Effect of input power on plasma expansion and ion acceleration in a radio-frequency plasma thruster

Abstract

Exploring the physics of low pressure plasmas expanding in a diverging magnetic nozzle, and the resulting acceleration mechanisms, plays an important role in the development of a new-type of electrode-less plasma propulsion systems. This study discusses the effects of input power on plasma expansion and ion beam acceleration in a magnetic nozzle electrode-less plasma thruster. The experiments were conducted in a radio-frequency magnetic nozzle plasma device at The University of Auckland with four different power configurations $P_{\mathrm{RF}}$. Different plasma diagnostics were used to measure the characteristics of the plasma plume. A planar Langmuir probe was used to measure the floating potential and ion saturation current both in the plasma source and in the expansion chamber. The potential drop in the plasma source was obtained with an emissive probe. A retarding field energy analyser was employed to evaluate the local plasma and ion beam potentials, the ion energy distribution functions, and to estimate the ion beam speed in the expansion region. Measurements showed that, as expected, increasing the power input resulted in a higher plasma and supersonic ion density, while the ion beam speed did not increase further for $P_{\mathrm{RF}}>100$  W. Interestingly, and contrary to the idealised physical model, the ion sonic transition did not occur at the magnetic nozzle throat, but instead close to the geometrical expansion point, i.e. near the interface between the source tube and the expansion chamber. This feature would result in a lower performance of the thruster given the reduced expansion ratio. An E-H mode change is also observed to occur in the device with increasing radio-frequency power that would help explain the different plasma characteristics observed at the 200 W transition point.