Taylor S. Matlock

Development and Initial Testing of a Magnetically Shielded Miniature Hall Thruster

The scaling of magnetically shielded Hall thrusters to low power is investigated through the development and fabrication of a 4-cm Hall thruster. During initial testing, the magnetically shielded miniature Hall thruster was operated at 275 V discharge voltage and 325-W discharge power. Inspection of the channel walls after testing suggests that the outer discharge channel wall was successfully shielded from high-energy ion erosion while the inner channel wall showed evidence of weaker shielding, likely due to magnetic circuit saturation. Scanning planar probe measurements taken at two locations downstream of the thruster face provided ion current density profiles. The ion current calculated by integrating these data was 1.04 A with a plume divergence half-angle of 30°. Swept retarding potential analyzer measurements taken 80-cm axially downstream of the thruster measured the most probable ion voltage to be 252 V. The total thruster efficiency was calculated from probe measurements to be 43% (anode efficiency of 59%) corresponding to a thrust of 19 mN at a specific impulse of 1870 s. Discharge channel erosion rates were found to be approximately three orders of magnitude less than unshielded Hall thrusters, suggesting the potential for a significant increase in operational life.

Magnetically Shielded Miniature Hall Thruster: Performance Assessment and Status Update

The magnetically shielded miniature Hall thruster, originally tested at the University of California, Los Angeles, underwent performance validation experiments at the Jet Propulsion Laboratory. The thruster was operated over a range of discharge voltages, from 150 V – 300V, and currents, from 1 A – 2.3 A. It was discovered that the thruster operated in two distinct modes which were dependent on the thruster’s temperature: a “jet” mode and a “diffuse” mode. At the nominal condition of 275 V and 1.2 A in the jet mode, a thrust of approximately 12 – 13 mN was measured by a thrust stand with an anode efficiency of approximately24%. At the same nominal conditions, the diffuse mode showed a thrust of 11 – 12 mN and an anode efficiency of approximately 21%. Characterization of the plume in both operating modes was accomplished using a shielded Faraday probe, a retarding potential analyzer, and an ExB probe. Discharge current oscillations on the order of 2.5 – 4 times the mean current were observed during jet mode operation, while the oscillations in the diffuse mode were on the order of 20% of the mean current. Results from the plume characterization, post-operation discharge channel inspection, and discharge oscillations, combined with the temperature-dependent mode shift, suggest that changes to the magnetic field strength and topology caused by saturation of the thruster’s magnetic circuit may be occurring at elevated operating temperatures.

A dc plasma source for plasma–material interaction experiments

A new device has been constructed for the investigation of interactions between engineered materials and a plasma in regimes relevant to electric propulsion and pulsed power devices. A linear plasma source, consisting of a hollow cathode, cylindrical anode, and axial magnetic field, delivers a 3 cm diameter beam to a biased target 70 cm away. The ion energy impacting the surface is controlled by biasing the sample from 0 to 500 V below the local plasma potential. This paper discusses the major aspects of the plasma source design and presents measurements of the plasma parameters achieved to date on argon and xenon. Experiments show that splitting the gas injection between the hollow cathode and the anode region provides control of the discharge voltage to minimize cathode sputtering while providing ion fluxes to the target in excess of 1021 m−2 s−1. Sputtering rate measurements on a non-textured molybdenum sample show close agreement with those established in the literature.

View factor modeling of sputter-deposition on micron-scale-architectured surfaces exposed to plasma

The sputter-deposition on surfaces exposed to plasma plays an important role in the erosion behavior and overall performance of a wide range of plasma devices. Plasma models in the low density, low energy plasma regime typically neglect micron-scale surface feature effects on the net sputter yield and erosion rate. The model discussed in this paper captures such surface architecture effects via a computationally efficient view factor model. The model compares well with experimental measurements of argon ion sputter yield from a nickel surface with a triangle wave geometry with peak heights in the hundreds of microns range. Further analysis with the model shows that increasing the surface pitch angle beyond about 45° can lead to significant decreases in the normalized net sputter yield for all simulated ion incident energies (i.e., 75, 100, 200, and 400 eV) for both smooth and roughened surfaces. At higher incident energies, smooth triangular surfaces exhibit a nonmonotonic trend in the normalized net sput...

Validation of a Plasma-Facing Surface Sputtering and Deposition View Factor Model

A computational model that will be used to study sputtering, deposition, and erosion of surfaces exposed to low energy, low density plasma environments has been developed and validated. Comparisons with sputter yield curve fits and experimental data show favorable agreement and indicate that the model can accurately capture the effects of incidence angle and energy on sputtering behavior. COMSOL simulations were also used to validate view factor and shadowing calculations. This computational model will be used in the future to test the effects of surface architecturing on gross sputter yield and surface erosion rate, and will serve to drive experimental efforts at UCLA.

An Investigation of Low Frequency Plasma Instabilities in a Cylindrical Hollow Cathode Discharge

A new plasma-material interactions testbed (refered to as the Pi facility) is under development at UCLA, which uses a 250 A lanthanum hexaboride (LaB6) hollow cathode coupled to a cylindrical copper anode to provide a plasma environment representative of electric propulsion (EP) and pulsed power (PP) devices to a target material nearly 1 m downstream. A steady ion bombardment energy for long duration testing is desired, which requires a steady (known) plasma potential adjacent to the target. Time-resolved plasma potential measurements reveal oscillations of ∼20% of the discharge voltage occuring near the anode exit at a fundamental frequency of 10-20 kHz, which is between the local ion cyclotron and lower hybrid frequencies (fci < f < flh). A 70-90 kHz oscillation dominates on-axis and couples non-linearly with the m=1, fundamental mode and a ∼60 kHz fluctuation that is localized just off-axis. The present article investigates these oscillations through intrusive probing across a range of discharge paramaters.