Mark A. Hopkins

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.

Self-Assembling Array of Magnetoelectrostatic Jets from the Surface of a Superparamagnetic Ionic Liquid

Electrospray is a versatile technology used, for example, to ionize biomolecules for mass spectrometry, create nanofibers and nanowires, and propel spacecraft in orbit. Traditionally, electrospray is achieved via microfabricated capillary needle electrodes that are used to create the fluid jets. Here we report on multiple parallel jetting instabilities realized through the application of simultaneous electric and magnetic fields to the surface of a superparamagnetic electrically conducting ionic liquid with no needle electrodes. The ionic liquid ferrofluid is synthesized by suspending magnetic nanoparticles in a room-temperature molten salt carrier liquid. Two ILFFs are reported: one based on ethylammonium nitrate (EAN) and the other based on EMIM-NTf2. The ILFFs display an electrical conductivity of 0.63 S/m and a relative magnetic permeability as high as 10. When coincident electric and magnetic fields are applied to these liquids, the result is a self-assembling array of emitters that are composed entirely of the colloidal fluid. An analysis of the magnetic surface stress induced on the ILFF shows that the electric field required for transition to spray can be reduced by as much as 4.5 × 10(7) V/m compared to purely electrostatic spray. Ferrofluid mode studies in nonuniform magnetic fields show that it is feasible to realize arrays with up to 16 emitters/mm(2).

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.

Assessment of plasma impedance probe for measuring electron density and collision frequency in a plasma with spatial and temporal gradients

Numerical simulations and experimental measurements were combined to determine the ability of a plasma impedance probe (PIP) to measure plasma density and electron collision frequency in a plasma containing spatial gradients as well as time-varying oscillations in the plasma density. A PIP is sensitive to collision frequency through the width of the parallel resonance in the Re[Z]-vs.-frequency characteristic, while also being sensitive to electron density through the zero-crossing of the Im[Z]-vs.-frequency characteristic at parallel resonance. Simulations of the probe characteristic in a linear plasma gradient indicated that the broadening of Re[Z] due to the spatial gradient obscured the broadening due to electron collision frequency, preventing a quantitative measurement of the absolute collision frequency for gradients considered in this study. Simulation results also showed that the PIP is sensitive to relative changes in electron collision frequency in a spatial density gradient, but a second broad...

Active Stabilization of a Magnesium Hall Thruster in Constant Voltage Mode

The direct-evaporation magnesium Hall thruster exhibits thermal runaway when operated in voltage-limited mode. This paper reports on a method of controlling runaway mass flow by manipulating the pre-set current and voltage limits of the anode power supply. By choosing proper voltage and current limits, voltage-limited operation was achieved for periods of greater than 9 minutes without any active control scheme. Natural cyclic transitions into and out of voltage-limited mode with a period of a few minutes were observed and are attributed to a time delay between onset of power increase and temperature rise in the propellant. By adjusting the current and voltage limits, both the amplitude and the frequency of the cyclic oscillations were reduced such that the thruster was semi-stable. From this semi-stable operating, point minor adjustments in the current to the electromagnets allowed true stabilization of the thruster discharge. Using this control scheme voltage-limited operation at 225 V and 5 A was sustained for more than an hour.

Evaluation of a Plasma Impedance Probe in a Time-varying Non-uniform Plasma

Numerical simulations and experimental measurements were combined to determine the ability of a PIP to measure plasma density and electron collision frequency in a plasma containing spatial gradients as well as time-varying oscillations in the plasma density. Collision frequency was to be measured by the width of the parallel resonance in the real part of plasma impedance-vs.-frequency, while plasma density was to be measured using the zero-crossing of the imaginary impedance-vs.-frequency at parallel resonance. Simulations of the probe characteristic in a plasma gradient indicated that the broadening the real part of plasma impedance due to the spatial gradient obscured the broadening due to electron collision frequency. Simulation results also showed that it may be possible to measure relative changes in electron collision frequencies in a spatial plasma gradient, but the broadening effect of the time-varying oscillations made collision frequency measurements impossible. The time-varying oscillations had the effect of causing multiple zero-crossings in the imaginary part of impedance at parallel resonance. Results of experiments and simulations indicated that the lowest-frequency zero-crossing represented the lowest plasma density in the oscillations and the highest-frequency zero-crossing represented the highest plasma density in the oscillations, thus the PIP probe was found to be an effective tool to measure both the average plasma density as well as the max and min densities due to temporal oscillations.

Demonstration of an Automated Mass Flow Control System for Condensable Propellant Hall-effect Thrusters

An automated mass flow control system for condensable propellant thrusters was demonstrated. The control system has the ability to arrest thermal runaway in a directevaporation feed system and stabilize the discharge current during voltage-limited operation of a magnesium-fueled Hall-effect thruster. The system supplemented plasma discharge heating at the evaporative anode with a resistive heater located behind the anode. Steadystate operation at constant voltage with discharge current variations less than 0.35 A was demonstrated for 60 minutes. A thrust of 44 mN was measured at a discharge voltage of 300 volts at 6 Amps, yielding a thrust-to-power of 24.4 mN/kW. A thrust of 50 mN was measured at a discharge voltage of 300 volts at 7 Amps, also yielding a thrust-to-power of 23.8 mN/kW. For a thruster operating at 2.1 kW, the steady-state supplemental heater power was 136 watts representing only 6% of the total system power.

Performance Comparison between a Magnesium- and Xenon-fueled 2 kW Hall Thruster

The performance metrics of a 2-kW-class thruster operated using magnesium propellant was measured and compared to the performance of the same thruster operated using xenon propellant. It was found that the magnesium thruster has thrust ranging from 34 mN at 200 V to 39 mN at 300 V with 1.7 mg/s of propellant. It was found to have 27 mN of thrust at 300 V using 1.0 mg/s of propellant. The thrust-to-power ratio ranged from 24 mN/kW at 200 V to 18 mN/kW at 300 volts. The specific impulse was 2000 s at 200 V and upwards of 2700 s at 300 V. The anode efficiency was found to be ~23% using magnesium which is substantially lower than the 40% anode efficiency of xenon at approximately equivalent molar flow rates. A comparison of the angular ion current distribution in the plasma beam revealed that the magnesium-fueled thruster has a much larger beam divergence than the xenon-fueled thruster.