Hall-effect thruster-cathode coupling : the effect of cathode position and magnetic field topology

Hall-effect thruster (HET) cathodes are responsible for the generation of the free electrons necessary to initiate and sustain the main plasma discharge and to neutralize the ion beam. The position of the cathode relative to the thruster strongly affects the efficiency of thrust generation. However, the mechanisms by which the position affects the efficiency are not well understood. This dissertation explores the effect of cathode position on HET efficiency. Magnetic field topology is shown to play an important role in the coupling between the cathode plasma and the main discharge plasma. The position of the cathode within the magnetic field affects the ion beam and the plasma properties of the near-field plume, which explains the changes in efficiency of the thruster. Several experiments were conducted which explored the changes of efficiency arising from changes in cathode coupling. In each experiment, the thrust, discharge current, and cathode coupling voltage were monitored while changes in the independent variables of cathode position, cathode mass flow and magnetic field topology were made. From the telemetry data, the efficiency of the HET thrust generation was calculated. Furthermore, several ion beam and plasma properties were measured including ion energy distribution, beam current density profile, near-field plasma potential, electron temperature, and electron density. The ion beam data show how the independent variables affected the quality of ion beam and therefore the efficiency of thrust generation. The measurements of near-field plasma properties partially explain how the changes in ion beam quality arise. The results of the experiments show that cathode position, mass flow, and field topology affect several aspects of the HET operation, especially beam divergence and voltage utilization efficiencies. Furthermore, the experiments show that magnetic field topology is important in the cathode coupling process. In particular, the magnetic field separatrix plays a critical role in impeding the coupling between cathode and HET. Suggested changes to HET thruster designs are provided including ways to improve the position of the separatrix to accommodate the cathode

Emission cross sections for neutral xenon impacted by Xe\u3csup\u3e+\u3c/sup\u3e and Xe\u3csup\u3e2+\u3c/sup\u3e

Ion impact emission cross sections for eleven transitions from the 5p56p configuration to the 5p56s configuration of neutral xenon occurring in the spectral region between 700 nm and 1000 nm have been measured experimentally. Collisions between both singly- and doublyionized xenon and neutral xenon have been studied. These cross sections are of primary use in the development of a spectrographic diagnostic for Hall effect thruster plasma. A detailed discussion of the experimental methods and the subsequent data reduction is included. The results are presented and the importance of these data for spectrographic emission models of Hall effect thruster plasmas is discussed

Ion-Collision Emission Excitation Cross Sections for Xenon Electric Thruster Plasmas

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76738/1/AIAA-33657-821.pd

Hall-effect thruster - Cathode coupling part I: Efficiency improvements from an extended outer pole

This is the first part of a two part paper in which we explore the effect of cathode position and magnetic field configuration on Hall-effect thruster (HET) performance. In this part, the effect of the cathode position on the efficiency of the thruster is explored. The study consists of two experiments in which we compare the efficiency of an HET in such a way as to change the external magnetic field topology with respect to the cathode position to its original configuration. We show the importance of the magnetic field separatrix, a surface which divides the magnetic field lines into internal and external regions. The modifications provide a means to improve cathode coupling while keeping the cathode outside of the damaging ion beam. The improvement in cathode coupling results in improved thruster performance. Specifically, better cathode coupling voltages, lower beam divergences, and an improvement in efficiency of up to 10 percentage points are noted

Effect of cathode position on Hall-effect thruster performance and near-field plume properties

Thruster efficiency and plume properties, including ion energy distributions and beam current density profiles, of a Hall-effect thruster are compared when the cathode is placed in nine radial positions relative to the thruster and at three cathode mass flow rates. The cathode was mounted such that its orifice normal was at a right angle to the thruster axis. Plume properties were measured at a radius of 250 mm from the intersection of the thruster axis and the exit plane. Cathode coupling voltage, beam divergence, and voltage utilization are strongly affected by cathode position

Hall-effect thruster-cathode coupling, Part II: Ion beam and near-field plume

This is the second part of a two-part paper in which the effect of cathode position and magnetic field configuration on Hall-effect thruster performance is explored. The effect magnetic field topology has on the coupling between a Hall-effect thruster and its cathode has been studied. With two Hall-effect thruster configurations, each with a different external field topology, the cathode is positioned across a range of radial distances and the effect on performance, ion beam, and near-field plasma is investigated. The importance of the magnetic field separatrix, a surface which divides the magnetic field lines into internal and external regions, is shown. In particular, total efficiency improvements of up to seven percentage points are seen when placing the cathode near the separatrix as opposed to locations further away of the thruster. Analysis of the thruster telemetry, ion beam current distribution, and ion energy distribution functions properties show the effect of the cathode position on the efficiency loss mechanisms of beam divergence, current utilization, ion velocity distribution, voltage utilization, and cathode coupling. Measurements of near-field plasma potential, electron temperature, and electron density provide help to explain why these efficiency improvements come about and add insight into the cathode coupling processes. As the cathodeis moved radially away from the thrusterup to 250 mm, the cathode coupling efficiency decreasesbyupto10 percentage points. Furthermore, the near-field plasma potential increases by up to 30 V, and this is correlated with a decrease in beam divergence efficiency of up to 15 percentage points. Copyright © 2011 by Jason D. Sommerville

Energy-Loss mechanisms of a low-discharge-voltage hall thruster

The findings of the probe study and thrust stand measurements performed on an Aerojet BPT-2000 Hall thruster at discharge voltage from 100 to 300 V for 3 to 5 mg/s flow of xenon were examined. Voltage utilization efficiencies were obtained by a four-grid retarding potential analyzer (RPA) probe placed 0.55m downstream from the thruster faceplate at 0.15, and 30° to the thruster centerline. The Faraday probe was enclosed in an alumina sheath with an outer diameter of 4.75 mm. Energy efficiency can further be separated into voltage utilization efficiency and current efficiency, where the voltage utilization efficiency is the percentage of the anode-cathode potential that the ions are accelerated through and the current efficiency is the ratio of exhausted current to discharge current. The results showed that current levels were nearly constant for each mass-flow rate until the discharge voltage dropped below 200V, where the current increased rapidly

Efficiency analysis of a low discharge voltage Hall thruster

Power loss mechanisms for a 2 kW (nominal) Hall thruster operating at low discharge voltages were examined through thrust stand measurements and probe studies. Operating conditions included discharge voltages ranging from 100 V to 300 V and mass flow rates of 3 to 5 mg/s of xenon. Thrust stand measurements indicate a minimum thrust efficiency of 15% at 100 V at 3 mg/s and a maximum of 59% at 300 V and 5 mg/s. Retarding potential analyzer, emissive and Faraday probes were utilized to quantify multiple sources of inefficiency. The ratio of exhausted ion current to discharge current was found to be the dominant loss mechanism at low discharge voltage

Hall-effect thruster-cathode coupling, Part I: Efficiency improvements from an extended outer pole

This is the first part of a two-part paper in which the effect of cathode position and magnetic field configuration on Hall-effect thruster performance is explored. In this part, the effect of the cathode position on the efficiency of the thruster is explored. The study consists of two experiments performed on a Hall-effect thruster with its outer pole modified so as to change the external magnetic field topology with respect to the cathode position. In these experiments, the efficiency of the modified thruster is compared with the efficiency of the Hall-effect thruster in its original configuration. The importance of the magnetic field separatrix, a surface which divides the magnetic field lines into internal and external regions, is shown. The modifications provide a means to improve cathode coupling while keeping the cathode outside of the ion beam, which can erode the cathode components. The improvement in cathode coupling results in improved thruster performance, specifically improved cathode coupling voltages (the potential between the cathode and facility ground) of approximately 3% of the discharge voltage, increased beam divergence efficiencies on the order of 5 percentage points, and an improvement in total efficiency of up to 10 percentage points are noted. Copyright © 2011 by Jason D. Sommerville