10th ITPA meeting on SOL & divertor physics, Avila, Spain, Jan 7-10, 2008 Arne Kallenbach 1/15...

10th ITPA meeting on SOL & divertor physics, Avila, Spain, Jan 7-10, 2008 Arne Kallenbach 1/15

Prediction of wall fluxes and implications for ITER limiters

Max-Planck-Institut für Plasmaphysik

Arne Kallenbach, ASDEX Upgrade Team


Topics of this talk: guidelines on load specifications

steady state particle main chamber fluxes from spectroscopy

estimates of connected power fluxes and decay lengths

contribution due to ELMs (enhancement factor)


Current ITER Guidelines (PID V3.0): Only radiation and CX load to first wall, 0.5 MW/m2

diffusive transport between ELMs

blobby transport between ELMs(radial outward convection)

ELM SOL transport (like large blobs)

- parallel drift towards high-field side- strong recycling around inner X-point

additional players in particle transport:

transport and drifts lead toparallel heat fluxes in far SOL


Main chamber spectroscopy at ASDEX Upgrade

Ralph Dux


Innner and outer wall plasma-surface interaction in AUG from CII spectroscopy: very sensitive on in-out alignment

inner heat shield major recycling region except plasma close to outer limiter lower inner wall flux dominated from inner divertor upper inner wall flux has radial e-folding length ~ 2-3 cm

R lim

8 m2

0.3 m2


How to estimate the stationary power flows on the limiters

1) estimate the total radial ion outflux*

2) estimate the deposited energy per electron-ion pair

3) estimate the effective wetted area or peak load and decay length

limiter power flux density

average value from different models, be conservative and use upper end

*IO calculates in terms of parallel power fluxes and decay lengths


AUG edge density profiles from Li-beam

H-mode

recyclingrises

Radial SOL particle flux in ITERansatz with effective D:

= D dn/dr

D = 3 m2/s this value typical for SOL wing in many devices

dn/dr = 21019 m-3 / 0.05 m = 1.21021 m-2 s-1

total main chamber ion influx: multiply with 1/3 of plasma surface area

F ITER = 680 m2 1/3 ~ 31023 s-1

1) Total radial ion wall flux in ITER [i] scaling like diffusive transport

Transport balloons around outer midplane:

conservative,can be larger


3.21023 s-1

1) Total radial ion wall flux in ITER [ii] some alternative ways of estimation

a) Same flux density as in AUG discharge with high similar fGreen, P/R, and absolute density, scaled with area

b) Same flux density as in AUG discharge with high similar fGreen, P/R2 and absolute density, scaled with area 16

c) Same flux density as in JET discharge with high similar fGreen, P/R2 and absolute density, scaled with area (best use 4 MA, 25 MW discharge)

ITER: 100 MW/6 m, not possible in AUG, scale P0.24 [NF 42 (2002) 1184]

ITER: 100 MW/36 m2, 7.5 MW in AUG, AUG # as above

AUG 21015/17, 7.5 MW, ne=1020 m-3, =21022 1/s, drXP=3 cm

4.41023 s-1

JET 70054, 3.5 MA, 24 MW, 1e20, midplane H2072 m2 main=7.21022 s-1

2.91023 s-1 Over all, 3(1-5)1023 s-1

seems reasonable estimate


100 eV per e-i pair 1.6 MW

2) Energy per electron ion pair

Te in the SOL wing of a high density H-mode discharge is typically 5-10 eV,Ti tends to be moderately higher

We assume for ITER Te= 10 eV, Ti= 20 eV

standard model for sheath power deposition (negl. secondary el. emission)

P= ei (2Ti + 3Te + Erec) + ee 2Te 100 e

per 11023 part/s


ITER quick guess: 18 protruding ribs, height 5 m, 0.05 m wetted width 4.5 m2 for HFS and LFS each (good alignment required !)

3) effective wetted area and resulting loads

The wetted area depends on actual wall design !

wetted width depends on decay length in limiter shadow

Of course, the upper X-point region takes more power and must be strengthened

3 1023 ions/s 4.8 MW charge exchange is expected to increase this number by 10-20 % ELMs contribute to recycling fux by factor 1.5 radiation is expected to contribute < 0.2 MW/m2 some contrib. by fast ion losses on LFS

overall, expected peak loads about 1 MW/m2

not problematic, but safely 1 MW/m2 would allow to avoid active cooling


ELM contributions to average particle influxes:small for D, C, dominnat for W

Outer limiter

ELM-cycle averaged,D and C fluxes increasedby ~ 1.5

but: 70 % of the W influxdue to ELMs (increased yield)

R. Dux


Decay length depends on connection length to limiters

measurements inAUG limiter shadowby H.W. Müller

Increasing the number of limiters can reduce the power load.However: the decay length shortens with reduced connection lengthand more precise alignment will be required


ITER expects negligible loads on inner wall- does the existence of a 2nd sep. shield the inner wall ?

No !

further investigations needed on inner wall load close DN dRXP= 3 mm


Comparison to previous estimate based on JET-AUGRecycling scaling (Tarragona meeting, July 2005)

new insight: predominantly HFS recycling multiply with S/4 only: Rtot= 1024 s-1 (ne,sol= 4.7 1019)

Strong dependence of total recycling on ne,line-av (power 4) If pellets are needed to reach 1020 m-3 in ITER, this number comes down:If ITER produces ne= 7.5 1019 by recycling only, Rtot= 3 1023, ne,sol= 2.6 1019


Conclusions

Main chamber recycling occurs predominantly on the high field side and on wall structures touching the innermost flux surfaces

Effect supposed to be connected to strong drifts towards HFS

Strong plasma wall interaction with the inner wall close to DN operation is not understood: fluxes close to the separatrix or ExB drifts around upper X-point ?

Expected total particle fluxes 3 2 1023 part/s, power fluxes ~ 5 MW

How will the ITER FW will look like ?


ELMs:Simple size scaling and effect to wall materials

Size scaling based on empirical findings: natural type-I ELM size ~ 10 % of pedestal energy, 3.5 % of plasma energy ELMs carry 30 % of the power flux simple algebra: PELM= 0.3 Ploss = 0.3 Wtot/E = 0.035 fELM Wtot = fELMWELM

fELM = 8.6/E

ITER Sc. 1: Wtot = 353 MJ, E=3.4 s AUG typ.: 0.8 MJ, 0.1 s fELM = 2.5 Hz, WELM= 12 MJ 80 Hz, 28 kJ

controlled ELMs: if fELM is changed, WELM scales ~ 1/fELMITER PID: uncontrolled ELMs fELM = 1 Hz, WELM= 15-20 MJ controlled ELMs fELM = 5 Hz, WELM= 3-4 MJ

o.k.


ELMs:Simple size scaling and effect to wall materials

Divertor peak power load: ITER PID assumed effective wetted divertor area of 7.5 m2 (w= 0.1 m along targets) resulting maximum loads were 2.7 MJ/m2 (uncontr.), 0.5 MJ/m2 (contr.)

Material properties:

melting/ablation limits: Be 20, W 60, CFC 60-70 [MJ m-2s-0.5]

example: ELM 1 MJ/m2, 0.5 ms duration 45 MJ m-2s-0.5

recent lab exps. (Russian-EU collab.) suggest limit below0.7 MJ/m2 both for W and CFC (fatigue, crack formation) reduce peak load by factor 0.5

1556 K 3683 K 3640 K (subl.)

the maximum allowed ELM was controlled to 4 MJ

latest changes: no ELM power broadening p 5 mm (fact 2/3) in-out asymmetry 2:1 – (fact ¾), recover factor 2 safety margin 0.5 0.25 MJ/m2 ?

maximum “ELM” ~ 1 MJ too pessimistic – ignores large p inner div


Open points = possible AUG contributions1) midplane inter-ELM power width

midplane Te decay length scales ~ machine size

A. Kallenbach et al., ITPA SOL&Div Topical Group, PSI 2004

expected power width 2/7 Te

considerably broader widths observed in divertor (mapped to omp)good topic for future AUG / inter-machine exps (L. Horton)

AUG: 1.3 mm omp

10th ITPA meeting on SOL & divertor physics, Avila, Spain, Jan 7-10, 2008 Arne Kallenbach 1/15...

Documents

Transcript of 10th ITPA meeting on SOL & divertor physics, Avila, Spain, Jan 7-10, 2008 Arne Kallenbach 1/15...