Characterization of a 2 MHz magnetically expanding RF plasma source for thruster development*

A 2 MHz band RF plasma source with a 14 cm diameter is developed to improve the performance of the helicon plasma thruster (HPT). The previous study has demonstrated that the performance of the helicon plasma thruster can be improved in terms of the thruster efficiency, by enlarging the source tube diameter. Here the source tube diameter is enlarged up to 14 cm, while the 9.5 cm diameter tube has been used previously. When a high-density plasma is produced inside the source, it is difficult for RF field to penetrate into the central plasma due to a skin effect. Therefore, a 2 MHz RF generator is chosen as the power supply. In the present study, the plasma density, the RF power transfer efficiency, and the ion energy distribution function, are measured. The results demonstrate that the high-density plasma above 10¹² cm^-3 is generated, and that the electrons are heated in the core, implying the penetration of the RF field into the plasma core. In the presently developed source, the RF power transfer efficiency above 90% is successfully obtained. Downstream of the open source exit, the supersonic ion beam is observed, implying that the RF frequency does not impact the ion acceleration process.