Runaway electron mitigation by 3D fields in the ASDEX Upgrade experiment

Disruption-generated runaway electron (RE) beams represent a severe threat for tokamak plasma-facing components in high current devices like ITER, thus motivating the search of mitigation techniques. The application of 3D fields might aid this purpose [1,2]; recently it was investigated also in the ASDEX Upgrade experiment by using the internal active saddle coils (termed B-coils). n =1 resonant magnetic perturbations (RMPs) have been applied by the B-coils, in a RE specific scenario, before and during disruptions which are deliberately created via massive gas injection [3]. The application of RMPs seemingly changes the dynamics of the disruption and results in a significantly reduced current and lifetime of the generated RE beam. A similar effect is observed also in the hard-x-ray (HXR) spectrum, associated with runaway electron emission, characterized by a partial decrease of the high energy content when RMPs are applied. The strength of the observed effects strongly depends on the upper-to-lower B-coil phasing, i.e. on the poloidal spectrum of the applied RMPs, which has been reconstructed including the plasma response by the code MARS-F [4]. In particular, the runaway beam is reduced or almost suppressed when the radial field resonant with the pre-disruption edge q profile is maximized. A crude vacuum approximation fails in the interpretation of the experimental findings: despite the relatively low ? (< 0.5%) of these discharges, a modest amplification (factor of ~2) of the edge kink response occurs, which has to be considered to explain the observed suppression effects. These results do not only contribute to explain similar experiments performed in existing tokamaks, where RMPs have been applied by external coils but - combined with disruption prediction methods - might be relevant for mitigation techniques in future fusion devices.