Modeling runaway electron losses due to tearing mode activities in HL-3

Losses of runaway electrons (REs) due to magnetohydrodynamic (MHD) perturbations in a recent HL-3 discharge are numerically modeled by a linear stability code combined with a relativistic particle guiding-center orbit tracing module. The MHD perturbation is identified as (predominantly) the n = 1 ( n is the toroidal mode number) tearing mode (TM) with large 3/1 and 2/1 Fourier components. The 2/1 islands are found significantly larger than the 3/1 counterpart. As the perturbation level associated with the TM progressively increases, the 3/1 island chains disappear and transition to the edge field stochasticity occurs at about 10 G field perturbation, according to the modeling. The tendency of RE losses, due to the three consecutive TM bursts observed in the experiment, is qualitatively captured by test particle tracing. Modeling finds about 0.7%, 3% and 25% RE loss fractions associated with these three bursts, assuming that the corresponding TM perturbation levels match that of the Mirnov data. High-energy REs are subject to less losses at low perturbation levels, due to the stronger inward shift of the particle orbits. Only full field line (or drift orbit) stochasticity facilitates losses of high-energy ( ∼ 10 MeV) REs.