Dean R. Massey

Progress on the Development of a Direct Evaporation Bismuth Hall Thruster

It has been demonstrated that using segmented anodes for thermal regulation of anodes is possible by controlling where the discharge current attaches to. Thrust, Isp and efficiency measurements were taken on a segmented Hall thruster in order to ascertain what effect moving the discharge current attachment has on thruster performance. Overall, very little change in thrust, specific impulse and efficiency were measured across the operating spectrum when running on xenon. Thermal measurements were also taken but it was found that anode power density needed to be substantially increased to achieve the temperatures necessary for operation on bismuth. The anode face area was subsequently reduced and using a unique dualpropellant distributor, this work reports on experiments to use a xenon discharge as a “jump start” mechanism to provide waste heat necessary to initiate direct bismuth evaporation. Using the shim electrodes and magnetic fields for temperature control, the thruster is operated entirely on bismuth after a xenon warm-up stage.

Development of a Vaporizing Liquid Bismuth Anode for Hall Thrusters

Bismuth metal vapor Hall thrusters may have superior performance and economic characteristics when compared to xenon. From increased efficiency to reduced propellant and testing costs, bismuth seems to have a bright future. Of paramount importance when developing a practical bismuth device is the mechanism by which the propellant flow is controlled. This paper reports on an effort to use waste heat from the thruster to control the evaporation of a reservoir of liquid bismuth maintained within the discharge chamber. Research done thus far indicates that mass flow control can be achieved via a segmented anode configuration that serves as a thermostat to control input power into the bismuth reservoir. Thermal modeling has indicated that sufficient thermal gradients can be maintained between anode segments. Laboratory testing on xenon development thrusters validates the scheme to control reservoir temperature through discharge current sharing.

Development of a Magnesium and Zinc Hall-effect Thruster

§This paper describes what are believed to be the first demonstrations of Hall-effect thrusters operating on magnesium and zinc propellant. Pathfinding experiments were performed using consumable anodes that were machined from solid magnesium and zinc, which sublimated under the heat load from the discharge plasma and delivered propellant gas to the thruster. Therefore the magnesium and zinc anodes served as the acceleration electrode and also served as the propellant supply. A retarding potential analyzer was used to obtain plume diagnostics during early operation of the experiments, showing reasonable acceleration of the propellant ions. Two main issues were expected and encountered with the solid magnesium and zinc anodes – 1) the zinc anode displayed localized melting causing liquid zinc to accumulate in the discharge channel and 2) the crude scheme did not feature any means to actively control the sublimation rate of the metal propellant. A new porous anode with internal propellant reservoir was designed and built that could be refilled with either propellant, eliminating liquid intrusion into the discharge channel. A scheme developed earlier for bismuth thrusters was employed wherein shim anodes were implemented to shift discharge current to and from the main anode to control the main anode temperature and hence the metal propellant sublimation rate. Results are reported showing stable operation of a thruster using a porous anode with magnesium propellant for more than 100 minutes. Also demonstrated was the ability of the shim anode scheme to actively control the propellant mass flow rate.

Development and Testing of a Prototype Bismuth Cathode for Hall Thrusters

Using bismuth in place of gases such as xenon for Hall thruster propellant could potentially offer both physical and economical gains. As research continues to develop Hall thrusters that are fueled with bismuth, it will become advantageous to maintain one propellant supply rather than multiple supplies for the anode and cathode. The recent development of a bismuth Hall thruster at Michigan Tech, operated using a xenon LaB6 cathode, provided a motive to explore the feasibility of developing an entire bismuth system. This paper provides a background on the development and operation of a bismuth vapor LaB6 cathode. Comparisons of operating parameters are provided for the cathode running on xenon and bismuth propellants along with a description of the mass flow technique used. Complications in determining and controlling the mass flow rate are presented as well.

Bismuth Hollow Cathode for Hall Thrusters

B ISMUTH has several qualities that make it well suited for development as a Hall-thruster propellant. When compared with more conventional propellants such as xenon, bismuth holds significant advantages from both an energetics (lower ionization energy) [1] and cost standpoint. In addition, there are significant ground-test-facility cost savings, because bismuth does not require the use of cryogenic pumps. Unlike traditional propellants, bismuth is solid at room temperature; thus, the exhausted bismuth solidifies on the room-temperature vacuum chamber walls, and consequently the entire vacuum chamber becomes an effective pumping surface. With this in mind, operating a high-power bismuth Hall thruster would require only enough pumping speed to keep up with facility outgassing and minor vacuum leaks. However, there are some difficulties that need to be addressed when using a condensable propellant. Some of the issues include sustaining elevated temperatures for bismuth evaporation, regulating bismuth mass flow, mechanical limitations inherent with using refractory metal components, and bismuth plating of thruster and spacecraft components. Recently, there have been three new programs to develop bismuth Hall thrusters, with the first successful demonstration occurring in the spring of 2005 [2–4]. In this work, the bismuth thruster was operated using a xenonLaB6 cathode. The encouraging results of the bismuth thruster motivated a study to examine the feasibility of an all-bismuth system using a bismuth cathode. In addition to all of the physical and economical gains, it would be advantageous to incorporate a bismuth cathode to eliminate the need for multiple propellant supplies on an eventual flight unit. In 2005, a functioning prototype bismuth cathode was developed and a limited number of operating characteristics were reported [5]. The primary goals of the present research were to evaluate the operating characteristics of a bismuth LaB6 cathode at different mass flow rates, compare bismuth data with xenon and krypton performance, and to reduce the amount of power required for cathode operation.

Effect of Segmented Anodes on the Beam Profile of a Hall Thurster

The effect of the addition of shim electrodes on the discharge in a Hall thruster was investigated. A BPT-2000 magnetic circuit was retrofitted with a segmented anode with thermal measurement capabilities. Current was shared between shims and main anode by changing the voltage on the shim. A Faraday probe was used to measure ion current density. The acquired beam profiles were compared to develop a relationship between current sharing and thruster plume divergence.

Performance and Active Thermal Control of a 2-kW Hall Thruster with Segmented Electrodes

The performance and plume characteristics of a 2-kW Hall thruster with segmented anodes for thermal control of the main anode are characterized. Diagnostics performed included thrust, specific impulse, and efficiency, as well as the ion current density in the plume. The apparatus included an inverted-pendulum thrust stand for performance measurements and a shielded Faraday probe for ion current density characterization. Data were taken at several operating points and at several ratios of current to the main anode vs the shim electrodes. The data show a general increase in centerline current density, thrust, specific impulse, and efficiency as discharge current is moved to the shims. Thermal measurements show up to 50° C control authority in the main anode temperature.

Performance Characteristics of a LaB6 Bismuth Cathode for use with Hall Thrusters

*† ‡ Interest in the development of bismuth Hall thrusters has brought about a need to examine cathodes that are capable of running on bismuth at relatively high discharge currents. Recently it has been shown that it is possible for a LaB6 hollow cathode to function using bismuth as a propellant. For this paper, an investigation into the operating characteristics of a bismuth cathode running at constant mass flow was performed. Also reported is a comparison of the cathode using bismuth propellant with both krypton and xenon propellant at the same mass flow and molar flow rates.

Performance Characterization and Ion Energy Analysis of a 2-kW Hall Thruster with Segmented Anodes

The goal of this paper is to describe the change in performance and ion energy distribution in a 2-kW Hall thruster operated with segmented anodes. The effect on the performance and ion energy distribution as discharge current is shifted from the main anode to the shim electrodes is examined. Performance was largely unaffected by shifting current from the main anode to the shims. A water cooled ExB probe was utilized to determine the ion energy distribution. A change in the dominant singly charged ion energy was observed when the shim electrode voltage was varied. Measurable increases in singly charged xenon velocities correlated with slight performance gains. The thruster geometry examined here has been proposed for control of the temperature of a bismuth propellant feed system. No significant change in thrust, Isp, or efficiency indicates that a bismuth thruster could be operated and thermally controlled using segmented anodes without significant impact on the performance.

Vertically Aligned Carbon Nanotubes as the Sputter Resist in Space Propulsive Systems

Two-types of vertically aligned multi-walled carbon nanotubes (VA-MWNTs) are evaluated as the protective coatings against ion erosion in electric propulsion systems. A series of experiments have been conducted to understand the erosion rate and erosion mechanism of these VA-MWNTs. These experiments were carried out with Xe propellant at an ion current density of 5 mA/cm 2 . We found that the erosion rates of both types of VA-MWNTs were changing with time. Such a nonlinear erosion process is explained according to a possible erosion mechanism.