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

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... etic 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. § Research Assistant, PPPL, jgayoso@pppl.gov * Principal Research Physicist, PPPL, yraitses@pppl.gov ** Professor, Princeton University and PPPL, fisch@princeton.edu F a) b) Figure 3. Angular ion current density distribution measured for different CHT thrusters: a) the effect of the magnetic shield, cathode placement (P1 and P2 positions) and the cathode-keeper discharge (current overrun, (CO)) for the permanent magnet CHT thrusters; b) a comparison of the plume for CHTpm and CHT with electromagnet coils (CHTem) of the magnetic mirror-type (direct) configuration. All thrusters were operated at the discharge voltage of 250 V and Xenon gas flow rate of 3.5 sccm. The cathode placements P1 and P2 are shown in Figs. 1 and 2. For the CHTem thrusters, the cathode placement was different: the cathode outlet orifice was located approximately 5.4 cm away radially and 2 cm downstream from the center of the channel exit. 14