Richard E. Huffman

Ultrahigh-Speed Imaging of Hall-Thruster Discharge Oscillations With Krypton Propellant

The discharge oscillations of a 200-W Hall thruster, with krypton propellant, was captured optically with an ultrahigh-speed camera providing information up to 500 kHz. The sequential images provide a temporal 2-D description of the plasma field, which illustrate the direct emission (i.e., visible light emitted by the plasma discharge) of the plasma increasing and decreasing within the thruster channel. These periodic fluctuations of intensity have a frequency of 31 kHz, which is 10% less than the measured breathing-mode frequency utilizing xenon propellant under the same conditions.

Experimental Study on the Induced Velocity of a Three Potential Sliding Discharge DBD Actuator.

A comparison between a sliding discharge DBD actuator with grounded and AC biased sliding electrode driven at kHz frequency is presented. The induced velocity in the bulk ow has been recorded with PIV and compared for multiple potential splits between two test con gurations. The applied potential to the sliding electrode a ects the initial jet velocity and thickness. Peak jet velocity increases with increasing bias on the sliding electrode until that electrodes bias reaches the ionization thresh-hold of the bulk ow. Any further increase in potential results in a small secondary discharge which acts as a boundary layer trip, then steers the jet away from the wall. The highest induced velocity improvement observed was, for a 9 kV sliding electrode bias at 41 mm downstream, 22% greater than the base case and the induced wall jet thicknesses varied from 21% taller for the zero bias con gurations to 10% thinner than the base case for the 8 kV sliding electrode bias.

An Experimental Investigation of Accurate Particle Tracking in Supersonic, Rarefied Axisymmetric Jets

Research indicates that atomized liquid seed particles will be small enough to accurately track the flow dynamics of rarefied supersonic nozzles. However, the accuracy of the particle tracking must be quantified. This article describes the process used to determine the accuracy of particle based velocimetry measurements. Measurements taken with seed particle based tracking techniques are compared to a molecular tracer technique called molecular tagging velocimetry to confirm if the particles accurately track the flow. Particle based velocity measurements are confirmed to track the mean flow features to within 5% of the molecular tracer based velocimetry technique, however the particle based measurements do not accurately track shocks.

Ultrahigh Speed Images of Hall Thruster Azimuthal Instabilities

Visible emission of the azimuthal plasma instabilities were observed within the plume of a low-power Hall thruster utilizing ultrahigh speed imaging. The captured images provide time-dependent and spatial information on the azimuthal instability in the axial-radial plane of the thruster, which have not been well characterized. These images are then compared with observations made based on their probe measurements of azimuthal instabilities termed spoke instabilities. Data from the images show that the spoke instabilities are more complex than originally described and have additional features not detectable with probe measurements in a single plane.

µ-Hall Effect Thruster Characterization

Results of two methods to characterize a novel micro-Hall effect thruster are presented. The voltage-current characteristics of the thruster during steady state operation are reported. The exhaust plume is characterized using Faraday probe measurements and the beam efficiency is reported. Additionally, discussion is provided on the operational power limits for this thruster design, based on observed degradation to the boron nitride insulating material inside the discharge channel at power levels above the design point.

Development and Verification of a Pseudospectral Forward Model for Collisional Plasma Diagnostics

Plasma diagnostics such as microwave interferometry require a forward model to relate the measurement (phase and amplitude shift) to plasma properties (electron density and eective electron-neutral collision frequency). In the present work a two-way wave equation for the electric eld of an interferometer beam is discretized by a pseudo-spectral method of lines approach, and stabilized with Lanzcos -factor smoothing. A Chebyshev pseudospectral method is employed that is suitable for modeling a non-periodic domain. This numerical solution of the forward model also allows arbitrary material property gradients to be accurately treated in contrast to simpler analytical approaches (e.g. Appleton-Hartree). The phase and amplitude shift of the interferometer beam is assumed to be caused only by electron currents and electron-neutral collisions in a Lorentz model of the electron motion. The numerical implementation is formally veried using the method of manufactured solutions (MMS). The capabilities of a computer algebra system (CAS) are leveraged throughout for code generation and verication. The model is applied to diagnosing a dielectric barrier discharge in air.

The effect of plasma on the flow features of an axisymmetric jet

The effect of a radio frequency capacitively charged plasma field on a series of axisymmetric flowfields is investigated in this paper. Particle image velocimetry is utilized to examine the variation of compressibility on axisymmetric jets of Nitrogen gas through a series of perfectly matched converging diverging nozzles and varying pressure ratios. Mean flow velocity measurements as well as turbulent fluctuating parameters are examined to determine differences in the flow between baseline (plasma off) and plasma on. Additionally, a series of more turbulent axisymmetric flowfields, created by developing flow in a constant diameter pipe, are investigated for plasma-turbulence interactions. Finally, electrode designs are modified in an attempt to increase plasma interaction with the flowfield. Results generally indicate that the plasma interaction with the shear layer is responsible for heating the shear layer and delaying jet-core breakdown.