INVESTIGATION OF LOSS MECHANISMS IN A PERMANENT MAGNET ELECTRODELESS RADIO FREQUENCY PLASMA THRUSTER

Abstract

Radio frequency (RF) electrodeless thrusters are high-efficiency and flexible propellantplasma generation devices with potential for both station-keeping and deep-space missions. In this dissertation, a low-power RF electrodeless thruster was designed and tested in a vacuum chamber using a flush-mounted wall probe, emissive probe, and retarding potential analyzer (RPA). The design utilizes permanent ring magnets and an upstream axial magnet. An optional downstream ring magnet (“throat magnet”) was introduced to form an electromagnetic funnel intended to improve ion confinement and reduce wall interactions. The wall probe and emissive probe measurements provide the first quantitative mapping of floating potential in this configuration and demonstrate that magnetic topology directly modifies the near-wall potential structure in the capacitive regime of low power RF discharges. Wall probe data revealed localized negative potentials on the order of 10-40 V near regions where modeled magnetic-field lines intersect the chamber wall, indicating magnetized-electron losses. Emissive probe measurements taken several centimeters off-axis showed higher saturated floating potentials when the throat magnet is installed, reflecting enhanced electron confinement and stronger local sheath potentials. RPA measurements detected decreases in measured current and energies moving downstream through the device in all throat and no-throat magnet configurations. The combined data shows that there is systematic offset between ion energies inferred from the RPA and plasma potentials measured by the emissive probe in overlapping spatial regions. This bias is attributed to RF oscillations being introduced from the radiating antenna into the plasma. This discrepancy is attributed to RF-induced oscillations of the plasma potential: while the emissive probe measures a time-averaged potential, ions are created during higher-potential phases of the RF cycle and retain this energy as they propagate downstream. As a result, the RPA detects ion energies that exceed those expected from the time-averaged plasma potential. The findings clarify the mechanisms that are produced in beam formation and transport in this class of permanent-magnet electrodeless thrusters as well as providing guidance for future designs aimed at improving confinement, reducing wall losses, and enabling effective ion acceleration.