In traditional gridded electrostatic ion thrusters, positively charged ions are generated from a plasma discharge of noble gas propellant and accelerated to provide thrust. A separate electron source, typically a neutralizer cathode that consumes propellant, is required in the propulsion system to neutralize the ion beam after it exits the thruster, thereby maintaining overall charge balance. However, if high-electronegativity propellant gases are used, a plasma discharge can result that consists of both positive and negative ions. Such an electronegative plasma thruster has the ability to generate thrust with a quasi-neutral ion-ion plume, thus allowing for the elimination of the neutralizer cathode subsystem, reduction of propulsion system complexity, and improvement of system lifetime and operational flexibility.
As with other electric propulsion systems, an electronegative plasma thruster has a thrust-to-power ratio capability directly proportional to its thruster efficiency. An important factor that affects thruster efficiency is the energy cost required to ionize the propellant. Demonstrating an ability to reduce the propellant ionization cost would directly aid in increasing the thruster efficiency and the corresponding thrust-to-power ratio capability.
This work focuses on reducing propellant ionization costs and improving thruster efficiency by enhancing RF power deposition into the discharge plasma. This is achieved through the implementation of two source region modifications. The first adds a ferrite toroid to a large-width, solid copper RF antenna. The addition of the ferrite toroid will reduce the required driving voltages necessary to achieve particular RF power levels as compared to an RF antenna where the ferrite is absent for the same corresponding RF power level. The second modification improves the delivery of neutral gas propellant into the source region. The propellant is mixed external to the thruster body, which allows for better control of the propellant gas mixture ratio and uniformity during thruster operations. Utilizing an external gas distributor, propellant flow is divided among six radial gas lines that feed the propellant gas mixture into the plasma source region directly in front of the ferrite-enhanced RF antenna. These two modifications work together to facilitate ionization by creating a high-density, uniform flow in the area of influence of the enhanced RF antenna, leading to reductions in ionization costs and improved thruster efficiency.