The role of kinetic instabilities in formation of the runaway electron current after
argon injection in DIII-D
Lvovskiy, A.; Paz-Soldan, C.; Eidietis, N. W.; Dal Molin, A.; Du, X. D.; Giacomelli, L.; Herfindal, J. L.; Hollmann, E. M.; Martinelli, L.; Moyer, R. A.; Nocente, M.; Rigamonti, D.; Shiraki, D.; Tardocchi, M.; Thome, K. E.
Kinetic instabilities in the MHz range driven by runaway electrons (REs) have been
observed for the first time during the current quench (CQ) in disruptions triggered
by massive injection of argon in DIII-D. These instabilities are well-correlated with
intermittent RE losses in the beginning of RE current formation. The runaway current
phase is not observed when the power of instabilities exceeds a threshold. Novel measurements
of the RE distribution function during the CQ indicate that the instabilities appear
when RE energy (E-RE ) exceeds 2.5-3 MeV, the number of modes grows linearly with
E-RE , and their frequencies lie in the range 0.1-3 MHz, below the ion cyclotron frequency.
Possible plasma waves exciting by REs in this region are proposed. Increase of the
amount of injected argon decreases the E-RE and increases the success rate of the
runaway current formation, while increase of the pre-disruption plasma current acts
in the opposite direction. No dependence on the pre-disruption core electron temperature