
JET migration accounting (I)JET migration accounting (I)Use spectroscopic methods + modelling to compute C sources
EDGE2D/NIMBUS DIVIMP/OSMSimulation of CIII emission intrinsic sources
Divertor C-source = 5-10 x Wall source
Carbon recycles
1 ton/year
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 25 of 30
~400g C
JET migration accounting (II)JET migration accounting (II)Make balance for period 1999-2001 with MarkII
GasBox divertor: 14 hours plasmain diverted phase (50400 s, 5748 shots)
450g C (CIII)
Spectroscopy + ModellingPost mortem surface analysis
• Deposition all at inner target
• Net erosion at main walls
• No significant divertor erosion
Likonen et al, JNM 337-339 (2005) 60, Matthews et al., EPS 2003
215 kg/year strong T co-deposition
Very similar result for AUG, but overall C-balance more complexMayer et al, JNM 337-339 (2005) 119
(1 year = 3.2 x 107 secs)
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 26 of 30
W+
W0
Tungsten migration in AUGTungsten migration in AUG2002-2003 Campaign: ~1.4 hours in diverted phase (4680 s, 1205 shots)
1.3x1018s-1
0.5x1017s-1
1.1x1017s-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 337-339 (2005) 10
Larger Larmor radius helps at higher mass
~1.5 kg/year
W-coated: (40% of total
area)
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 27 of 30
Material choices for the next stepMaterial choices for the next step
An ITER-like first wall at JET
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 28 of 30
Current materials choice for ITERCurrent 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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 29 of 30
An ITER-like wall in JETAn ITER-like wall in JETOption 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
Option 1 Option 2
Be
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 30 of 30
ConclusionsConclusions
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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 31 of 30
Reserve slides
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 32 of 30
A. Herrmann, AUG
Herrmann et al, P1.006 Mon.
ELMs might also erode the main wallsELMs 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:Eion > 1 keV on ITER erosion problem, even for high Z wall
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 33 of 30
Can SOL ion flows transport material?Can SOL ion flows transport material?
B
ErxB, pxB
Ballooning
Pfirsch-Schlüter
Divertor sink
ExB
Simplified – shown in the poloidal plane only
Poloidal
Parallel
Yes, but picture is complex – theory and experiment not yet reconciled
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 34 of 30
Toroidal limiters: 22 7
Total: 19-20 2.7-5.5
Neutralisers: 1 1-2
Bumper: 1 ?
“Obstacles”: 6 0.5
Pump ducts: 0.02 ?
Pumped out: 1-2 0.2-2
Carbon balance: TEXTOR, Tore SupraCarbon balance: TEXTOR, Tore SupraCarbon 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: 10 1
TEXTOR TS
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
Dufour et al, P5.002 Friday
Low sticking – also AUG
R. A. Pitts, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 35 of 30
Similar observations at JETSimilar observations at JET
Coad et al., JNM 313-316 (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, Paper I2.001, 28/06/2005 32nd EPS Conference, Tarragona, Spain 36 of 30
~400g C22g Be
JET migration accounting (II)JET migration accounting (II)Make balance for period 1999-2001 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 337-339 (2005) 60, Matthews et al., EPS 2003
215 kg/year strong T co-deposition
Very similar result for AUG, but overall C-balance more complexMayer et al, JNM 337-339 (2005) 119