RF power transfer efficiency and plasma parameters of low pressure high power ICPs

Inductively coupled radio frequency (RF) ion sources operating at 1 MHz under the condition of a low gas pressure of 0.3 Pa are the basis of negative hydrogen/deuterium ionbased neutral beam injection systems of future fusion devices. The applied high RF powers of up to 75 kW impose considerable strain on the RF system and so the RF power transfer efficiency eta becomes a crucial measure of the ion source's reliability. eta depends on external parameters such as geometry, RF frequency, power, gas pressure and hydrogen isotope. Hence, eta along with the plasma parameters are investigated experimentally at the ITER prototype RF ion source. At only 45%-65% in hydrogen and an increase of around 5% in deuterium, eta is found to be surprisingly low in this ion source. The power that is not coupled to the plasma is lost by Joule heating of the RF coil (similar to 26%) and due to eddy currents in the internal Faraday screen (similar to 74%). The matching transformer adds up to 8 kW of losses to the system. The low values of eta and the high share of the losses in the Faraday screen and the transformer strongly suggest optimization opportunities. At high power densities well above 5 W cm(-3), indications for neutral depletion as well as for the ponderomotive effect are found in the pressure and power trends of eta and the plasma parameters. The comprehensive data set may serve for comparison with other RF ion sources and more standard inductively coupled plasma setups as well as for validating models to optimize RF coupling.