Internal measurements of magnetic fluctuations in long-lived post-disruption runaway electron beams on DIII-D

Post-disruption runaway electron (RE) beams are generated on DIII-D to understand their underlying physics and to devise mitigation techniques for future devices such as ITER. Such beams on DIII-D are generally stable due to a high edge safety factor, q(a) > 10, with essentially no MHD activity detected by the edge sensing coils. Internal measurements with Faraday-effect polarimetry using the Radial Interferometer Polarimeter (RIP) reveal a steady, continuous band of fluctuations (f < 20 kHz). To help confirm the magnetic origin of these fluctuations, q(a) ~ 5 - 7 plasmas were produced in recent experiments where bursts of MHD were detected by both the edge sensing coils and polarimetry. These bursts are associated with RE loss, and synchrotron emission from the RE-beam appears to show a two-lobed poloidal structure that may be related to an m = 2 mode. We now seek to understand the origin of the fluctuation band and to accentuate it and the bursts for enhanced RE loss. This will fold in an effort already underway to reconstruct the RE beam equilibrium based on RIP magnetic-field constraints in EFIT. Measurement of the internal magnetic field is not otherwise possible in RE beams.