William A. Hargus

Near Exit Plane Velocity Field of a 200-Watt Hall Thruster

This work presents the near exit plane velocity field of a 200-W laboratory xenon Hall thruster at a single operating condition with a 250-V anode potential. The ionized propellant velocities were measured using laser-induced fluorescence of the 5d[4] 7/2 -6p[3] 5/2 excited state xenon ionic transition at 834.72 nm. Ion velocities were interrogated from the acceleration channel exit plane to a distance 107 mm from the exit plane (3.3 exit plane diameters). Both axial and radial velocities were measured. A nearly uniform axial velocity profile of approximately 13,800 ± 500 m/s (130 ± 10 eV) was measured at the thruster exit plane. The maximum axial velocity, measured 107 mm from the exit plane, was 16,800 m/s (192 eV). The ion flow exiting the thruster acceleration channel mixes downstream due to both the coaxial thruster geometry and a possible ion-acoustic shock. This behavior appears in regions where multiple, or broadly distributed, radial and axial velocity components occur. These regions also exhibit broadened fluorescence line shapes, likely indicative of collisions between the various velocity populations as well as possible ionization of background neutral, and correspond to the brighter, more visible portions of the plume. This region has been previously identified as a possible ion-acoustic shock. This hypothesis appears consistent with the low radial velocity ion populations measured in this more luminous portion of the plume. In addition, a limited study at five off-nominal conditions near a region of high insulator erosion indicates that the impinging ion energy on the protruding center pole boron nitride insulator is predictably changed by flow rates and anode potentials; however, it also appears to vary significantly with applied magnetic field strength.

A low-power, linear-geometry Hall plasma source with an open electron-drift

The operating characteristics of a linear geometry Hall plasma source scaled to operate in the 50 to 100 Watt power range are described. Two thruster acceleration channels are implemented-one of alumina and one of boron nitride. Differences in operation with the two channel materials are attributable to differences in the secondary electron emission properties. In either case, however, operation is achieved despite the lack of a closed electron current drift in the Hall direction, suggesting that there is an anomalous axial electron mobility, due to either plasma fluctuations or collisions with the channel wall. Strong low frequency oscillations in the discharge current, associated with the depletion of propellant within the discharge, are seen to appear and vary with changes in the applied magnetic field strength. The frequency of this oscillatory mode is higher than that seen in larger (and higher power) discharges, due to the decreased residence time of the propellant within the channel. Linear geometry Hall thrusters permit simpler magnetic circuit configurations and enable stacking of multiple thrusters to provide modular arrays.

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

Preliminary Plume Characterization of a Low-Power Hall Thruster Cluster

In an effort to understand the technical issues related to running multiple Hall effect thrusters in close proximity to each other, testing of a cluster of four Busek BHT-200-X3 devices has begun in Chamber 6 at the Air Force Research Laboratory. Preliminary measurements have shown that the variations in the discharge currents of the four thrusters are synchronized, possibly due to cross talk through the thruster plumes. Measurements of plasma density, electron temperature, and plasma potential in the thruster plumes obtained using a triple Langmuir probe are presented. Anomalously high electron temperatures were recorded along the centerline of each thruster. Collisionless, magnetosonic shock waves induced by the ion-ion two-stream instability are proposed as a possible cause of the high temperatures. The unperturbed ion velocity distribution along the centerline of a Hall thruster is shown to be unstable and a simple geometric model is presented to illustrate the qualitative changes in plasma properties expected across the proposed shock. Estimates using this model show that relatively large changes in electron temperature are consistent with small changes in electron number density across a shock. Qualitative arguments are presented indicating that collisionless shocks are unlikely to form as a result of clustering multiple thrusters.

Background Pressure Effects on Ion Velocity Distribution Within a Medium-Power Hall Thruster

5 torr). In addition to varying the background pressure, the radial magnetic field of the thruster was varied (by a factor of 2) between low- and high-strength configurations. The low- strength configuration produced large-magnitude anode current oscillations, whereas the high-strength configuration produced small current oscillations. Ion axial velocity distribution function peaks were used to approximate ion energy and, in turn, axial electric field strength. Acceleration profiles of the tested thruster operatingconditionswerecompared.Highbackgroundpressureoperationwasobservedtoshifttheionacceleration regionupstreaminthedischargechannel. Thewidthofthevelocitydistributionscorrelatedstrongly tothemagnetic field strength. The high magnetic field strength configuration produced narrow velocity distribution functions, whereas the low magnetic field strength configuration led to a broad velocity distribution.

Ion Velocity Measurements Within the Acceleration Channel of a Low-Power Hall Thruster

This paper presents axial ion velocity measurements within the acceleration channel of the 200-W Busek Company Inc. BHT-200 laboratory Hall thruster derived from laser-induced fluorescence measurements of the 5d[4]7/2 - 6p[3]5/2 xenon-ion excited-state transition. Acceleration-channel-centerline ion velocities were measured for one nominal and six related cases. These six cases were chosen to be representative of small variations of the applied propellant flow, magnetic field, and discharge potential from the nominal condition. These deviations in operating parameters translate into changes in the plasma density, electron transport, and applied electric field, respectively. The effect of varying the magnetic field, hence influencing the electron transport, is to adjust the location of the internal ion acceleration. Increasing the anode propellant flow, which proportionally increases the plasma density and also influences the electron transport, appears to shift the acceleration upstream. Increasing the discharge potential increases ion acceleration proportionally. Examinations of the fluorescence traces, which have been previously shown to be representative of the ion velocity distributions, are also undertaken. From these data, it is possible to estimate internal axial electric fields and identify regions of ion acceleration and creation.

Laser-induced fluorescence velocity measurements of a diverging cusped-field thruster

Abstract : Measurements are presented of time-synchronized ion velocities at three points within the acceleration channel and in the plume of a diverging cusped field thruster operating on xenon. Xenon ion velocities for the thruster are derived from laser-induced fluorescence measurements of the 5d[4]7/2-6p[3]5/2 xenon ion excited state transition centered at lambda = 834.72 nm. The thruster is operated in a high current mode, where the anode discharge current is shown to oscillate quasi-periodically. A sample-hold scheme is implemented to correlate ion velocities to phases along the current cycle. These time-synchronized measurements show that ionization and acceleration regions of the discharge shift in position over the course of a current cycle.

Interior and exterior laser-induced fluorescence and plasma measurements within a Hall thruster

Abstract : We describe results of a study of emissive-probe-based plasma potential measurements and laser-induced fluorescence velocimetry of neutral and singly ionized xenon in the plume and interior portions of the acceleration channel of a Hall thruster plasma discharge operating at powers ranging from 250 to 725 W. Axial ion and neutral velocity profiles for four discharge voltage conditions (100, 160, 200, and 250V) are measured as are radial ion velocity profiles in the near-field plume. Axial ion velocity measurements both inside and outside the thruster as well as radial velocity measurements outside the thruster are performed using laser-induced fluorescence with nonresonant signal detection. Neutral axial velocity measurements are similarly performed in the interior of the Hall thruster with resonance fluorescence collection. Optical access to the interior of the Hall thruster is provided by a 1-mm-wide axial slot in the outer insulator wall. The majority of the ion velocity measurements used partially saturated fluorescence to improve the signal-to-noise ratio. Probe-based plasma potential measurements extend from 50 mm outside the thruster exit plane to the near anode region for all but the highest discharge voltage condition. For each condition, the axial electric field is calculated from the plasma potential, and the local electron temperature is determined from the difference between the floating and plasma potentials. These two sets of measurements delineate the structure of the plasma and indicate that the ionization and acceleration regions are somewhat separated. Also, these measurements indicate a region of low electric field near the thruster exit, especially at the higher discharge voltages. This region of near constant potential (low electric field) may be a result of oscillations, which enhance the local plasma conductivity.

Background Pressure Eects on Internal and Near-eld Ion Velocity Distribution of the BHT-600 Hall Thruster

Abstract : Presented is a study of the effects of chamber background pressure on the ion axial velocity distribution within the discharge chamber and in the near-field of the Busek BHT-HD-600 xenon Hall effect thruster. Ion velocity distributions were measured along the acceleration channel centerline using laser-induced fluorescence of the 5d[4](sub 7/2)-6p[3](sub 5/2) xenon ion excited state transition. Measurements were taken at the lowest possible chamber background pressure and a pressure that was a factor of two higher. In addition to varying the background pressure, the magnetic field of the discharge chamber was switched between two configurations. The radial magnetic was set to a low and high strength case, which produced two different anode current oscillatory regimes. Ion axial velocity distribution function peaks were used to approximate ion energy and axial electric field strength to compare the acceleration profiles of the tested thruster operating conditions. Increasing background pressure shifted the ion acceleration region upstream in the discharge chamber. The width of the velocity distributions correlated strongly to the radial magnetic field strength. The high magnetic field case data showed narrower peaks.

Ion velocity and plasma potential measurements of a cylindrical cusped field thruster

Measurements of the most probable time-averaged axial ion velocities and plasma potential within the acceleration channel and in the plume of a straight-channeled cylindrical cusped field thruster operating on xenon are presented. Ion velocities for the thruster are derived from laser-induced fluorescence measurements of the 5d[4]7/2-6p[3]5/2 xenon ion excited state transition centered at λ=834.72nm. Plasma potential measurements are made using a floating emissive probe with a thoriated-tungsten filament. The thruster is operated in a power matched condition with 300 V applied anode potential for comparison to previous krypton plasma potential measurements, and a low power condition with 150 V applied anode potential. Correlations are seen between the plasma potential drop outside of the thruster and kinetic energy contours of the accelerating ions.