Observation of High Frequency Emission from the MST Tokamak Plasma in the Presence of Runaway Electrons*

Local magnetic fluctuations of high-frequency emissions ranging from 37 MHz up to 5.5 GHz have been measured in a runaway-dominated MST tokamak plasma with a magnetic probe inserted into the plasma, up to <!--[if gte msEquation 12]> m:val="lin"/>r style='mso-bidi-font-style:normal'>a style='mso-bidi-font-style:normal'>~0.8 <!--[if gte msEquation 12]> m:val="lin"/>r style='mso-bidi-font-style:normal'>a style='mso-bidi-font-style:normal'>~0.8 r/a = 0.8, and digitized on a high-speed oscilloscope (up to 12.5 GHz). The emissions are observed in several frequencies that correspond to low and high Whistler bands; frequencies which would otherwise be hidden to probes outside the plasma. The magnetic emission and radial x-ray emissions bursts are correlated, consistent with pitch-angle scattering of energetic electrons. Hard x-ray measurement, using CdZnTe detector show high energy photons up to about 1 MeV. Runaway electrons are generated throughout the discharges at plasma density less than 0.03x10¹⁹ m³, toroidal magnetic about 0.14 T, and plasma current in the range of 50-60 kA. Whistler mode have been observed with toroidal magnetic field as low as 900 gauss. The emission frequencies scale with the magnitude of the toroidal field. The parallel and perpendiculars wave number have been measured as a function of frequency. The polarization of different frequency modes have also been measured. Modes at frequency 37 MHz and the 3.2 GHz are elliptically polarized. The high frequency branch of the Whistler is a backward propagating wave, consistent with anomalous Doppler.