ExB Probe: Determines ion’s charge state

The ExB probe, otherwise know as Wien filter, is a mass spectrometry device.  The ExB probe acts as a velocity filter.  It can separate ion species according to their velocities.  The ExB probe typically is used in front of a more elaborate mass spectrometer.  The Lorentz Force describes the effect of electric and magnetic fields as they simultaneously act on a charged particle.  An ExB probe uses uniform, perpendicular electric and magnetic fields that are both perpendicular to the velocity vector.  Consequently, the two fields create opposing forces on the particles.  For a given velocity, these fields can be adjusted so that there is no net force on the particle, which will then travel to the detector without being redirected by the fields.  Since particle acceleration in the thruster is dependent on charge, the ion speed will be proportional to its charge state.  Because of this, the ExB probe can distinguish the ions with different charge states.  By varying the electric or magnetic field, one can select the ions to be collected by the probe's particle detector.

1.            Kim, S., and Gallimore, A.D., "Plume Study of a 1.35 kW SPT-100 Using an ExB Probe, "AIAA-99-2423, 35th Joint Propulsion Conference, Los Angeles, CA, June 1999.

2.            Kim, S.W., "Experimental Investigations of Plasma Parameters and Species-Dependent Ion Energy Distribution in the Plasma Exhaust Plume of a Hall Thruster," Ph.D. Dissertation, University of Michigan, 1999.