Plasma Characterization for RF Ion Thrusters via Correlated Impedance*

Invasive, physical probes used to make measurements of the discharge chamber plasma parameters can influence the local plasma environment leading to un-reliable measurements. When the discharge is an inductively coupled plasma, the addition of rf noise requires that rf compensation be applied to those probes and an additional source of measurement uncertainty. Maximizing gridded ion thruster life and performance requires reliable data of the discharge plasma environment. Correlations of discharge plasma parameters with impedance measurements using optical emission spectroscopy (OES) techniques during ground-testing provides a method for monitoring and control of the grid potentials to optimize performance of an rf-sourced gridded ion thruster. By measuring the forward and reflected voltage waveforms and measuring in detail the no-plasma condition impedances, we were able to correlate the plasma parameters estimated through the OES with impedance components calculated with the voltage waveforms. Initial analysis has identified trends in electron temperature, a key variable in determining the performance of the ion optics of a gridded ion thruster, with the calculated plasma impedance. The research demonstrates the potential of using rf impedance parameters to monitor and optimize the performance of rf-driven ion thrusters. This approach enables better in-situ diagnostics and adjustments, crucial for long-term space missions, lays the groundwork for advanced EP systems that are more efficient and reliable. Additionally, this approach can support a propellant agnostic plasma source for fusion applications.