Thomas E. Markusic

Thrust stand for electric propulsion performance evaluation

An electric propulsion thrust stand capable of supporting testing of thrusters having a total mass of up to 125kg and producing thrust levels between 100μN to 1N has been developed and tested. The design features a conventional hanging pendulum arm attached to a balance mechanism that converts horizontal deflections produced by the operating thruster into amplified vertical motion of a secondary arm. The level of amplification is changed through adjustment of the location of one of the pivot points linking the system. Response of the system depends on the relative magnitudes of the restoring moments applied by the displaced thruster mass and the twisting torsional pivots connecting the members of the balance mechanism. Displacement is measured using a noncontact, optical linear gap displacement transducer, and balance oscillatory motion is attenuated using a passive, eddy-current damper. The thrust stand employs an automated leveling and thermal control system. Pools of liquid gallium are used to deliver ...

Performance of a low-power cylindrical hall thruster

Recent mission studies have shown that a Hall thruster which operates at relatively constant thrust efficiency (45-55%) over a broad power range (300W - 3kW) is enabling for deep space science missions when compared with slate-of-the-art ion thrusters. While conventional (annular) Hall thrusters can operate at high thrust efficiency at kW power levels, it is difficult to construct one that operates over a broad power envelope down to 0 (100 W) while maintaining relatively high efficiency. In this note we report the measured performance (I(sub sp), thrust and efficiency) of a cylindrical Hall thruster operating at 0 (100 W) input power.

Ablative Z-Pinch Pulsed Plasma Thruster

The design, performance, and basic features of ablative pulsed plasma thrusters based on the z-pinch configuration are discussed through a series of experiments and numerical simulations. The motivation stems from the promise of the z-pinch configuration for increasing the thrust-to-power ratio and mass utilization efficiency above those of ablative thrusters with a conventional rectangular geometry. The performance of a series of ablative z-pinch pulsed plasma thrusters is characterized using a swinging-gate thrust stand and mass ablation measurements. The performance measurements are complemented by additional experimental diagnostics (current monitoring and high-speed photography) and numerical modeling in order to gain an understanding of the acceleration mechanism and provide direction for future design iterations. Three iterations in the design of the thruster result in thrust-topower ratios ranging from 12‐45 µN/W, with specific impulse and thrust efficiency values spanning 240‐760 s and 2‐9%, respectively. Numerical simulations show reasonable quantitative agreement with the experimental data and predict the existence of an optimal thrust chamber aspect ratio, which maximizes the thrust-to-power ratio.

Measurements of current sheet canting in a pulsed electromagnetic accelerator

The phenomenon of current sheet canting in pulsed electromagnetic accelerators is the departure of the plasma sheet (that carries the current) from a plane that is perpendicular to the electrodes to one that is skewed or tipped. Developing an understanding of current sheet canting is important because it can detract from the propellant sweeping capabilities of current sheets and, hence, negatively impact the overall efficiency. In the present study, photographic, magnetic, and laser-interferometric diagnostics were implemented to study current sheets in an experimental pulsed electromagnetic accelerator, using a variety of propellants and pressure levels. The interferometric measurements were used to quantify the canting angle during the stable phase of propagation. The canting angle was found to increase with the atomic mass of the propellant and the current sheet was always found to tilt such that the anode current attachment leads the cathode attachment.

Numerical Modeling of a Pulsed Electromagnetic Plasma Thruster Experiment

We assessed the behavior and capabilities of a two-dimensional magnetohydrodynamic modeling code with a new tabular equation of state model for high-power pulsed plasma thrusters. The numerical results enabled detailed interpretation of the experimental data from a ∼10-kJ pulsed coaxial plasma accelerator with ablative breech insulator polytetrafluoroethylene. Our analysis showed that the initial conditions and ablation rate are critical to the behavior of the current sheet evolution in the numerical model. Qualitative agreement was obtained between the two-dimensional numerical model and experimental results, but there were significant quantitative discrepancies. The two-dimensional model indicates that some of the current remains trapped near the breech, because of the ablation of material off the breech boundary, which appears to be supported by the experimental data. The sensitivity of the model to initial prefill density (small in comparison with the ablated mass) and ablation rate suggests that future modeling efforts involving high-power ablative fed pulsed plasma thrusters using the approach discussed in the paper should include a first-principles ablation model, together with density, temperature, and ablation rate estimates from the experimental apparatus.

Integrated Liquid Bismuth Propellant Feed System

A prototype bismuth propellant feed and control system was constructed and operated in conjunction with a propellant vaporizer. An electromagnetic pump was used in this system to provide fine control of the hydrostatic pressure, and a new type of in-line flow sensor was developed in an attempt to provide an accurate, real-time measurement of the mass flow rate. High-temperature material compatibility was a driving design requirement for the pump and flow sensor, leading to the selection of Macor for the main body of both components. Post-test inspections of both components revealed no degradation of the material. In separate proof-of-concept experiments, the pump produced a linear pressure rise as a function of current that compared favorably with theoretical pump pressure predictions, with a pressure of 10 kPa at 30 A. Preliminary flow sensor measurements have been made at a bismuth flow rate of 6 mg/s ± 6%. A real-time controller was successfully used to control the entire system, simultaneously monitoring all power supplies and performing data acquisition duties.

Effects of ignition on discharge symmetry in gas-fed pulsed plasma thrusters

We present the results from a new diagnostic tool for investigating the efiects of ignition on the discharge symmetry in gas-fed pulsed plasma thrusters (GFPPTs). Using a fast-framing camera, up to 16 individual pictures of a single GFPPT discharge can be captured at frequencies as high as 20 MHz. This high framing rate capability allows the discharge initiation and current sheet formation to be visualized with a large degree of temporal resolution. Varying the frame rate, number and size of pictures per discharge, as well as the trigger delay sent to the camera allowed multiple discharges at difierent stages to be visualized. Various conflgurations of GFPPT electrode geometry and discharge ignition systems were photographed in operation. It is shown that the large surface ∞ash-over ignitors used in early GFPPT designs are unreliable in providing a constant, symmetric discharge initiation. Smaller semi-conductor spark gap ignitors used in later GFPPT designs have demon

Electromagnetic Pumps for Liquid Metal-Fed Electric Thrusters

Prototype designs of two separate pumps for use in electric propulsion systems with liquid lithium and bismuth propellants are presented. Both pumps are required to operate at elevated temperatures, and the lithium pump must additionally withstand the corrosive nature of the propellant. Compatibility of the pump materials and seals with lithium and bismuth were demonstrated through proof-of-concept experiments followed by post-experiment visual inspections. The pressure rise produced by the bismuth pump was found to be linear with input current and ranged from 0-9 kPa for corresponding input current levels of 0-30 A, showing good quantitative agreement with theoretical analysis.

Galium Electromagnetic (GEM) Thruster Concept and Design

We describe the design of a new type of two-stage pulsed electromagnetic accelerator, the gallium electromagnetic (GEM) thruster. A schematic illustration of the GEM thruster concept is given. In this concept, liquid gallium propellant is pumped into the first stage through a porous metal electrode using an electromagnetic pump. At a designated time, a pulsed discharge (approx. 10-50 J) is initiated in the first stage, ablating the liquid gallium from the porous electrode surface and ejecting a dense thermal gallium plasma into the second state. The presence of the gallium plasma in the second stage serves to trigger the high-energy (approx. 500 J), second-stage pulse which provides the primary electromagnetic (j x B) acceleration.

Visualization of current sheet evolution in a pulsed plasma accelerator

High-speed photography and magnetic field probes were used to visualize and study the formation and propagation of current sheets in a pulsed plasma accelerator. Magnetic field measurements complement photographic records, as the latter indicate the location of the plasma but not necessarily current, whereas magnetic field data provide an unambiguous picture of the current location, but do not directly yield the location of the plasma. The observed current sheet evolution was found to be rich in features and has produced fundamental insight into the phenomena of current sheet canting and trailing wake formation.