Cathode effects on operation and plasma plume of the permanent magnet cylindrical hall thruster

The use of permanent magnets instead of electromagnet coils can be advantageous for low power Hall thrusters. Previous measurements revealed that the miniaturized cylindrical Hall thruster (CHT) with permanent magnets and electromagnet coils operate rather differently.¹ In particular, the plasma flow from the permanent magnet CHT (CHTpm) has a halo shape with a majority of high energy ions flowing at large angles with respect to the thruster centerline. It was suggested that a strong axial magnetic field outside the permanent magnet CHT causes this unusual shape of the plasma plume. The use of a magnetic shield was shown to restore a conic shape of the plasma flow and to significantly narrow the plume.² New result reported in this paper is that when the magnetic field magnitude outside the permanent magnet thruster is sufficiently reduced by the magnetic shield, 1) the cathode placement with respect to the magnetic separatrix and 2) the auxiliary cathode-keeper discharge can affect to some degree the plasma plume of the CHTpm. With the cathode placement at the magnetic separatrix, an additional plume narrowing can be achieved by running the cathode keeper discharge without a significant degradation of the current utilization efficiency. This cathode effect on the plume angle is however not as strong as the effect of the magnetic shield.