MHD instabilities in benign termination of high-current runaway electron beams in the JET and DIII-D tokamaks

A promising solution to mitigate runaway electron (RE) beams is the low-Z benign termination scheme, in which an MHD instability terminates the RE beam benignly. Dedicated JET tokamak experiments reached higher pre-disruptive plasma and RE currents than other machines and highlighted challenges in achieving terminations of high pre-disruptive currents of I_p > 2.5 MA without producing significant heat loads to the plasma-facing components. The analysis presented in this work focuses on the nature of the terminating MHD events to understand the instability dynamics that distinguish benign from non-benign terminations. This is addressed through a systematic analysis of magnetic sensor data from about 50 RE discharges conducted between 2019–2023. It is found that unsuccessful termination in JET happens at low edge safety factors q_a = 2 with rather weak MHD events, after undergoing MHD events at higher rational q_a values that were not able to deconfine the RE beam sufficiently. Our analysis suggests more peaked RE current densities for the high-I_p cases, which could make them more MHD stable, ultimately terminating non-benignly by preventing the instability from growing large enough to safely dissipate the RE beam. These observations are compared to similar analysis results from DIII-D. Understanding the unique behavior of the variety of observed MHD effects is required for extrapolating the benign termination scenario to future devices.