The effects of radio-frequency wave heating on quiescent runaway-electron plasmas in DIII-D*

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 between the REs and waves in the background plasma to be studied. Electron-cyclotron heating (ECH) at 110 GHz was applied to such a plasma in 100ms pulses of varying power, resulting 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. The effect of externally-applied helicon waves on QRE plasmas was also investigated. The launched helicon waves were not found to have any obvious or significant effect on the runaway electrons, though further analysis is underway.