Non-intrusive measurements of plasma impedance in an electron-cyclotron resonance thruster

We present a general method allowing for direct measurements of the complex impedance of radio frequency devices that need to be powered by an external power supply unit, making use of a dual-directional coupler and a vector network analyzer. This method is used to measure the impedance of an operating electron-cyclotron resonance thruster, which is then calibrated using a custom triple-short calibration kit to retrieve the impedance of the plasma within the chamber. The measurements, realized without disturbing the thruster operation, show that the plasma impedance ranges between 55 and 90 Ω in resistance and between −30 and 20 Ω in reactance. An equivalent circuit model is finally discussed, aiming at estimating plasma parameters through these impedance measurements.