Kink instabilities of the post-disruption runaway electron beam at low safety factor

Abstract

Realization of a high-current (approaching 1 MA) post-disruption runaway electron (RE) beam in DIII-D yields controlled access to very low edge safety factor (qa) conditions. This enables unique observation and study of low-order kink instabilities in post-disruption plasmas where the current is carried entirely by relativistic REs. The conventional external kink stability boundary (in terms of qa and internal inductance, ℓi) is found to accurately predict the operational space of the RE beam, with qa limited to ≈2. Kink instabilities appear with a characteristic growth rate of a few tens of microseconds (which is comparable to the Alfven time) and ultimately cause complete loss of the RE population on a similar time-scale. This characteristic RE loss time is significantly faster than observations away from the qa ≈ 2 stability limit and implies both higher peak heat loading but also less chance of destructive magnetic to kinetic energy conversion via RE beam regeneration. With large enough kink amplitude no RE beam regeneration is observed, indicating the magnetic to kinetic energy conversion was inhibited. Instability structure analysis reveals that early instabilities at high qa () are likely internal or resistive kinks (at higher poloidal mode number), while at qa ≈ 2 the most destructive instabilities are either internal or external kinks with low-order poloidal mode number (m = 2). The HXR loss magnitude is found to be proportional to the perturbed magnetic field and exhibits a helical spatial pattern. These observations are novel for present-day tokamaks yet will potentially be very common in high current tokamaks such as ITER, where predicted RE beam equilibrium evolutions cross the qa ≈ 2 stability boundary.