Parametric study of Alfvénic instabilities driven by runaway electrons during the current quench in DIII-D

Abstract

To avoid or mitigate runaway electron (RE) beams in ITER, RE-driven instabilities are actively studied as a complimentary technique to massive material injection. In this work we report experimental dependencies of Alfvénic instabilities driven by REs during the current quench in DIII-D on plasma and material injection parameters. These instabilities, observed in the frequency range of 0.1–3 MHz, correlate with increased RE loss and thus may play a role in non-sustained RE beams. It was found that as the toroidal magnetic field (BT) decreases, the RE population becomes more energetic, the energy of instabilities increases, and no RE beam is observed when the maximum energy of REs exceeds 15 MeV (or when BT is below 1.8 T). Analysis of disruptions at plasma core temperature (Te) of 1 keV and 8 keV shows that the RE population is much less energetic (with the maximum energy of only about 3 MeV) when Te is high, and no instabilities are observed in this case. Besides disruptions above caused by Ar injection, cases with Ne and D2 injections were also studied. Both Ne and D2 injections cause no sustained RE beams, however, by different reasons. Measurements of the instability polarization indicates that it is of predominantly compressional nature at the edge which is consistent with modeling suggesting excitation of compressional Alfvén eigenmodes. However, drive of global Alfvén eigenmodes is also possible at low frequencies.

Resonant interaction between runaway electrons and the toroidal magnetic field ripple in TCV

“…Only higher-order modes can be accessed, but the corresponding ripple strength and, thus, scattering efficiency will be limited. Nevertheless, other RE-driven [34][35][36] or deliberately induced [21,[37][38][39][40] highfrequency instabilities could be invoked to manipulate RE energies through pitch angle scattering induced synchrotron losses, in order to minimise damage to the plasma-facing wall and underlying components [41]. The present work provides direct experimental evidence of such RE momentum-space engineering.…”

Characteristics of the Alfvénic activity during the current quench in ASDEX Upgrade

“…In the contribution by Lvovskiy et al [24,25], the duration of MHD activity and the loss of the initial RE seed are connected, thus the consequent runaway current I RE is affected. To analyse this connection for ASDEX Upgrade, linear regressions are created.…”

“…While compressional Alfvén eigenmodes (CAEs) are used as a possible explanation for the observations at DIII-D [24,25,39,40] the modes observed on ASDEX Upgrade appear in different frequency ranges. We therefore propose a different potential driving mechanism for the post-disruption modes, which are identified as global Alfvén eigenmodes (GAEs).…”

“…Plasma waves might contribute to the avoidance of the formation of strong RE beams, as the level of magnetic perturbations [21] and Alfvénic activity in post-disruption plasmas [22,23] can have a strong influence on a subsequent runaway current formation. In the contributions by Lvovskiy et al [24,25], they present instabilities observed during disruptions in the tokamak DIII-D, which could deconfine the initial seed electrons, hence suppressing the formation of a strong RE beam.…”