Effects of Electron Cyclotron Heating on Runaway Electrons and Runaway-Electron-Driven Modes in DIII-D plasmas*

In the quiescent-runaway electron (QRE) regime on DIII-D, a beam of runaway electrons (RE) is created and controlled within a background Ohmic plasma for several seconds, allowing complex wave-particle interactions. Electron-cyclotron heating (ECH) at 110 GHz was applied to QRE plasmas in on/off pulses of varying duration and power. The ECH resulted primarily in a decrease in the number of REs, as inferred by hard x-ray emission. The gamma ray-imaging diagnostic shows the ECH caused a suppression of low-energy runaways (~5-10MeV) and non-trivial fluctuations in high-energy REs (>10MeV), in line with scintillators suggesting a decrease in the overall RE population. Fast magnetic loops also show a decrease in the excitation of RE-driven whistler waves while the characteristic synchrotron-emission 'crescent' from the RE beam all but vanishes after a few 100ms of ECH. Periodic decreases in the loop voltage caused by the ECH heating the background plasma partially describe these effects, but not completely. A Frontier Science Experiment will be conducted to investigate the exact mechanism behind the ECH-induced RE losses, using a range of turbulence/fluctuation diagnostics. The effect of keeping the loop voltage constant, of using current-drive ECH or just regular heating ECH, and whether any density limits and power thresholds exist for this effect, will be investigated.