During helium-fuelled (He) discharges on the J-TEXT tokamak, high-frequency kinetic instabilities driven by runaway electrons have been observed during the plasma current (Ip) ramp-up phase for the first time using the ion cyclotron emission diagnostic system. Two distinct instability branches appear sequentially, superimposed on an intermittently occurring broadband spectrum associated with fast pitch angle scattering. One branch exhibits a frequency spacing of 3.5 MHz, whereas the other shows a spacing of 9 MHz, and both branches contain multiple harmonics. In this series of He discharges with Ip = 80–140 kA, statistical analysis indicates that these instabilities are preferentially excited at low toroidal magnetic field. When the instabilities occur, the production of fast electrons and runaway electrons, together with their radial losses, is observed. Analysis of the hard x-ray energy spectrum suggests that the particle population responsible for driving these high-frequency instabilities is most likely runaway electrons. The generation of runaway electrons is correlated with an elevated loop voltage. In discharges where high-frequency instabilities are successfully excited, the Ip ramp rate is significantly reduced.
The effects of kinetic instabilities on the electron cyclotron emission from runaway electrons