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Modelling of ELM mitigation at JET

Modelling of ELM mitigation at JET.

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Modelling of ELM mitigation at JET

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  1. Modelling of ELM mitigation at JET F Koechl, R Albanese, R Ambrosino, G Corrigan, L Garzotti, H-S Kim, J Lönnroth,P T Lang, E de la Luna, M Mattei, F Maviglia, D C McDonald, V Parail, F Rimini,G Saibene, E R Solano, M Valovič, I Voitsekhovitch, A Webster, S Wiesen,JET EFDA contr. & ITM-TF ISM Group. 25th SPPT - Prague, 18th-21st June 2012

  2. JINTRAC code suite:

  3. JINTRAC code suite: • Integrated modelling with JET code suite JINTRAC to better understand: • Vertical kick ELM trigger mechanism • Density depletion in mitigated regimes • Interaction pellet fuelling  ELM mitigation

  4. ELM mitigation: Edge Localised Modes (ELMs) are repetitive bursts of the edge plasma: • Large ELMs in ITER would accelerate erosion of plasma facing components! • Tolerable energy density for ITER divertor target plates: < 0.5 MJm-2 • This requires ELM size reduction by a factor 20! Quiet phase Strong heat flux to the divertor Pressure & current build up at the edge Pressure & current suddenly collapse • Various methods for ELM size control under investigation at JET: • Gas fuelling • Resonant magnetic perturbation (RMP) • Magnetic ELM pacing (plasma vertical kicks) • ELM pacing by pellet injection

  5. Plasma vertical kicks at JET: #77640 @ 59.29 s • Sudden vertical plasma displacement by application of large voltages to the Vertical Stabilization circuit (max. fkick 60 Hz). • Effects of kicks on the plasma: • Change in plasma shape / volume • Evolution of edge current profiles • Plasma reaction to kicks: • Desired: ELM triggering for large enough kick perturbation • Side effect: Density reduction (“pump-out”) at high fELM 7 cm The plasma moves down and shrinks Dz< 2 cm E. de la Luna, IAEA 2010

  6. Kick cycle modelling: • Simulation shows that pressure-driven instabilities are not triggered by kicks, but current driven instabilities are enhanced by low shear. • We observe flattening of shear due to kicks: 57.300s 57.304s 57.308s 57.312s ~ ELM time 57.316s 57.320s Current perturbation propagating inwards Shear flattening at ELM trigger time

  7. Density depletion with kicks: Reduction in density at higher ELM frequency, with mild degradation in confinement: ne lin. core fELM6 Hz, experiment fELM40 Hz, experiment ne lin. edge Te ped

  8. Density depletion with kicks: Experimental trends can be reproduced with JINTRAC, same simulation conditions except for fELM and ELM amplitude (adjusted to DWELM): ne lin. core fELM6 Hz, experiment fELM6 Hz, simulation fELM40 Hz, experiment fELM40 Hz, simulation ne lin. edge Te ped

  9. Density depletion compensated by pellets in JET: JINTRAC simulations, density maintained by pellet injection: Density feedback control ne lin. avg. Wth Degradation in plasma confinement! H98y,2 fELM6 Hz, fELM40 Hz, fELM40 Hz +pellets Te ped

  10. Pellet fuelling  ELM mitigation in ITER: JINTRAC simulations of pellet fuelling in ITER ELM-mitigated regime: ne avg. Pellets:. ne ped. ELMs Pellet plasmoid drift allows for deeper particle penetration & improved fuelling efficiency! Te ped. H.-S. Kim Fuelling pellets trigger ELM bursts, reducing fuelling efficiency! Wth no plasmoid drift with plasmoid drift

  11. Summary: • Summary: • Kick-triggered ELMs can be reproduced assuming peeling mode (current driven) instabilities (pressure perturbation too small to reach critical gradient for natural ELMs). • – Shear modification due to combination of current reduction close to the edge + induced current close to top of pedestal may be responsible for ELM triggering and could explain ELM trigger time delay. • Density depletion in mitigated regime appears to be natural consequence of different location of heat and particle sources and enhanced pumping efficiency; “mitigated” ELM mitigation due to change in SOL conditions. • Pellet injection might help to recover initial density, but leads to a degradation in confinement. • Fuelling efficiency of shallow pellet fuelling may decrease due to prompt pellet triggered ELMs and cause temporary variation of fELM.

  12. Backup slides

  13. Effect of kicks on power depos.: Electron power to inner target / pumped neutrals: fELM = 6 Hz, simulation fELM = 40 Hz, simulation Mitigated ELM impact on divertor Enhanced pumping until lower ne level is reached Steady-state pumping conditions

  14. Effect of kicks on SOL: SOL contour plots (t ~ 60.0s): fELM = 6 Hz: nD Te fELM = 40 Hz: nD Te Reduced target protection at low ne / high Te reduced ELM mitigation efficiency!

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