Influence of Field Topology on Magnetically Shielded Hall Thruster Plume Divergence

The use of magnetically shielded geometries has been demonstrated to drastically reduce the rate of erosion on Hall thruster channel walls. However, the switch to a magnet-ically shielded configurations can increase plume divergence contributing to a decrease in efficiency and thrust. To investigate methods to mitigate the increase in plume divergence, a magnet-ically shielded hall thruster with an adjustable field shape has been built. The thruster, with a nominal power range of 150 to 500 W, has been tested at a single operating point of 250 V, 1 A, across a range of field positions. The thruster operation at 250 W is in line with other thrusters of similar power producing 16 mN of thrust at 1450 s of specific impulse with an anode efficiency of 45% and total efficiency of 29%. The plume divergence of the thruster has been measured across a range of field positions with a swept Faraday probe. Varying the central pole of the thruster, and with it the average angle of the magnetic field in the acceleration region, has been shown to drastically change the plume divergence. The divergence observed varies from wide to over-focused with a 2 mm change in pole position increasing the full-width-half-maximum by 180%.