Jason M. Makela

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 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.

Regenerable Field Emission Cathode for Spacecraft Neutralization

This research investigates the discharge characteristics of a field emission cathode for use in electric propulsion that has the ability to be regenerated when the emitter tip becomes damaged. Emitter tip regeneration is achieved by taking advantage of Taylor cone formation from an operating liquid―metal ion source. Tip formation is accomplished by solidifying, or quenching, the ion-emitting cone to preserve the sharp protrusion so that it can then be used for electron emission. Electron emission I-V curves were taken after tips were formed by quenching the liquid―metal ion source at ion discharge currents ranging from 1 to 25 μA. Fowler―Nordheim modeling was then used to estimate the emitter tip radii of each quenched liquid―metal ion source. Results of the Fowler―Nordheim modeling were promising, showing the ability to regenerate tips and to control the features of the resulting tips by varying the ion current during the quench process. The set of experiments that are reported demonstrated the regeneration process of emitter tip radii ranging from approximately 30―45 nm from a tip quenched at 2 μA down to tip radii of 15―22 nm when quenched at 25 μA.

Re-generable Field Emission Cathodes for Low-Power Electric Propulsion

The research reported here explores the possibility of field-emission cathodes for use in EP that have the ability to be re-generated when the emitter tip becomes damaged. The method for re-generation takes advantage of Taylor cone formation in an effort to solidify, or quench, an operating liquid-metal-ion-source (LMIS) to preserve the sharp Taylor cone tip for use as a field-emission cathode. Electron emission I-V curves were taken after Taylor cones were formed by quenching the LMIS at different discharge currents. It is shown that quenching the LMIS at as low as 2 μA produced an increase in electron discharge current as compared with the unquenched emitter, 53 nA as compared with 25 nA at an extraction voltage of 2.7 kV. When the ion emission current at quench was increased to 3 μA and then 25 μ μ μ μA, the discharge that was measured increased to 210 nA for the 3 μA emitter and 1.02 μA for the 25 μ μ μ μA emitter at an extraction voltage of 2.7 kV. Fitting the electron emission I-V characteristics using Fowler-Nordheim theory indicated tip radii as small as 80 nm were formed during the LMIS quenching process.

Operating Characteristics of a Re-generable Field Emission Cathode for Low-power Electric Propulsion

This research investigates the discharge characteristics of a field-emission cathode for use in Electric Propulsion (EP) that has the ability to be re-generated when the emitter tip becomes damaged. Emitter tip re-generation is possible by using Taylor cone formation from an operating liquid-metal ion source (LMIS) in an effort to solidify, or quench, the ion emitting cone to preserve the sharp protrusion so that it can then be used for electron emission. Multiple electron emission I-V curves were taken after tips were formed by quenching the LMIS at ion discharge currents ranging from 1 to 25 μA. Fowler-Nordheim modeling was then used to estimate the emitter tip radii of each quenched emitter and showed that emitter tip radii decreased from about 45 nm from a tip quenched at 2 μA down to about 15 nm when quenched at 25 μA. Also, a 46 hour test was done using a single emitter that was quenched at 15 μ μ μ μA. Electron I-V curves were taken at the start, middle, and end of the test that showed no degradation in tip performance.

Mass Flow Control in a Magnesium Hall-effect Thruster

*† ‡ § , The research reported in this paper examined methods of operating a Hall-effect thruster on solid magnesium propellant at discharge voltages in excess of 200 volts. Mg vapors were supplied through evaporation of propellant feedstock within the thruster anode. A constantcurrent discharge was used to achieve steady evaporation from three different anodes, each having a different vapor escape area. It is shown that the stable discharge voltage can be increased by reducing the vapor escape area of the source. It was shown that by decreasing the open area of the anode from 2.77 x 10 -4 m 2 to 6.93 x 10 -5 m 2 the maximum discharge voltage of the thruster increased from 184 volts to 341 volts. The ability of inert (nonevaporative) shim anodes to stabilize the thruster by increasing or decreasing heat flux to the main anode was also shown.

Re-generable Field Emission Cathodes Part I: Surface Morphology of Emitter Apex

This is the first part of a two-part paper that focuses on a field-emission cathode for use in Electric Propulsion (EP) that has the potential for very long lifetime due to its ability to be re-generated when the emitter tip become damaged. The field-emitting tips were formed by the application of an ion-extracting electric potential applied to a heated indium-coated tungsten needle, known as a liquid metal ion source (LMIS). The LMIS is then cooled, freezing in a solid nanotip at the apex. When the modified needle was then subjected to electron-extracting potentials stable and long-lived electron emission was observed. The focus of this investigation was to operate and quench a LMIS at ion emission currents from 2 to 25 A to acquire micrographs of the surface morphology as a function of the ion emission current at quench. The LMISs were also operated at selected ion emission currents for 1’s to 10’s of seconds between quenching to observe the temporal change in emitter tip surface morphology as a function of ion emission current. Micrographs of the quenched emitter tips yielded Taylor-cone-shaped structures. The quenched emitters exhibited multiple nanoprotrusions on the surface of the micro-scale Taylor cone, which were capable of electron field emission.

Mobility Studies of a Pure Electron Plasma in Hall Thruster Fields

An electron trapping apparatus was constructed in order to study electron dynamics in the defining electric and magnetic fields of a Hall-effect thruster. The approach presented here decouples the cross-field mobility from plasma effects by conducting measurements on a pure electron plasma in a highly controlled environment. Dielectric walls are removed completely eliminating all wall effects; thus, electrons are confined solely by a radial magnetic field and a crossed, independently-controlled, axial electric field that induces the closed-drift azimuthal Hall current. Electron trajectories and cross-field mobility were examined in response to electric and magnetic field strength and background neutral density. Without wall effects or neutral plasma effects mobility is presumed to follow the classical mobility model. In the present research, measurement techniques are investigated, and results are verified against the classical model. Preliminary findings suggest that that the apparatus and techniques used will be valid for mobility studies in more complex field environments.