Plasma Potential Measurements in the Discharge Channel of a 6-kW Hall Thruster

In an effort to understand the role of neutral flow dynamics in the operation of Hall thrusters, a floating emissive probe was used to measure the plasma potential, electric field, and electron temperature in the discharge channel of a 6-kW Hall thruster operating at 300 V and anode flow rates of 10, 20, and 30 mg/s. To isolate the effect of propellant flow rate, the cathode flow fraction and magnetic field topology were held constant while varying magnetic field strength to minimize discharge current while maximizing thrust. The probe was inserted and removed from the discharge channel by a high-speed positioning system at nine radial locations. The plasma potential profiles showed little variation in the radial direction, leading to negligible radial electric field and ion focusing towards channel centerline. The peak axial electric field occurred near the exit plane, moving downstream 35% of the channel length for each addition of 10 mg/s. The electron temperature calculated by the difference in plasma and floating potentials agreed well with Langmuir probe measurements at 10 mg/s, but results at 20 and 30 mg/s displayed poor agreement. The Langmuir probe peak electron temperature decreased by about 5 eV and moved downstream 3-5% of the channel length for each addition of 10 mg/s. The results indicate a strong dependence of peak plasma property magnitude and location on neutral collisions.