Fully kinetic simulations of a magnetic nozzle radiofrequency plasma thruster using an open axial boundary condition

Fully kinetic simulations of a magnetic nozzle radiofrequency (rf) plasma thruster are conducted to investigate the plasma expansion in the magnetic nozzle. The magnetic nozzle accelerates the plasma through a divergent magnetic field and imparts the net momentum to the plasma. Our previous simulations assumed the Dirichlet boundary condition at a plasma exit surface, resulting in the sheath generation near the boundary. The simulated plasma expansion toward the sheath successfully validates the experimental results, because the experiment also involves the sheath near the chamber wall. However, the plasma expansion toward the sheath is not suitable for evaluating the space operation, in which the plasma does not involve the sheath and expands infinitely. In addition, it was reported that the plasma dynamics such as a polytropic index depended on the sheath existence. In this study, a Neumann boundary condition and low-energy electron reflection are applied to eliminate the sheath in the magnetic nozzle acceleration region as an open boundary. The simulated plasmas show different polytropic indexes depending on the boundary conditions.