1/11

Selected aspects of type-I ELM power load in ASDEX Upgrade (and comparison to JET)

07/05/2007, ITPA Meeting (DIVSOL/PEP Joint Session), Garching

T.Eich

Brief history: Assessment of ELM power load Temporal evolution of divertor power deposition In/Out asymmetry of divertor target load

2/11

Thermal material ablation limit

• For assessment of ELM thermal load we need to know:

energy (per ELM, fraction deposited on inner/outer target)

power decay width ( λp, toroidal asymmetry, ELM structure)

power deposition time scales (tparallel, tperpendicular)

IRfff

R

C

TE

divertoroutin

divELMIR

material

ELMtolerableELM

tolerable

/

2

Radiation, see talk by R.A.PittsWall load, talk by W.Fundamenski

3/11

Experimental observation

• Data taken from USN discharges• Non tilted (divertor) target tiles with virgin surfaces installed• Always pairs of discharges with both field direction• Data taken early in the campaign• Only some hundred plasma seconds, surface layer should be much smaller than for standard divertors (JET,AUG)• No chance for surface layers to grow asymmetric• Shunt measurements for target currents available

4/11

Parallel transport time scales

2

,

2

,

2

,,, exp1)(

tttAtp oioioi

oioi

outerinnerouterinner LL //

Fit of target response:Maxwellian distribution of particle energies with Te and otherwise only geometry (and amplitudes)

W.Fundamenski, PPCF 2006, p. 109

Works well for low density cases

5/11

Field dependence of power asymmetryNormal field (#16725)Reversed field (# 16724) T.Eich, PPCF 47, p.815

In normal field more ELM power on inner divertor than on outer

6/11

In/Out asymmetries

A simple ELM parallel transport model is hard to get in line with a

larger fraction of energy found on inner divertor (in normal field)

Power deposition integrated over ELM duration on the outer/inner

divertor target giving ELM target energies: Eouter, Einner

The sum, the difference and the ratio of these ELM energies:

Eouter + Einner, Eouter - Einner, Einner / Eouter

Target current onto inner and outer target integrated over ELM

duration gives (net) charge CELM flowing through targets due to the ELM

All presented data are based on coherent ELM averaging (≈20 ELMs)T.Eich, Journal Nucl.Mater., in print

7/11

‘normal’ ion B x grad(B) direction

• More energy (power) deposited on inner target than on outer target

• Charge (current) → net negative charge flows through outer target

• Charge (current) for inner and outer target are equal (within error bars) in absolute size and opposite in sign

AUG upper divertor

‘hot Te’

‘cold Te’

8/11

Observation: target with net negative charge flowing through that target receives less energy during ELMs

‘reversed’ ion B x grad(B) direction

• More energy (power) deposited on outer target than on inner target

AUG upper divertor

• Charge (current) → net negative charge flows through inner target

‘cold Te’

‘hot Te’

9/11

ELM energy difference vs. charge difference

• The difference of ELM deposited energy on the targets is well correlated with charge flowing through the targets during ELMs

• Both quantities switch sign with field direction

• Slope of the line for the two field directions is different

line goes through zero

Cha

rge

(As)

for ‘normal’ field

As

kJ

C

EE

ELM

innerouter

5

5

1outer

inner

E

E

Diagnostical artefacts (i.e. surface layer) are neglegible

10/11

Comparison JET and ASDEX Upgrade

AUG JET

• JET & AUG + (ELM target energy < 100kJ) : 1 ≤ Einner / Eouter ≤ 2

• Only JET + (ELM target energy > 100kJ) : Einner / Eouter ≈ 2

Focusing on ‘normal’ field:

11/11

Summary

• Temporal evolution of power deposition can be well approxiamted by a simple transport model (maxwellian distribution of particles)

• Amplitudes of the ELM power load are not understood

• ELM energy difference between the targets is well correlated to a net target charge flowing through the targets during an ELM

• In absence of a charge difference, the ELM divertor target energy gives a balanced in/out ratio fitting to the convective transport model

• There appears an unidentified parameter for Eouter-Einner which seems to play a less significant role for large ELMs (>100kJ) at JET

12/11

Same JET data as on previous slide

Unclear if pedestal or target properties cause variation

13/11

8 field lines started from =0 (poloidal) and =101,79,58, 43,19,350,327,314. ~21: n=360/ 21=17.

Spatial ELM structure

14/11

Field line tracing in equilibrium field

Outer path of field lines approaches upper X-point

Inner path of field lines goes by upper and lower X-point

Quasi toroidal symmetric

Largest toroidal asymmetry

15/11

Temporal evolution

DOC-L current and gas scan