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 (q a ) 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 q a and internal inductance, ℓ i ) is found to accurately predict the operational space of the RE beam, with q a 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 q a ≈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 q a ( 4 ⪆ ) are likely internal or resistive kinks (at higher poloidal mode number), while at q a ≈ 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 q a ≈ 2 stability boundary.

Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability

“…Same result has been shown via estimations of the internal inductance l i (3) based on the plasma equilibrium reconstruction [65]. External kink instabilities leading to large and eventually complete RE loss have been recently studied for a high-current (1 MA) RE beam in DIII-D [66].…”

“…On the contrary, MHD unstable RE current profiles are likely best avoided since large-scale MHD instabilities can lead to minor and eventually major disruption of the RE beam as discussed in Ref. [66].…”

Recent DIII-D advances in runaway electron measurement and model validation

“…Recent realization of a nearly MA-level RE plateau on DIII-D enables thorough diagnosis of the phenomenology encountered when approaching the tokamak external kink limit at edge safety factor (q a ) of 2 [78]. While difficult to achieve in present experiments, low q a operation is very likely for ITER RE beams owing to the larger RE currents expected in that device [79].…”

Three-Dimensional Nonlinear Modeling of MHD Instabilities for Low-q Plasma on J-TEXT

“…For high current operation, there is a hard limit imposed by ideal kink modes which are unstable for q edge < 2. In the experiment, the disruption behavior associated with kink or tearing mode instabilities could be observed in the low-q plasma when q edge approaching the value of two [2,3]. Recently researches which focus on disruption avoidance show one of the active feedback control techniques that using small non-axisymmetric magnetic field could suppress these unstable MHD instabilities [4].…”