Characterization of an Ion Thruster Neutralizer

T HE high-power electric propulsion (HIPEP) thruster was designed and built to address the need for a long-life, highspecific-impulse option to satisfy requirements for missions to the outer planets and the outer edge of the solar system [1,2]. These powerful thrusters would use a nuclear reactor source to provide the necessary electrical energy. Subcomponents of such ion thrusters must be capable of operating up to 10 years, a typical thrusting-time upper limit [2]. The work presented here focuses on the hollow, cathode-based, plasma-bridge neutralizer subcomponent [3]. As the name implies, the purpose of the neutralizer is to provide space charge and current neutralization of the ion beam. Investigated herein is the neutralizer operating behavior over a wide range of thruster operating conditions. This investigation provides insight into the neutralizer operating envelope and associated flow-rate margin, as well as the impact of the neutralizer-produced plasma on other thruster subcomponents. In conducting this investigation, neutralizer voltage and plasma characteristics as functions of flow rate were determined. The neutralizer characterization is necessary to determine the optimum flow rate for neutralizer operation. Ideally, this would be the lowest flow rate at which the neutralizer can still operate in the spot mode. The hollow cathode spot mode is a quiet, low-voltage discharge condition characterized by the visible presence of a luminous spot of intense ionization and excitation in the orifice of the cathode [4,5]. Although theminimization of neutralizer flow can improve the specific impulse, it can also have deleterious effects on the neutralizer assembly lifetime. A flow rate that is too low forces the neutralizer to operate in the so-called plume mode in which high keeper voltages and often unstable operation prevail. In this mode, a distinct and visible luminous plume emanating from the cathode is typically observed. The plume is produced by electron excitation of neutrals external to the cathode. Large keeper-tocathode, peak-to-peak voltage oscillations are commonly measured in this mode [4,5]. These large voltages can drive sputter erosion of the neutralizer assembly, giving rise to cathode-orifice-channel enlargement and orifice-plate texturing [5–7]. Over time, a neutralizer can transition from the spot to the plume as it undergoes natural erosion of the orifice or through deposition processes within the orifice channel while operating at a fixed flow rate [6]. Such transitioning phenomenawere observed both during theNASASolar Electric Propulsion Application Readiness (NSTAR) flight and during the long duration testing of the NSTAR flight spare [8]. The neutralizer flow rate was increased to transition the neutralizer out of this undesirable operating mode [8]. Again, the plume mode is to be contrasted with spot-mode operation, which is characterized by quiet, stable operation and low discharge voltages and associated reduced wear rates [4]. Diagnostics used to monitor the neutralizer operating mode have typically involved tracking the behavior or change in thruster operating parameters such as keeper voltage or keeper peak-to-peak voltage oscillation magnitude. Indeed, during the NSTAR program, a 5 V or greater peak-to-peak keeper oscillation voltage was defined as the initiation of the plume mode. In the work presented here, in addition to monitoring keeper voltage changes, electrostatic probes co-located with the neutralizer were used to document the change in neutralizer operating mode in response to flow-rate changes. These neutralizer plasmameasurements also provide insight into the energy and magnitude of ion flux incident upon the neutralizer assembly during neutralizer characterization. Witness plates, coated with a layer of tantalum of a known thickness, were placed near the neutralizer assembly. The witness plates were used to determine if extreme, poorly focused, off-axis ion beamlets were present. Such beamlets could impinge upon the neutralizer keeper barrel, reducing its overall service life.