Preionization and heating of plasma at the electron cyclotron frequency in JIPP T-II stellarator

Experimental studies of electron cyclotron preionization and heating are carried out in JIPP T-II torus by injecting a power of 36 kW at the frequency of 35.5 GHz. Preionization effectively decreases the loop voltage required to start a joule heating and eliminates strong spikes in the signals of electron density, light emission as well as hard X-ray which are due to runaway electrons at the initial breakdown phase of joule heating. Experiments on electron cyclotron heating of stellarator plasma in ordinary modes show a heating efficiency of 1.6 eV/kW, and the absorption rate of microwave power is estimated to be around 50% from power balance. (author)