Characterizing bursty magnetic fluctuations in long-lived post-disruption runaway electron beams on DIII-D using internal measurements

Internal measurements in post-disruption runaway electron (RE) beams on DIII-D reveal a low-frequency band of density and magnetic fluctuations (f < 20 kHz) as well as bursts of MHD activity. The bursty MHD, with toroidal mode number n = 1, occurs when the edge safety factor, q(a), is between 2.2 – 9, leading to minor loss of REs. Motivated by the need to understand the stability of such RE beams with the goal of devising mitigation techniques for future devices such as ITER, we use internal measurements from the Radial Interferometer Polarimeter (RIP) to measure directly both the equilibrium and fluctuating density and magnetic field. The origins of the bursts and the fluctuation band are yet unknown, though such bursts have been identified as resistive/internal kink modes in previous experiments by others on DIII-D and JET. We find no obvious correlation between the size of a burst, as measured by external hard-x-ray detectors, and q(a). Synchrotron emission during these bursts shows a spatial structure to the loss of REs. In RE-dominated equilibria where measurement of internal magnetic field is not otherwise possible, EFIT equilibrium reconstructions using the RIP-measured magnetic field as a constraint are also carried out. Interestingly, RIP also measures a broader band of density fluctuations, not seen by the CO2 interferometer, in non-disruptive plasmas where REs are formed at plasma startup.