A flight-model SPT-100 Hall thruster from the Fakel Experimental and Design Bureau was characterized with respect to performance and plume properties at The Aerospace Corporation and at L-3 Electron Technologies, Inc. over vacuum facility background pressures ranging from 2.3×10 -4 to 9.5×10 -3 Pa (1.7×10 -6 to 7.1×10 -5 Torr). Thrust and xenon mass flow rate measured at both facilities agreed to within the measurement uncertainties. Thrust decayed exponentially, while flow rate increased linearly with decreasing pressure. Using simple curve-fits for extrapolation, thrust, specific impulse, efficiency, and flow rate were projected to change by -4, -10, -14, and 7% respectively at zero pressure relative to 6.7×10 -3 Pa (5.0×10 -5 Torr). A decrease in net ion accelerating voltage and an increase in plume divergence were identified as contributors to decreased thrust with decreasing pressure. The amplitude of discharge current oscillations decreased linearly with decreasing background pressure, while the center frequency of the oscillations also decreased, from 26 to 16.5 kHz, but in non-linear fashion. Neutral density near the exit plane of Hall thrusters can be quite low, O(10 18 ) m -3 , such that background neutrals from the residual gas in the vacuum chamber may significantly influence ionization and conduction there.
Performance, plume properties, and discharge stability of a 9 cm diameter laboratory model Hall thruster were measured at vacuum chamber background pressures ranging from 1e-6 to 6e-5 Torr. The thruster was operated at constant discharge powers of 300 and 500 W while background pressure was varied from the minimum achievable value to values 10, 20, and 40 times greater than the minimum at each power level. Discharge current oscillation amplitude generally increased with increasing background pressure, and the sensitivity of oscillation amplitude to background pressure increased with increasing applied magnetic field strength. The fundamental frequency of the discharge current oscillations varied with background pressure. Thrust, specific impulse, and efficiency declined as stability degraded, primarily as a result of increased electron cross-field mobility, broadening of the ion energy distribution, and increased multicharged ion production. Ingestion of background neutrals appeared to lengthen the ionization zone and shift it toward the anode. NomenclatureF = thrust α = thrust correction due to multicharged ions β = thrust correction due to off-axis flow J D = discharge current V D = discharge voltage m = ion mass q = elementary charge J n+ = beam ion current in n th charge state (n = 1, 2, 3) g = acceleration due to Earth's gravity at sea level η c = current utilization efficiency η V = voltage utilization efficiency η m = mass utilization efficiency n n = thruster exit plane neutral density m = total anode flow rate A e = thruster exit plane area k = Boltzmann's constant T n = neutral temperature at thruster exit plane
An investigation of a fully cylindrical Hall thruster was performed using laser induced fluorescence (LIF) to measure ion velocity profiles in the plume. The measurements confirm a previously reported 9% increase in the exhaust energy when the cathode keeper draws an excess current (overrun mode). Furthermore, the velocity directions in the plume remain relatively unchanged for the cusped and direct magnetic field configuration in both overrun and nonoverrun modes. Previously reported plume narrowing in the overrun mode was confirmed and found to be due to the shift of the acceleration and ionization regions toward the anode. The electric field inferred from the LIF measurements allowed calculation of the electron E×B drift. Close to the centerline of the thruster, electrons drift azimuthally with velocity decreasing away from the centerline, thus creating shear. This shear can be a source of plasma instabilities and influence electron transport. Further away from the centerline, electrons drift in the opposite direction with their velocity increasing with increasing radius. In that region, electrons rotate without shear.
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