Studies of a TE/sub 04/ mode, wideband, triplet gyrotron amplifier with harmonic multiplication

Abstract

In a linear divertor plasma simulator NAGDIS-I1 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(S1T) inverter [l], 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.Figure1 shows 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 3MHz, 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 3MHz. 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 20kW at a frequency below 3MHz. Initial experimental results of the hydrogen plasma heating will be shown.A new triplet gyrotron amplifier, which we call Gyrotriotron, has recently been developed. The device utilizes a 1-2-2 harmonicmultiplying scheme and operates with a short Ku-band gyro-TWT as input stage, a longer Ka-band gyro-TWT as output stage and a multiunit stagger-tuned cluster-cavity as intermediate stage.Electronic coupling of adjacent stages is realized through beam ballistic bunching in the radiation-free drift spaces between the stages. A mode converter/filter type of interaction circuit is employed for each stage to minimize spurious mode competitions. Theoretical studies show that in addition to the features of lower magnetic field requirement and lower input frequency, high gain, high efficiency and wide bandwidth are expected for this three stage harmonic multiplying gyro-amplifier. A proof-of-principle Gyrotriotron tube with TE04 mode output was designed, fabricated and tested. This presentation provides the device development and an overview of some gyrotriotron-related special techniques that are detailed in the other two poster papers.[l], [2] References: * This WO& is suppoRed by the DoD MURl for innovative vacuum electronics under the grant AFOSR-F49620-9%1-0297.