Investigation of a Gallium MPD Thruster with an Ablating Cathode

Arc impedance, exhaust velocity, and plasma probe measurements are presented. The thruster is driven by a 50 s pulse from a 6.2 m pulse forming network, and gallium is supplied to the discharge by evaporation of the cathode. The arc voltage is found to vary linearly with the discharge current with an arc impedance of 6.5 m . Electrostatic probes yield an exhaust velocity that is invariant with the discharge current and has a peak value of 20 km/s, which is in reasonable agreement with the value (16 1 km/s) calculated from the mass bit and discharge current data. Triple probe measurements yield on axis electron temperatures in the range of 0.8-3.8 eV, electron densities in the range of 1.6 10 21 to 2.1 10 22 m 3 , and a divergence half angle of 16 . Measurements within the interelectrode region yield a peak magnetic eld of 0.8 T, and the observed radial trends are consistent with an azimuthally symmetric current distribution. A cathode power balance model is coupled with an ablative heat conduction model predicting mass bit values that are within 20% of the experimental values.