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A (Re-) examination of electron motion in Hall thruster fields

This paper reports on an analytic and numerical study of electron motion in E- and B-field configurations representative of Hall thruster discharge chambers. Electron dynamics are modeled using collisionless guiding center theory to predict drift velocities and resulting particle trajectories. A simplified field consisting of a magnetic monopole with a constant axial E-field is used to identify fundamentals of electron motion in Hall thrusters. The monopole study uncovers an unexpected collisionless cross-field mobility that is inherent to radial magnetic fields, with a velocity comparable to the azimuthal ExB drift. Data obtained using an in-situ probe technique within the discharge chamber of the NASA 173Mv1 are used to represent conventional Hall thruster topologies. Collisionless guiding center trajectories are computed for the 173Mv1 field data. These models show the importance of the magnetic mirror force and the radial component of the electric field in closing the azimuthal electron drift. Magnetic field design criteria are proposed that may outline a method for improving thruster efficiency by reducing ion wall losses and beam divergence.