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

Abstract

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 and recently was investigated also in the ASDEX Upgrade experiment by using the internal active saddle coils (termed B-coils). Resonant magnetic perturbations (RMPs) with dominant toroidal mode number n = 1 have been applied by the B-coils, in a RE specific scenario, before and during disruptions, which are deliberately created via massive gas injection. The application of RMPs affects the electron temperature profile and seemingly changes the dynamics of the disruption; this 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 to runaway electron emission, characterized by a partial decrease of the energy content below 1MeV 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. A crude vacuum approximation fails in the interpretation of the experimental findings: despite the relatively low β (< 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.PACS numbers:

Runaway electron experiments at COMPASS in support of the EUROfusion ITER physics research

“…7 reveals that under these conditions a strong kink mode amplication due to plasma screening appears, as it is indicated by amplication of components at |m| higher than those of rational surfaces (shown by red symbols). This is also in agreement with observations in [54]. Next, experiments scanning the amplitude of the RMP were performed.…”

“…The role of the phase was determined in a separate scan of the four possible ∆Φ angles. It has been observed in the experiment that the most pronounced eect of RMP on CDR, similarly to [54], is associated with the strongest coupling of screened RMP to the equilibrium rational surfaces. Furthermore, comparison of vacuum perturbation spectrum to the one including plasma response in Fig.…”

High resolution gamma-ray spectrometer with MHz capabilities for runaway electron studies at ASDEX Upgrade

“…Besides MGI experiments, HXR measurements have been performed also in experiments on the use of RMPs as a mitigation technique. 4,19 As for the MGI case, we have observed changes in the maximum RE energy, particularly when the RMP had the largest effect on the RE beam. An additional difficulty in these types of experiments was due to the pronounced counting rate which, in some cases, approached 10 MHz, at the limit of the gain stability of the instrument.…”

“…The gamma-ray spectrometer developed for ASDEX Upgrade has been used to measure the HXR spectrum from REs in experiments on mitigation both by MGI 3 and by the application of RMPs. 4 Here we do not present a complete summary of these experiments, but we rather limit ourselves to show an example of results for discharge #34084 on RE mitigation by MGI with single argon injection. Figure 3 shows an overview of the plasma parameters in the discharge.…”

“…1,2 In order to limit the risks for ITER operations, different mitigation techniques have been developed and experiments are presently ongoing to study their impact on the REs. Since 2015, an experimental scenario to generate REs in a reproducible way has also been developed at the ASDEX Upgrade tokamak and studies of RE mitigation by means of massive gas injection (MGI) 3 or resonant magnetic perturbations (RMP) 4 are being conducted. An important goal of these experiments is to validate theoretical models with data from existing machines in order to increase the confidence on their predictions for ITER.…”