3rd-Harmonic operation of SIT inverter RF source for ICRF heating in the divertor plasma simulator NAGDIS-II

Summary form only given, as follows. In a linear divertor plasma simulator NAGDIS-II the ICRF slow wave heating is employed to heat both the ion and electron. The rf power source used here is a Static Induction Transistor(SIT) inverter, which can generate about 15 kW in CW mode at a gate driving frequency range of 0.25-1.7 MHz. An output current at the fundamental frequency is usually used for induction heating in the rf inverter power supply. In the present experiment the 3rd harmonic current is used for plasma heating in order to extend the operational frequency range of the rf inverter. In the present experiment characteristic performance of the SIT inverter is investigated in the cases of fundamental and 3rd harmonic operations. We show the frequency dependance of the DC to RF conversion efficiency of the SIT inverter. The conversion efficiency is about 95% when the resonance frequency is tuned at the fundamental wave. When that frequency is tuned at 3rd harmonic wave, the conversion efficiency decreases linearly with increasing the frequency. At the frequency below 3 MHz, the conversion efficiency is more than 50%, which is equivalent or a little higher than that of conventional linear amplifiers. The maximum output power is about 5 kW at 3 MHz. The use of the 3rd harmonic wave beyond the upper frequency limit of the SIT gate driving frequency can extend the operational frequency range of SIT rf inverter in the practical rf power level of several kW without any improvement of the gate driver circuit. We have a plan to use four SIT inverter power supplies to heat hydrogen plasmas in NAGDIS-II. The total power is expected to be about 20 kW at a frequency below 3 MHz. Initial experimental results of the hydrogen plasma heating will be shown.