Alec D. Gallimore

Internal plasma potential profiles in a laboratory-model Hall thruster

The Plasmadynamics and Electric Propulsion Laboratory High-speed Axial Reciprocating Probe system is used in conjunction with a floating emissive probe to measure plasma potential in the discharge chamber of the P5 Hall thruster. Plasma potential measurements are made at a constant voltage, 300 V, at two different discharge current conditions: 5.4 and 10 A. The plasma potential contours for the 5.4 A case indicate that the acceleration region begins several millimeters upstream of the exit plane, extends several centimeters downstream, and is uniform across the width of the discharge chamber. The 10 A case is similar to the 5.4 A case with the exception that the acceleration region is shifted downstream on centerline. Axial electric field profiles, computed from the measured potential, show a double peak structure in the 5.4 A case, indicating a zone of ion deceleration. Perturbations to the discharge current are shown to correspond spatially with the location of the peak electric field indicating that th...

High-specific impulse hall thrusters, Part 2 Efficiency analysis

Performance and plasma measurements of a high-specific impulse (2000‐3000 s) Hall thruster were analyzed using a phenomenological performance model that accounted for a partially ionized plasma containing multiply charged ions. Anode efficiency over discharge voltages of 300‐900 V ranged from 57 to 69%, which corresponded to 89‐97% voltage utilization, 86‐90% mass utilization, 77‐81% current utilization, and 97‐99% charge utilization. Although the net decrease of efficiency due to multiply charged ions was at most 3%, the effects of multiply charged ions on the discharge current could not be neglected because the increase of the discharge current with voltage was primarily due to the increasing fraction of multiply charged ions. This and the fact that the maximum deviation of the electron current from its average value was only +5/−14% illustrated how efficient operation at high-specific impulse was enabled through the regulation of the electron current with the applied magnetic field. The electron Hall parameter, defined by acceleration zone plasma properties, was nearly constant with voltage, decreasing from an average of 210 at 300 V to an average of 160 between 400 to 900 V.

Performance Characteristics of a 5 kW Laboratory Hall Thruster

Abstract : The University of Michigan and United States Air Force Research Laboratory have jointly developed a 5 kW class Hall effect thruster. This thruster was developed to investigate, with a variety of diagnostics, a thruster similar to that specified by IHPRPT goals. The configuration of this thruster is adjustable so that diagnostic access to the interior of the thruster can be provided as necessary, and to allow for the exploration of various thruster geometries. At nominal conditions, the thruster was designed to operate at 5 kW with a predicted specific impulse of 2200 s. The actual operating parameters at 5 kW were 2326 s specific impulse, with 246 mN of thrust at an efficiency of 57%. These conditions are comparable to those of thrusters under commercial development, making the information learned from the study of this thruster applicable to the understanding of its commercial counterparts.

High-Specific Impulse Hall Thrusters, Part 1: Influence of Current Density and Magnetic Field

A laboratory-model Hall thruster with a magnetic circuit designed for high-specific impulse (2000‐3000 s) was evaluated to determine how current density and magnetic field affect thruster operation. Results have shown for the first time that a minimum current density and optimum magnetic field shape exist at which efficiency will monotonically increase with specific impulse. At the nominal mass flow rate of 10 mg/s and between discharge voltages of 300 and 1000 V, total specific impulse and total efficiency ranged from 1600 to 3400 s and 51 to 61%, respectively. Comparison with a similar thruster showed how efficiency can be optimized for specific impulse by varying the shape of the magnetic field. Plume divergence decreased from a maximum of 48 deg at 400 V to a minimum of 35 deg at 1000 V, but increased between 300 and 400 V as the likely result of a large increase in discharge current oscillations. The breathing-mode frequency continuously increased with voltage, from 14.5 kHz at 300 V to 22 kHz at 1000 V, in contrast to other Hall thrusters where a sharp decrease of the breathing-mode frequency was found to coincide with increasing electron current and decreasing efficiency. These findings suggest that efficient, high-specific impulse operation was enabled through the regulation of the electron current with the applied magnetic field.

The Effects of Nude Faraday Probe Design and Vacuum Facility Backpressure on the Measured Ion Current Density Profile of Hall Thruster Plumes

The effects of dissimilar probe design and facility backpressure on the measured ion current densities of Hall thrusters are investigated. JPL and GRC designed nude Faraday probes are used to simultaneously measure the ion current density of a 5 kW Hall thruster in the Large Vacuum Test Facility (LVTF) at the University of Michigan. The probes are located one meter from the exit plane of the Hall thruster, which is operated over the range of 300-500 V and 5-10 mg/s. In addition, the effect of facility background pressure is evaluated by varying the nominal pumping speed from 70,000 l/s to 240,000 l/s on xenon, corresponding to backpressures of 4.3x10-6 Torr to 2.3x10-5 Torr, corrected for xenon. Detailed examination of the results has shown that the GRC probe measured a greater ion current density than the JPL probe over the range of angular positions investigated for each operating condition. Yet, both probes measure similar thruster plume profiles for all operating conditions. Because all other parameters are identical, the differences between ion current density profiles measured by the probes are contributed to material selection and probe design. Moreover, both probes measured the highest ion current density near thruster centerline at the lowest facility pumping speed. A combination of charge exchange collisions and vacuum chamber gas ingestion into the thruster is believed to be the cause of this phenomenon.

Efficiency Analysis of a Hall Thruster Operating with Krypton and Xenon

Krypton has recently become the focus of attention in the Hall thruster community because of its relatively large specific impulse compared with xenon and its potential to operate with comparable efficiencies. However, before krypton can be considered a viable propellant choice for missions, the performance gap between xenon and krypton must be reduced. A series of diagnostic measurements are taken for xenon and krypton propellant using the NASA-173Mv1 Hall thruster and the results are analyzed using a phenomenological performance model. The combined use of experiments and modeling enables a direct comparison of several efficiency components for each propellant to be made. With this method, it is possible to pinpoint the exact causes for the efficiency gap between xenon and krypton. It is also possible to see the effect of the magnetic field topology on Hall thruster performance and where gains are being made due to the magnetic field. Although there is a large series of competing components that differentiate krypton and xenon performance, the largest factors that dictate the efficiency difference between krypton and xenon are kryptons inferior propellant utilization and beam divergence. For xenon, the propellant utilization is 5-10% higher and the beam divergence efficiency is approximately 8% higher.

Effect of Backpressure on Ion Current Density Measurements in Hall Thruster Plumes

The effects of facility backpressure and localized electric fields on the measured ion current densities of Hall thrusters are investigated. Langmuir probe measurements are taken in the near-field plasma surrounding a nude Faraday probe, which is located 1 m from the exit plane of the University of Michigan/U.S. Air Force Research Laboratory P5 Hall thruster. The thruster is operated at an anode flow rate of 5.30 mg/s, at backpressures of 1.5 × 10 −3 Pa (1.1 × 10 −5 torr) and 4.8 × × 10 −4 Pa (3.6 × × 10 −6 torr), corrected for xenon. The effect of the facility backpressure is clearly seen in the wings of the plume. A combination of charge-exchange collisions and vacuum chamber gas ingestion into the thruster is believed to be the cause of this phenomenon. The Langmuir probe results indicate that the electric fields near the nude Faraday probe are functions of facility backpressure and the angle from the thruster centerline. The plasma potential measured within 20 mm of the probe varied by no more than 3V .Thus, the electric fields near the nude Faraday probe are not large enough to explain the increased collection of charge-exchange ions at elevated facility background pressures and large angles from the thruster centerline.

Development of a high-speed, reciprocating electrostatic probe system for Hall thruster interrogation

The use of electrostatic probes to measure local plasma parameters inside the discharge chamber of a Hall thruster presents significant difficulties. The high-temperature, dense plasma, and Hall current in the accelerating channel heat the probe rapidly causing ablation of probe material, which perturbs thruster operation and reduces probe lifetime. Results are presented which show the extent of perturbation to discharge current, cathode potential, and thrust for the case where probe material is ablated. A simple thermal model of probe material heating is developed and ablation times for a typical probe configuration are presented. Using the results of the thermal model, a high-speed axial reciprocating probe (HARP) system was developed to enable probe survival and reduce thruster perturbations during interrogation of the discharge chamber of a Hall thruster. Results using the HARP system are presented showing a significant reduction in thruster perturbation. The results also indicate that a mechanism other than material ablation is contributing to perturbation of the thruster. Based on emissive probe data, the tungsten conductor appears to provide a low impedance path between magnetic field lines, enhancing electron transport to the anode.

Plasma Properties in the Plume of a Hall Thruster Cluster

The Hall thruster cluster is an attractive propulsion approach for spacecraft requiring very high-power electric propulsion systems. Plasma density, electron temperature, and plasma potential data collected with a combination of triple langmuir probes and floating emissive probes in the plume of a low-power, four-engine Hall thruster cluster are presented. Simple analytical formulas are introduced that allow these quantities to be predicted downstream of a cluster based solely on the known plume properties of a single thruster. Nomenclature A = area of one electrode AS = surface area of sheath surrounding an electrode B = magnetic field strength E = electric field strength e = electron charge kb = Boltzmann’s constant me = electron mass mi = ion mass n = electron number density n0 = reference density Te = electron temperature Te,0 = reference electron temperature Vd2 =v oltage measured between triple probe electrodes 1 and 2 Vd3 =v oltage applied between triple probe electrodes 1 and 3 V f = floating potential γ = ratio of specific heats δ = sheath thickness λD = electron Debye length φ = plasma potential φT = thermalized potential Subscript j = contribution from an individual thruster

The Role of Magnetic Field Topography in Improving the Performance of High-Voltage Hall Thrusters

Investigations of high-voltage Hall thrusters have indicated a peak in the efficiency versus voltage characteristic that limited the maximum efficiency to specific impulses of less than 3000 s. This peak is believed to be primarily a trait of modern magnetic field design, which is optimized for discharge voltages of 300 V. The NASA-173M has been operated at 300-1000 V and 5 mg/s to investigate whether performance improvements could be realized through in situ variation of the magnetic field topography through the use of an auxiliary trim coil. Without the trim coil, a peak in the efficiency characteristic was observed at 600 V. The results with the trim coil energized indicate there is always some performance benefit to altering the magnetic field topography. Above 400 V, efficiencies were maintained at >50% and above 900 V, specific impulses >3000 s were demonstrated while using the trim coil. The largest gains in performance were observed at 1000 V, where the thrust, specific impulse, and efficiency improved by 10 mN, 200 s, and 5.5%, respectively, to 165 mN, 3360 s, and 51.5%. The results demonstrate that the peak in the efficiency characteristic observed without the trim coil can be mitigated when the magnetic field topography is tailored for high-voltage operation. Analysis of the magnetic field from numerical simulations has identified several important factors contributing to the performance benefits with trim coil operation.