Influences on Langmuir Probe Measurements by an ECR Thruster with Magnetic Nozzle

Ensuring reliability in the measurement and interpretation of plasma diagnostics is essential for characterizing thruster concepts in the growing field of electric propulsion. Langmuir probes are often used to determine the properties of low temperature plasmas. However, not much research is found on Langmuir probe measurements on electron cyclotron resonance thrusters with magnetic nozzle, especially no detailed studies on variable probe orientations with respect to magnetic field lines. The question arises which differences can be shown in the electron energy distribution regarding the probes orientation and in what way it is possible to interpret probe alignment consequences on determined plasma characteristics. We perform our study as follows: We measure in two orientations, parallel and orthogonal to magnetic field lines, at different operating states of the thruster (variable power and volume flow settings). The Langmuir probe data shows clearly a dependency on the probes orientation. We use the measured ion energy distribution of the retarding potential analyzer as a comparison tool to the determined electron energy distribution of the single Langmuir probe measurements. Due to the determined anisotropic non-Maxwellian distribution mainly visible in parallel orientation we conclude a parallel orientation of Langmuir probes with respect to magnetic field lines is preferable for ECRT with MN.