r. a. pitts et al., o-8 18 th psi, toledo, spain 27 may 2008 the impact of large elms on jet...

R. A. Pitts et al., O-8 18th PSI, Toledo, Spain 27 May 2008

The Impact of large ELMs on JET

Presented by R. A. PittsCRPP-EPFL, Switzerland, Association EURATOM-Swiss Confederation

on behalf of JET Task Force E and JET EFDA Contributors

18th International Conference on Plasma-Surface Interactions, Toledo, Spain, 23-26 May 2008


G. Arnoux1, S. Brezinsek2, M. Beurskens1, T. Eich3, H. G. Esser2, W. Fundamenski1, A. Huber2, B. Gulejova4, S. Jachmich5, A. Kreter2,

A. Loarte6, E. de la Luna7, J. Marki4, G. F. Matthews1, V. Philipps2, E. Solano7, M. F. Stamp1 and JET EFDA Contributors*

1Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK 2Institut für Energieforschung-Plasmaphysik, Forschungszentrum Jülich, Association

EURATOM-FZJ, Germany3Max-Planck-Institut für Plasmaphysik, IPP-EURATOM Association, D-85748 Garching,

Germany4CRPP-EPFL, Switzerland, Association EURATOM-Swiss Confederation

5LPP, ERM/KMS, Association Euratom-Belgian State, B-1000, Brussels, Belgium6ITER Organization, Cadarache, France

7Associacion Euratom/CIEMAT para Fusion, Madrid, Spain

*See appendix of M. Watkins et al., Fusion Energy 2006 (Proc. 21st Int. Conf. Chengdu, 2006) IAEA Vienna (2006)

with thanks to many co-authors


Outline

• Introduction• Experiment• Impact on the divetor

– Radiation– Surface temperatures

• ELM-wall interactions– Energy deposition– Comparison with theory

• Conclusions


Introduction - ELM size limit

Material damage poses a limit on the maximum ELM size tolerable on ITER

Current estimates indicate that ELM power fluxes must remain below ~0.5 MJm-2 at the ITER divertor targets (see J. Roth, paper R-1)

This implies an ELM energy loss, WELM ~ 1 MJ ~0.3% of stored energy in an ITER QDT = 10 burning plasma!

This is lower than any Type I ELM energy so far achieved mitigation strategies required. BUT …

JET Type I ELMs can approach 1 MJ study the effects on first wall surfaces and edge plasma

Important also in preparation for JET ITER-like wall and improved understanding of ELM SOL physics


Experiment

Vertical targets, MarkIIHD div.Ip = 3.0MA, B = 3.0Tq95 = 3.1, ~ 0.25, ~ 1.72

Fuelling scan producing ELMs with range of frequencies and amplitudes


Large ELMs at zero fueling

Input energy ~195 MJEnergy Tile 3,7: 24.6, 70.1 MJRadiated energy: ~82 MJn/nGreenwald ~ 0.4

D (inner)

PTOT (MW)

WDIA (MJ)

Te,ped

(keV)

ne,ped (1019m-3)

H98Y

Zeff (Brems)

Time (s)

#70226 – no gas fuelling

R. A. Pitts et al., ITPA, Garching, 2007

Mostly NBI


Lowest fuelling cases at ITER relevant *ped

WELM/Wped ~ 0.2 for largest ELMs

R. A. Pitts et al., ITPA, Garching, 2007

D (inner)

PTOT (MW)

WDIA (MJ)

Te,ped

(keV)

ne,ped (1019m-3)

H98Y

Zeff (Brems)

Time (s)

#70226 – no gas fuelling

ITER

Mostly NBI

Large ELMs at zero fueling


Radiation during large ELMs

Time (s)

#70225, low fuelling

D(inner)

WDIA (MJ)PRAD (MW)

Erad (MJ)

0.58 MJ1.08 MJ

0.85 MJ1.29 MJ

Strong in-out asymmetry in ELM induced radiation for high WELM probably due to layers on inner targets and preferential inboard deposition of ELM energy

More details: A. Huber et al, P2-24


In-out ELM radiation asymmetry

WELM = 0.85 MJWELM = 0.61 MJ

First ELM spikeonly

ForWELM 0.6 MJ radiation “spills over” separatrix – in-out radiation asymmetry reduced

>~

ERAD/WELM ~ 0.5 if WELM 0.6 MJ

Evidence for a break at larger WELM

<~

Up to 70% WELM radiated

More details: A. Huber et al, P2-24


Target IR data not of high enough quality in this more recent experiment to quantify tile surface temperatures

Return to shots from 2003

Slightly raised strike pts. compared with recent pulses

Slightly lower WELM

Vertical targets, MarkIISRP divertorIp = 1 – 3 MA, B = 1 - 3Tq95 ~ 3.1, ~ 0.253.0 MA discharge #62218 similar in parameters to more recent shot #70226

Target surface temperatures?



Vertical targets, MarkSRP div.Ip = 1 – 3 MA, B = 1 - 3Tq95 ~ 3.1, ~ 0.253 MA discharge #62218 similar in parameters to more recent shot #70226

D (inner)

PTOT (MW)

WDIA (MJ)

Te,ped

(keV)

ne,ped (1019m-3)

H98Y

Zeff (Brems)

Time (s)

#62218

EL

M a

vera

gin

g p

eriod



Max. Tsurf far from sublimation

Suggest thermal decomposition and ablation of inner target layers accounts for strong radiation asymmetry

Inter-ELM power loads higher at outer than inner as usual

Clear affect of surface layers on inner target

For largest ELM (~0.7 MJ):Max Tsurf (outer) ~ 1150ºCMax Tsurf (inner) ~ 875ºC

See also talks by A. Kreter, I-3 and T. Eich, O-17

T. Eich


How much ELM energy to walls?

Main chamber IR camera too slow to follow single ELMs and filaments very asymmetric toroidally and poloidally

68193, 57 s

Make energy balance for a single outboard poloidal limiter during H-mode phase, assume:Only ELM filaments can deposit energy on limitersNo energy to upper dump platesNo energy deposited in compound phasesSame energy on 16 limiters

For more on ELM filament wall interactions see posters by: A. Alonso, P1-39M. Jakubowski, P1-24D. Moulton, P2-35


How much ELM energy to walls?

Main chamber IR camera too slow to follow single ELMs and filaments very asymmetric toroidally and poloidally

68193, 57 s

20.016 s 17.405 s

111213Make energy balance for a

single outboard poloidal limiter during H-mode phase, assume:Only ELMs can deposit energy on limitersNo energy to upper dump platesNo energy deposited in compound phasesSame energy on 16 limiters

∑E

tile (15 tiles)


Wall loading and ELM size

68193, 57 s

Pulse No.

gas

(1022e-/s)

No. ELMs (MJ) (MJ) (kJ)

70221 1.47 133 29.7 1.49 224 5.3

70222 1.24 87 23.9 1.02 275 4.3

70223 0.89 50 18.0 0.85 360 4.7

70224 0.38 16 8.34 0.71 521 8.8

70225 0 30 14.9 1.37 497 9.2

70226 0 24 12.7 1.49 528 11.8

Ip = 3.0 MA, B = 3.0 T, gas scan. Separatrix-midplane outer wall gap fixed at ~5.0 cm. WELM estimated for first ELM peak only

(%)ELM

LIM

W

E

ELMW LIME ELMW

For fixed wall gap, on average, larger ELMs deposit more energy on limiters.

Pulse No.

gas

(1022e-/s)

No. ELMs (MJ) (MJ) (kJ)

70221 1.47 133 29.7 1.49 224 5.3

70222 1.24 87 23.9 1.02 275 4.3

70223 0.89 50 18.0 0.85 360 4.7

70224 0.38 16 8.34 0.71 521 8.8

70225 0 30 14.9 1.37 497 9.2

70226 0 24 12.7 1.49 528 11.8

(%)ELM

LIM

W

E

ELMW LIME ELMW

How does wall energy fraction compare with theory?


Pedestal profiles (#70224)

Filament parallel energy loss model(W. Fundamenski, R. A. Pitts, PPCF 48 (2006) 109)

Assume ELM filament released in pedestal region with constant radial speed.

Propagate to walls and track power exhaust due to parallel energy loss

Pedestal width ~4 cm

Reasonable assumption: filament is born in the mid-pedestal region


Compare with filament model

Mid-pedestal:Te,0 = Ti,0 ~ 800 eVne,0 ~ 3.01019 m-3

ped ~ 4 cm

vELM = 600 ms-1 from previous JET studies

)(2

30,0,00 ie TTnW

W’ = 0.094(model)

W’ = 0.088(experiment)

Very good agreement given the approximations!



)(2

30,0,00 ie TTnW

0.094

Pedestal top:Te,0 = Ti,0 ~ 1500 eVne,0 ~ 5.01019 m-3

Separatrix:Te,0 = Ti,0 ~ 200 eVne,0 ~ 1.01019 m-3

vELM = 600 ms-1

0.037

0.394



)(2

30,0,00 ie TTnW

Mid-pedestalPedestal topSeparatrix

vELM = 600 ms-1

vELM = 1200 ms-1

A filament starting at the pedestal top with 2x higher vELM deposits the same energy at the limiter

Filaments starting at the separatrix must travel much more slowly (~180 ms-1)


How to extrapolate to ITER?Results indicate that larger ELMs travel faster consistent with interchange drive and sheath dissipation as mechanism for filament motion1

1 MJ (mitigated) ELMs on ITER deposit negligible energy fraction at wall

1O. E. Garcia et al., Phys. Plasmas 13 (2006), 2W. Fundamenski, JNM 363-365 (2007)

Use previous scaling from JET H-modes2 (vELM = 600 ms-1 for WELM/Wped ~ 0.12):

2/1

12.0

1600~

ITERped

ITERELM

JETmidped

ITERmidpedITER

ELM W

W

T

Tv

2/12/1

~~

ped

ELM

s

ELM

W

W

p

p

c

v

Natural ELM (~20 MJ) vELM = 1000 ms-1

Desired ELM (~1 MJ) vELM ~220 ms-1

20%

2.5%


Conclusions

• JET can access ELMs with WELM approaching 1 MJ at ITER relevant pedestal collisionality (but low n/nGW)

• Strong in-out divertor radiation asymmetry – up to 70% of the ELM energy drop can be radiated, mostly in the inner divertor volume

• Divertor surface temperatures too low for sublimation thermal decomposition and ablation of inner target co-deposited layers

• ELM filaments seen clearly at main chamber limiters

• Deposited energy fraction increases with ELM size

• Wwall/<WELM> = 9-12% for <WELM> ~ 0.5 MJ

• 1 MJ ELMs on ITER will deposit very small energy fraction at first wall if interchange driven filament velocity scaling applies

r. a. pitts et al., o-8 18 th psi, toledo, spain 27 may 2008 the impact of large elms on jet...

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