Plasma-Material Interactions for Electric Propulsion and Pulsed Power

Gary Li Chris Dodson Taylor Matlock Dan Goebel Richard Wirz

Degradation of plasma-facing materials is a significant problem in electric propulsion, pulsed power technologies, and fusion energy experiments. To reduce material degradation, new micro-engineered materials are being developed that reduce sputtering and thermal stress through an increased surface volume and area exposed to the heat flux, along with the use of micron-sized dendrites, nodules, or fibers that can deform independently.

A systematic investigation is underway at UCLA to characterize the plasma-material interactions (PMI) of these materials under plasma exposure through experiments within the Plasma Interactions (Pi) facility. Pi consists of a high-power lanthanum hexaboride hollow cathode DC plasma discharge, several transport coils to the target region, a target manipulator arm, and a high-speed, high-voltage circuit for biasing the target for simulating the conditions relevant to pulsed power technologies. Material samples are exposed to a range of plasma temperature, flux, and fluence levels. Existing diagnostics include Langmuir and emissive probes for obtaining plasma temperature, density, and potential, scanning electron microscopy for qualitative erosion assessment, and energy-dispersive x-ray diffraction spectroscopy for material composition. Several diagnostic upgrades are also being implemented to improve the quality of information obtained: a set of laser confocal microscopes are being installed for in situ, quantitative evaluation of sample morphology changes during plasma exposure; optical emission spectroscopy will provide information on the content and temperature of the plasma species (including sputtered material) in the near surface region; and vacuum ultra-violet spectroscopy will provide information of the neutral gas density (typically xenon).

To complement the experimental effort, a series of plasma models are being developed. A hybrid fluid/particle-in-cell (PIC) code DC-ION, developed initially for ion thrusters, is being modified to simulate the bulk plasma of the Pi facility to allow for optimization of the plasma discharge and magnetic field design. A fluid-based pre-sheath model will use the results from the bulk model and will couple to a full PIC sheath model in the near-surface region, which will determine the species currents to the wall and the sheath potential. The plasma models, coupled with thermo-kinetic material models being developed by professors Ghoniem and Kodambaka at UCLA, will self-consistently predict sputtering rates and changes to material and near-surface plasma properties.

References

[1] T S Matlock, D M Goebel, R Conversano and R E Wirz (2014) A dc plasma source for plasma–material interaction experiments