Extremely small-diameter, high-density, radio frequency, plasma sources and central gas feeding for next-generation electrodeless plasma thrusters

Radio frequency (RF) waves including helicon waves can readily produce high-electron-density (ne up to 1013 cm-3) plasmas with a broad range of external operating parameters. Various featured RF and helicon sources in a wide range of scales are suitable for plasma propulsion schemes. Electrodeless RF plasmas have no direct contact between electrodes and antennas, which enables long-life operation. However, one of the crucial problems is to reduce the plasma size for future applications in nano- and pico-satellites. Diagnostics of the plasma parameters in a small area should also be improved. Furthermore, to increase plasma performance, it is important to consider the radial electron density (ne) profile with an increasing upper limit, observed in high-density helicon sources due to the depletion of neutrals. This problem may be controlled by the location of neutral gas feeding and knowledge of the gas pressure distribution. Here, production of RF plasmas, with extremely small diameters from 3-mm down to 0.5-mm including 1-mm, was demonstrated, and characterization of ne and the electron temperature was performed with a collisional radiative model. Finally, to improve plasma performance such as ne and the thrust force, internal gas feeding was demonstrated using a developed Pirani gauge to measure neutral density.