High-density H-mode operation by pellet injection and ELM mitigation with the new active in-vessel saddle coils in ASDEX Upgrade

Abstract

Recent experiments at ASDEX Upgrade demonstrate the compatibility of ELM mitigation by magnetic perturbations with efficient particle fuelling by inboard pellet injection. ELM mitigation persists in a high density, high collisionality regime even with the strongest applied pellet perturbations. Pellets injected into mitigation phases trigger no type-I ELM like events unlike when launched into unmitigated type-I ELMy plasmas. Furthermore, the absence of ELMs results in improved fuelling efficiency and persistent density build up. Pellet injection is helpful to access the ELM-mitigation regime by raising the edge density beyond the required threshold level, mostly eliminating the need for strong gas puff. Finally, strong pellet fuelling can be applied to access high densities beyond the density limit encountered with pure gas puffing. Core densities of up to 1.6 times the Greenwald density have been reached while maintaining ELM mitigation. No upper density limit for the ELM-mitigated regime has been encountered so far; limitations were set solely by technical restrictions of the pellet launcher. Reliable and reproducible operation at line averaged densities from 0.75 up to 1.5 times the Greenwald density is demonstrated using pellets. However, in this density range there is no indication of the positive confinement dependence on density implied by the ITERH98P(y,2) scaling.Final version, 6.1.2012 2 IntroductionThe power load deposited by type-I ELMs onto the first wall and divertor presents a severe danger for ITER. Currently, there are several options for ELM mitigation: Operating with plasma scenarios which develop no or only benign ELMs, ELM pace-making to enhance the ELM frequency by at least a factor of 15 -30 in order to reduce individual ELM losses by this same factor, or ELM mitigation or ELM suppression with non-axisymmetric magnetic perturbations. Pace-making tries to reduce the type-I ELM size by triggering the instability through an external perturbation, e.g. pellet injection with a rate higher than the natural ELM frequency. Avoidance of type-I ELMs is preferred over ELM pace-making if it can be achieved without significant deleterious impact on the plasma performance. Nonaxisymmetric magnetic perturbations have been investigated in DIII-D where full suppression or at least significant mitigation of ELMs has been achieved over a wide operational range [1]. This successful first demonstration prompted further experiments at JET [2,3], NSTX [4] and MAST [5] with different perturbation field configurations. In theses experiments, quite different results with respect to ELM mitigation were obtained, and so far full ELM suppression has not been reproduced in any of them. Recently, the ASDEX Upgrade tokamak has been enhanced with a set of in-vessel coils [6] in order to study ELM amelioration, shed light on the physics involved, and to improve the extrapolation base for ITER. In a first stage of this enhancement, n=2 magnetic perturbations are found to allow suppressing type-I ELMs and replace th...

ELM pacing and high-density operation using pellet injection in the ASDEX Upgrade all-metal-wall tokamak

“…Line-averaged densities significantly beyond n Gw are achieved during the high-density phase. Again, as already reported in the previous study with in-vessel coil activation [51], this is due to the peaked density profiles generated. The Greenwald density imposes a limit, but only on the edge density.…”

Overview on plasma operation with a full tungsten wall in ASDEX Upgrade

“…In all discharges with successful ELM mitigation the residual core tungsten concentration is at the same level or lower than in comparable type-I ELMy phases [2]. Combined with pellet fuelling, densities 50% above the Greenwald density could be achieved in H-Mode discharges [63].…”

The H-mode density limit in the full tungsten ASDEX Upgrade tokamak

“…The centrally increased density does not have an effect on the confinement, but the density dependence of the confinement scalings (e.g. ITERH98P(y,2) 27 ) cannot be reproduced at these high densities 20 . Several experiments show that the Greenwald limit is determined to plasma edge parameters (e.g.…”