Measuring Electron Dynamics in the Magnetic Nozzle of an ECR Thruster

The diagnosis of plasma parameters, such as electron density and temperature, is a key issue in understanding and controlling electric propulsion systems. To date, electron density is mostly measured with electrostatic probes such as Langmuir probes, which can be quite invasive, or with spectroscopy methods. In this frame, we assess the performances of a novel resonant probe for the characterization of an ECR plasma thruster. The curling probe is a microwave resonant probe firstly proposed in 2011 by Liang et al., which has several advantages with respect to other diagnostics: compactness, robustness, embeddable in a reactor/thruster wall, and the possibility to perform density measurements through a dielectric wall. We present here some recent improvement of the curling probe diagnostic that includes a calibration procedure and a correction method capable of accounting for the electron-depleted plasma sheath that forms at the probe surface. We propose here the first totally non-intrusive time-resolved electron density measurement inside an ECR thruster source, and a 2D electron density mapping of the ECR magnetic nozzle. Measurements are used to investigate plasma expansion in the magnetic nozzle.