Numerical investigations of plasma distributions and their time variation in the expanding magnetic field

Magnetic nozzle RF plasma thrusters are candidates for high-power and long-lifetime thrusters because no electrodes are exposed to the plasma for its generation or acceleration. The thrust is produced by the azimuthal electron diamagnetic current and the radial component of the magnetic field. The axial momentum gain of electrons becomes dominant with increasing magnetic field strength, leading to the net axial momentum by the Lorentz force in the magnetic nozzle. However, the plasma should be detached from the applied magnetic field to obtain the net thrust. This mechanism is still unclear, and further studies are required. In this study, the plasma distributions and their time variation in the expanding magnetic field are numerically investigated to elucidate the detachment mechanism using a three-dimensional particle-in-cell simulation with the Monte Carlo collisions (PIC-MCC) method. A uniform plasma distribution was applied to the inlet, and the plasma distribution in the expanding magnetic field was calculated using open boundary conditions. The PIC-MCC results indicated that the plasma was expanded along the magnetic field lines, and the distribution seemed to rotate in the azimuthal direction. This plasma structure might cause the cross-field transportation of electrons and resultant detachment from the expanding magnetic field.