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

摘要

The power load deposited by type-I ELMs onto the first wall and divertor forms a severe threat for ITER. Currently, there are several options for ELM mitigation: (i) operating with plasma scenarios developing no or only benign ELMs, (ii) ELM pacing to enhance the ELM frequency at least by a factor of, 15 - 30, and (iii) ELM control with non-axisymmetric magnetic perturbations. Pacing tries to reduce the ELM size by triggering the instability through an external perturbation, e.g. by pellet injection with a rate higher than the natural ELM frequency. Suppression of type-I ELMs is preferred over ELM-pace-making if it can be achieved without significant deleterious impact on the plasma performance. A key issue: for H-mode operation with suppressed or mitigated ELM activity is whether core pellet fueling, a requirement in ITER, will trigger strong ELMs and thereby re-introduce excessive peak power loads. Full suppression, respectively significant mitigation of ELMs, has been achieved with in-vessel coil induced edge magnetic perturbation in DIII-D [1] covering a wide operational range. Pellet fueling, applied to compensate for the observed density pump out, however, caused recurring of the ELMs [2]. The all tungsten wall tokamak ASDEX Upgrade has been enhanced with a set of in-vessel coils in order to support ELM amelioration investigations in support of ITER. For fueling studies in this operational regime, also the pellet fueling system has been refurbished" and upgraded.