James M. Haas

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...

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.

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

Modeling of Stationary Plasma Thruster-100 Thruster Plumes and Implications for Satellite Design

A computational model of a stationary plasma thruster (SPT) has been developed using a quasineutral particle-in-cell/direct simulation Monte Carlo (PIC-DSMC) model. This model is based on theoretical work showing that the plume consists of a quasineutral plasma with collisionless electrons in which the magnetic field can be neglected. Details of the PIC DSMC method are presented as well as axisymmetric and three-dimensional results. Comparisons are made to new and previously reported experimental data. The model is shown to produce results similar to laboratory measurements of the ion current density and plume-induced sputter erosion rates. The model does not compare as well with retarding potential analyzer measurements of the ion energy distribution. The results confirm previous observations that measurements made in some ground facilities may substantially overpredict the amount of backflow current that will be experienced under operational conditions. A surface-sputtering model is used to predict the impact the plume has on solar array interconnects and to show the impact an SPT thruster could have on a communications satellite. The results show that the thruster should be canted with respect to the solar array, lowering its effective thrust and specific impulse.

Considerations on the role of the Hall current in a laboratory-model thruster

Hall current magnitude and spatial distribution are presented for the plasma discharge in the University of Michigan/Air Force Research Laboratory P5 5 kW laboratory-model Hall thruster. The data are calculated from direct, probe-based measurements of the electric field, static magnetic field, and charged particle number density. Thruster discharge voltage was fixed at 300 V and two current levels investigated: 5.4 A (1.6 kW) and 10 A (3 kW). The results indicate that, for both cases, the bulk of the Hall current is confined to a region centered several millimeters upstream of the exit plane and is asymmetric about the centerline of the discharge channel. At 1.6 kW, the axial plasma potential drop occurs over a much shorter distance, resulting in a more sharply peaked Hall current zone, as compared to the 3-kW case. Comparison of the Hall current and ion number density distribution suggests that the azimuthal electron drift may contribute significantly to the ionization process in the discharge channel. Integration of the Hall current over the entire discharge volume yields total current values that are a factor of 3.5-4.6 times larger than the discharge current. Estimates of the self-magnetic field induced by the drifting electrons indicate no significant modification to the applied magnetic field during thruster operation, at the power levels considered. Using the Hall current density distribution derived from probe measurements, the electromagnetic body force on the ions was calculated and compared to measured engine thrust for both power levels.

Air Force Research Laboratory high power electric propulsion technology development

Space solar power generation systems have a significant impact on Electric Propulsion (EP) technology development.1,2,3 Recent advances in solar cell, deployment, and concentrator hardware have led to significant reductions in component mass, thereby decreasing power generation system specific mass. Combined with maneuvering requirements for Air Force and DoD missions of interest, propulsive requirements emerge that provide direction for technology investments. Projections for near- to mid-term propulsion capabilities are presented indicating the need for thrusters capable of processing larger amounts of power (100 – 200 kW), operating at relatively moderate specific impulse (2000 – 6000 seconds) and high efficiency (≫ 60%), and having low propulsion system mass (≪ 1 kg/kW). Two technology areas are identified and discussed in the context of the above thruster constraints. Concentric channel Hall thrusters are an extension of a mature technology, offering operation over expanded power levels and lower propulsion system specific mass at state-of-the-art (SOTA) efficiencies. Field Reverse Configuration (FRC) thrusters are a specific type of pulsed inductive accelerator that have the potential to operate up to MW power levels, at propulsion system specific masses even lower than concentric channel Hall thrusters, and on a wider range of propellants. However, FRCs are currently less mature than the Hall thruster variants. Comparisons of candidate technologies are evaluated with VASIMR, a well publicized high power EP device currently under development.

An experimental investigation of the internal magnetic field topography of an operating Hall thruster

Magnetic field measurements were made in the discharge channel of the 5 kW-class P5 laboratory-model Hall thruster to investigate what effect the Hall current has on the static, applied magnetic field topography. The P5 was operated at 1.6 and 3.0 kW with a discharge voltage of 300 V. A miniature inductive loop probe (B-Dot probe) was employed to measure the radial magnetic field profile inside the discharge channel of the P5 with and without the plasma discharge. These measurements are accomplished with minimal disturbance to thruster operation with the High-speed Axial Reciprocating Probe system. The results of the B-Dot probe measurements indicate a change in the magnetic field topography from that of the vacuum field measurements. The measured magnetic field profiles are then examined to determine the possible nature and source of the difference between the vacuum and plasma magnetic field profiles.

Very-Near-Field Plume Investigation of the Anode Layer Thruster

The plasma properties of the very-near-e eld (10‐50 mm) plume of the D55 anode layer thruster (TAL) were measured. The D55 is the 1.35-kW TAL counterpart to the SPT-100 and was made by the Central Scientie c Research Institute of Machine Building of Kaliningrad, Russia. The thruster was tested in the 6 m diameter £ 9 m longvacuumchamberattheUniversityofMichigan’ sPlasmadynamicsandElectricPropulsionLaboratory,andthe diagnosticprobes werepositioned using a three-axis translation tablesystem. Water-cooled Hall probes, a Faraday probe, emissive probes, and langmuir probes were used to examine the near-e eld plasma properties. Water-cooled Hall probes were employed to exploretheeffectof the closed-drift current on theradial magnetic e eld. The change in the magnetic e eld during thruster operation was found to be less than 5% over the region examined, which indicated that the Hall current was limited to several tens of amperes. Evidence also indicated that the closed-drift current extended between 5 and 10 mm downstream of the anode. Ion current density proe les showed that the annular beam focuses within 40 mm of the thruster exit plane. Plasma potential measurements indicated that ion acceleration occurred primarily within 10 mm of the anode. The highest electron temperature measured in this investigation occurred immediately downstream of the anode, and the temperature decreased with axial distance from the thruster. The low-energy electrons were cone ned to the high-density core of the plasma beam.

Very near-field plume investigation of the D55

The plasma properties of the very near-field (10 to 50 mm) plume of the D55 anode layer thruster (TAL) were measured as part of an effort lead by NumerEx of Albuquerque, NM to model the processes within TALs. The D55 is the 1.35 kW TAL counterpart to the SPT-100 and was made by TsNUMASH of Kaliningrad, Russia. The thruster was tested in the 6 m diameter by 9 m long vacuum chamber at (lie Plasmadynamics and Electric Propulsion Laboratory (PEPL), and the diagnostic probes were positioned using a three axis translation table system. A Faraday probe, water-cooled Hall probes, emissive probes, and Langmuir probes were used to examine the near-field plasma properties. Water-cooled Hall probes were employed to explore the effect of the closed drift current on the radial magnetic field. The change in the magnetic field due to the Hall current was found to be less than five percent over the region examined. Ion current density profiles showed that the annular beam focuses within 40 mm of the thruster exit plane. Similarly, the electron temperature and number density radial profiles showed peaks near the discharge chamber at 10 mm axially, and the peaks moved toward the axis within 40 mm. The peak electron temperature decreased with axial distance, while the number density remained approximately constant over the very near-field region. Nomenclature