r. a. pitts crpp, association-euratom conf é d é ration suisse, epfl lausanne, switzerland

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,


Outline of the talkOutline of the talk

• Introduction

• The components of migration

• Global migration accounting

• Material choices for the next step

• Conclusions


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

=


● 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


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


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


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


Migration


erosion

=


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.)


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.


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)


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


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


Migration


erosion

=


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


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


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


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


Migration


erosion

=


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.


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


Global migration accounting


erosion

=


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


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


~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)


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)


Material choices for the next stepMaterial choices for the next step

An ITER-like first wall at JET


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


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


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 …….


Reserve slides


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


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


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


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


~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

r. a. pitts crpp, association-euratom conf é d é ration suisse, epfl lausanne, switzerland

Documents