Effects of magnetic nozzle strength and orientation on radio-frequency plasma expansion

Abstract

To improve the efficiency of radio-frequency magnetic nozzle plasma thrusters, it is important to better understand the coupling between plasma expansion and a convergent-divergent magnetic field. This study explores the effects of magnetic field strength and orientation on plasma expansion in a magnetic nozzle.
Two-dimensional measurements of the plasma characteristics obtained both in the source and in the expansion region are presented to investigate the influence of magnetic field strength on the formation of high-density conics in a symmetric magnetic nozzle. The measurements are repeated in a deflected magnetic nozzle using a novel magnetic steering system. Measurements of the ion saturation current and floating potential profiles are used respectively to qualitatively assess the plasma density distribution and the presence of high-energy electrons for the magnetic field configurations analysed. In the symmetric magnetic nozzle configuration, it is observed that the ion saturation current peaks on axis in the plasma source, but downstream of the nozzle throat, a double-peaked hollow profile is observed for all cases studied. The location of the high-density conics structure matches the most radial field lines that intersect the antenna and can freely expand downstream outside the source. Negative values of the floating potential are measured in the same peripheral regions, which could be a sign of the presence of high-energy electrons. When the magnetic field is deflected, the ion saturation current profile shows only a single peak centred around the bent field line that reconnects to the antenna. Again, a region of negative floating potential is measured at the location of the maximum ion current. Thus, it is shown how, independent of magnetic field strength and orientation, the magnetic field lines interacting with the antenna dictate the local plasma profiles downstream from the magnetic nozzle.