A low-current LaB₆ open-end knife-edge emitter hollow cathode for low-power Hall thrusters

Abstract

Heated thermionic cathodes based on LaB 6 emitters are known to consume a relatively large amount of power during the heating part of the ignition phase. Miniaturized electric propulsion systems, and in particular low-power Hall thrusters, rely on the operation of self-sustained cathodes at low-current emission, typically below 1 A. At PSAC/SPC, an open-end knife-edge LaB 6 emitter hollow cathode was developed and tested, denoted herein as the PSAC-KE cathode. Here, a detailed description of the new model's design and comparison to the older version, the PSAC cathode, is included. Electrostatic simulations suggested that the open-end emitter geometry allowed for an enhanced electric field within the emitter region during cathode startup and steady-state operation as compared to the orificed emitter PSAC cathode. The PSAC-KE cathode's thermal management was improved based on iterative thermal simulations driven by different cathode geometries and various combinations of materials. The new cathode was tested with xenon in diode mode with an external anode and triode mode with the external anode and the keeper electrode. Furthermore, results from the coupling tests with a low-power Hall thruster are presented. Cathode performance was monitored over a range of anode current from 0.1 to 1 A, mass flow rates from 0.057 to 0.3 mg s −1 and keeper current of 0.05, 0.1, and 0.15 A. The cathode's thermal design was assessed with thermocouple measurements and compared with thermal simulations and results obtained for the PSAC cathode. The preliminary tests showed that the PSAC-KE cathode achieved ignition at low heating power below 35 W, self-sustained operation at 1 A when in standalone mode, and < 1 A when against a low-power Hall thruster.

Ionization instability and turbulence in the plume of sub-ampere hollow cathodes with applied magnetic field

“…This axial magnetic field strength was previously measured at the cathode's location during the coupling tests with a low-power Hall thruster as shown in Ref. [27]. Measurements using the probe array described later are conducted at three locations along the cathode plume, z = 2.5, 4.5, and 6.5 mm, and at those location B max reaches 3.7, 4.5, and 5.6 mT respectively.…”

“…The PSAC-KE cathode [27] is used as a test-bench here. The cathode has a LaB 6 hollow cylindrical emitter with a knife-edge feature.…”

“…For a detailed description of the cathode, the interested reader is invited to consult with Refs. [24] and [27].…”

“…Sub-ampere thermionic cathodes operating at emission currents below 1 A are a class of electron emitters [13,26,27] employed with low-power Hall thruster systems, typically operating at < 250 W. Few studies focused on the discharge stability and mode transition in such cathodes [28,29] and the influence of an applied magnetic field on the operation of sub-ampere cathodes in standalone mode was never documented before. Here, an open-end knifeedge LaB 6 emitter cathode is tested in standalone mode with an external disk anode.…”

“…Here, an open-end knifeedge LaB 6 emitter cathode is tested in standalone mode with an external disk anode. A solenoid placed behind the anode is employed to produce an axial magnetic field with a strength at the cathode's keeper location equivalent to the measured value when the cathode is mounted next to a 200 W-class Hall thruster [27].…”