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

Recent studies have proven the viability of electrodeless, micro-wave (µW) powered plasma thrusters as an alternative to grid and Hall thrusters. Among these, electron-cyclotron resonance thrusters (ECRT) combine a good efficiency and a quite simple design, making them good candidates for the next generation of electric thrusters. A key point in improving the total efficiency of these devices is to maximize the coupling efficiency of µW by designing an impedance matching circuit. Using ECRA, the ECRT developed at ONERA, we propose to estimate both the transmission parameters of the thruster and the plasma impedance. To do so, we have designed a non-intrusive method to measure the µW reflection coefficient using a vector network analyzer and created a calibration kit specifically suited for use with ECRA that allows us to determine its scattering matrix. Thanks to these, we have measured the plasma impedance for a variety of thruster set points, and compared these measurements to classical wave propagation models. These results open the way to a better understanding of plasma behavior in ECRTs and, with suitable plasma models, may allow one to estimate plasma parameters through non-intrusive measurements.