Quantification of Unsteady Phenomena in Hall-Effect Thruster Discharges via Time-Resolved Terahertz Spectroscopy

This work proposes to develop a modified high-power Hall-Effect Thruster (HET) in the 5-10kW range with incorporated diagnostic apertures for integration of a THz-TDS system for the purpose of directly measuring plasma electron density variations within the HET with time resolution appropriate to quantify variation over the period of primary discharge current oscillations during HET operation. Terahertz time-domain spectroscopy (THz-TDS) is a recently demonstrated noninvasive plasma diagnostic technique to obtain electron density and collision frequency measurements. The technique has previously been demonstrated to be viable for plasmas in density ranges applicable to space solar electric and nuclear electric propulsion (SEP & NEP) systems of interest to the GO: Advanced Propulsion framework of the NASA STMD. Additionally, ceramic dielectric materials used in space propulsion such as boron nitride are known to be mostly transparent to THz pulses, and plasma parameters can be obtained for plasma which is bounded by these materials. An existing THz-TDS experimental workstation at Georgia Tech has been used to extract plasma properties in an inductively-coupled plasma. Modifications to this setup will be made to reduce the data collection time to nanoseconds or microseconds, sufficiently short to take multiple probe measurements within the period of the primary HET discharge oscillation or "breathing" mode. Following this, a HET system will be assembled at Georgia Tech for the purpose of collecting THz-TDS plasma measurements from within the discharge channel during excursions in HET operation to high voltage and high current density. Operating stably within this regime will be critical to advancement towards 100kW+ HET systems desired for interplanetary human and cargo transport missions utilizing SEP or NEP, but high discharge voltages and high current densities are known to contribute to unstable phenomena which reduce the efficiency of the HET system. Data collection from within the thruster discharge channel will help to develop the next generation of HET performance models and designs to help bridge the gap towards higher-power and more efficient HET systems.

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