Experimental studies of electron cyclotron pre-ionization and heating have been carried out in the JIPP T-II torus by injecting a power of 36 kW at the frequency of 35.5 GHz. Pre-ionization effectively decreases the loop voltage at the initial stage and eliminates strong spikes in the signals of electron density and of light and hard-X-ray emission which is due to runaway electrons in the initial breakdown phase of the Joule heating. From the measurement of the central electron temperature only, it is seen that electron cyclotron heating of a stellarator plasma with ordinary-mode radiation shows a heating efficiency of 1.6 eV·kW−1 and, from power balance considerations, the absorption rate of microwave power is estimated to be around 50%.
On the role of electron energy distribution function in double frequency heating of electron cyclotron resonance ion source plasmas