Understanding and Controlling of Plasma-Wall Interaction in Hall Thrusters: Segmented Hall Thruster
Hall thruster performance is strongly affected by the electric field distribution in the thruster channel. In conventional Hall thrusters, the electric field distribution is strongly coupled to the magnetic field configuration. Decoupling these distributions might, in fact, permit improvements regarding thruster efficiency, beam divergence, thruster lifetime and thruster compatibility with satellites. This decoupling can be implemented by the use of emissive and non-emissive conductive electrodes and dielectric spacers placed in the ceramic channel of the Hall thruster.
Direct control of the electric field along the channel can be accomplished with emissive electrodes. By emitting electrons from an equipotential metal surface, the plasma-electrode sheath can be almost eliminated and therefore, the magnetic field lines form precise equipotential surfaces. Hence, biased segmented electrodes should be able to establish a very abrupt potential drop in the plasma along the channel. In the case of non emissive electrodes, control of the electron temperature and the placement of the acceleration region in the channel can be gained due to lower secondary electron emission (SEE) from metal electrodes compared to the ceramic channel walls made typically from a boron nitride (BN) ceramic material and by shortening of the plasma through the sheath to equipotential surface of a conductive electrode. The electron temperature can be also controlled by ceramic spacers with different SEE properties. A combination of different electrodes and ceramic spacers along the channel may allow, for example, a favorable distribution of the electron temperature to reduce energy spread of the exhausted ions.
We already demonstrated that segmented electrodes can significantly reduce voltage potential drop outside the channel in defocusing fringing magnetic field and establish a favorable shape of equipotentials to reduce the plume angle as compared to non-segmented thruster configuration. This study was performed on a 1 kW laboratory Hall thruster, which was operated in a sub-kilowatt input power range in the large vacuum facility. Plasma diagnostics included various probes for measurements inside the thruster channel and in the thruster plume. The effect of segmented electrodes on the plasma flow and their applications for high power thrusters is now studied on a 3 kW laboratory Hall thruster.