Development of a Radio-Frequency Generator for RF Ion Thrusters

Abstract

To ensure the successful operation of a thruster using inductively-coupled plasmas (e.g. radio-frequency ion thrusters (RIT)), a high efficiency and high performance radio-frequency (RF) power supply is crucial. For this purpose, the supply needs to guarantee highly efficient RF signal generation and transfer to the thruster coil and furthermore an optimal power coupling between coil and plasma. In this paper we propose a high efficiency, high performance approach of generating RF power by using a resonant converter design. Due to a compensation of the load's inductance, resonant switching behavior becomes possible and thus, switching losses can be significantly reduced. The drive signals of the power semiconductors are generated by a switching frequency-adapting, load-controlled algorithm, which keeps up a quasi-resonant state. Due to high-speed tracking of the resonance frequency and phase, RF generation can be adjusted to altered load conditions within a few RF-cycles. A digital FPGA-based implementation guarantees precise period determination and control signal generation, since the programmed control-algorithm is executed in hardware. The proper functioning of the developed radio-frequency generator (RFG) concept is verified through performance mappings, recorded when supplying a RIM-4 RF ion thruster developed at the University of Giessen. In the long run, this RFG concept shall be employed for driving RIT more efficiently.