Measurement and identification of azimuthal current in an RF plasma thruster employing a time-varying magnetic field

We report two-dimensional temporally resolved measurements of the magnetic field and the azimuthal current in a radio-frequency plasma thruster with a time-varying magnetic field. The measured azimuthal current cannot be explained by the classical cross-field transport theory, which has been used in modeling of inductive electrodeless plasma thrusters; rather, it is fairly close to the estimated current using parallel electrical conductivity. In the analysis in the collisionless limit, the comparison of the measured azimuthal current and the diamagnetic current implies the non-negligible contribution of the azimuthal electron E × B drift current in the azimuthal current, in contrast with the case of conventional helicon plasma thrusters. This result indicates the establishment of the strong in-plane perpendicular electric field, as observed in the previous experiment [H. Sekine, H. Koizumi, and K. Komurasaki, Phys. Plasmas 27, 103513 (2020)]. Based on the experimental results, we build a hypothesis on the wall-charging-induced in-plane perpendicular electric field and the in-plane current-free condition. By analyzing the electron momentum equation under this condition, the electrical conductivity in the azimuthal direction agrees to the parallel conductivity, explaining the present result well.