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

Selected aspects of type-I ELM power load in ASDEX Upgrade (and comparison to JET). T.Eich. Brief history: Assessment of ELM power load Temporal evolution of divertor power deposition In/Out asymmetry of divertor target load. 07/05/2007, ITPA Meeting (DIVSOL/PEP Joint Session), Garching.

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

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  1. Selected aspects of type-I ELM power load in ASDEX Upgrade (and comparison to JET) T.Eich • Brief history: Assessment of ELM power load • Temporal evolution of divertor power deposition • In/Out asymmetry of divertor target load 07/05/2007, ITPA Meeting (DIVSOL/PEP Joint Session), Garching

  2. 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) Radiation, see talk by R.A.Pitts Wall load, talk by W.Fundamenski

  3. 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. Parallel transport time scales Fit of target response: Maxwellian distribution of particle energies with Te and otherwise only geometry (and amplitudes) Works well for low density cases W.Fundamenski, PPCF 2006, p. 109

  5. Field dependence of power asymmetry Reversed field (# 16724) Normal field (#16725) T.Eich, PPCF 47, p.815 In normal field more ELM power on inner divertor than on outer

  6. 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. ‘normal’ ion B x grad(B) direction AUG upper divertor ‘hot Te’ ‘cold Te’ • 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

  8. ‘reversed’ ion B x grad(B) direction AUG upper divertor ‘cold Te’ ‘hot Te’ • More energy (power) deposited on outer target than on inner target • Charge (current) → net negative charge flows through inner target Observation: target with net negative charge flowing through that target receives less energy during ELMs

  9. 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 Charge (As) for ‘normal’ field Diagnostical artefacts (i.e. surface layer) are neglegible

  10. Comparison JET and ASDEX Upgrade JET AUG Focusing on ‘normal’ field: • JET & AUG + (ELM target energy < 100kJ) : 1 ≤Einner / Eouter≤ 2 • Only JET + (ELM target energy > 100kJ) : Einner / Eouter≈2

  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-Einnerwhich seems to play a less significant role for large ELMs (>100kJ) at JET

  12. Same JET data as on previous slide Unclear if pedestal or target properties cause variation

  13. Spatial ELM structure 8 field lines started from =0 (poloidal) and =101,79,58, 43,19,350,327,314. ~21: n=360/ 21=17.

  14. Largest toroidal asymmetry Quasi toroidal symmetric 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

  15. Temporal evolution DOC-L current and gas scan

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