Spectroscopic measurements of argon plasma discharges for collisional radiative modeling of Hall thruster-relevant plasma conditions

Argon is increasingly used as the working gas in plasma electric propulsion systems for spacecraft, such as Hall thrusters. Non-invasive measurements of this kind of argon plasma are important in understanding plasma conditions and ramifications for thruster performance, and they are essential since direct probe measurements of a thruster plasma are impractical due to disturbing the plasma conditions and overheating the probe. Optical emission spectroscopy (OES) is one such non-perturbing diagnostic approach, but to yield measurements of electron temperature and density it requires an accompanying collisional radiative (CR) model of atomic processes. The CR model itself must be validated using direct Langmuir probe measurements of similar plasma conditions that are amenable to probe insertion. A new CR model has been developed for thuster-relevant argon plasma conditions based on the existing framework developed at NASA's Jet Propulsion Laboratory for xenon. Using new upgrades to an existing plasma production setup at the University of Alaska Anchorage, argon discharge data can now be taken over an expanded range of parameters, which will allow for more effective validation of the CR model. An ultraviolet-visible spectrometer (200-800 nm) is used to record argon emission spectra during radio frequency (RF) discharges and during afterglow. The RF discharges are combined with and compared to anode layer ion source DC discharges, and observed differences in OES measurements are discussed. The spectroscopic lines of sight coincide with physical Langmuir probe measurement locations, to facilitate the CR model development. A magnetic coil set is also employed to influence plasma conditions and yield a broader set of measurements.