First experiments with non-axisymmetric magnetic perturbations, toroidal mode number n = 2, produced by newly installed in-vessel saddle coils in the ASDEX Upgrade tokamak show significant reduction of plasma energy loss and peak divertor power load associated with type-I Edge Localized Modes (ELMs) in high-confinement mode plasmas. ELM mitigation is observed above an edge density threshold and is obtained both with magnetic perturbations that are resonant and not resonant with the edge safety factor profile. Compared with unperturbed type-I ELMy reference plasmas, plasmas with mitigated ELMs show similar confinement, similar plasma density and lower tungsten impurity concentration.

The compatibility of ICRF (Ion Cyclotron Range of Frequencies) antenna operation with high-Z plasma facing components (PFCs), needs improvement to keep ICRF as a good candidate for heating and current drive system in a fusion reactor. ASDEX Upgrade (AUG) with its tungsten (W) first wall and ICRF system allows to study ways to do this. A noticeable improvement of the ICRF operation with W-wall can be achieved by forcing low plasma temperature conditions at the PFCs. These conditions can be fulfilled by increasing plasma-antenna clearance and by strong gas puffing, thus approaching the conditions ITER ICRF antenna plans to operate at. W sputtering during ICRF can be significantly decreased when the intrinsic light impurity content is decreased. However, an additional improvement is required for further reduction of the high-Z impurity sputtering during ICRF in the present and for the future devices. The improved theoretical modelling of ICRF antenna near-fields shows that the RF voltages along the magnetic field lines may originate from RF currents on the antenna box to a large extent, and not directly from antenna straps and their RF magnetic flux. Experimental results in AUG corroborating this picture are described. The calculations for future antenna design show that a reduction of the antenna box contribution can be achieved by extending the antenna box parallel to the magnetic field and increasing the number of toroidally distributed straps with (0π...π0) or (0π...0π) phasing.

Abstract. The ASDEX Upgrade tokamak is currently being enhanced with a set of invessel saddle coils for non-axisymmetric perturbations aiming at mitigation or suppression of Edge Localised Modes (ELMs). Results obtained during the first experimental campaign are reported. With n = 2 magnetic perturbations, it is observed that type-I ELMs can be replaced by benign small ELM activity with strongly reduced energy loss from the confined plasma and power load to the divertor. No density reduction due to ELM mitigation (density "pumpout") is observed. ELM mitigation has, so far, been observed in plasmas with different shape, heating powers between a factor of 2 − 8 above the H-mode threshold, different heating mixes and, therefore, different momentum input. The ELM mitigation regime can be accessed with resonant and non-resonant perturbation field configurations. The main threshold requirement appears to be a critical minimum plasma edge density which depends on plasma current. So far it is not possible to distinguish whether this is an edge collisionality threshold or a critical fraction of the Greenwald density limit.

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