Cavity Enhanced Thomson Scattering Diagnostic for Electron Measurements in Weakly Ionized Discharges

The cavity enhanced Thomson scattering (CETS) technique represents a new laser diagnostic for the measurement of electron properties (density and temperature) in electric propulsion (EP) devices. Electron measurements are critical, but conventional probe-based measurements are challenging and, even with careful design and implementation, result in significant measurement error. The CETS technique is based on a new version of laser Thomson scattering (LTS) that relies upon a high-power beam, generated in an optical build-up cavity, to increase the scattered photon counts and signal levels. The new approach is enabled by recent technological advancements (narrow linewidth fiber lasers, commercial laser locking hardware, and dielectric super mirrors) that allow intra-cavity beam powers as high as ~10-100 kW, which are several orders of magnitude greater than what is typically used. In this way, the detection limit will be lowered to ~1010 to 109 cm-3, which will enable measurements of electric propulsion devices, such as Hall thrusters. The technique will have high spatial and temporal resolution such that dynamic processes, such as the 'breathing mode', can be studied. Hall thrusters have been acknowledged by NASA in the In-Space Propulsion Systems Roadmap section 2.2.1.2.2 (Hall thrusters) as a technology that will directly support current and future NASA mission goals. Development of physics-based models and a more fundamental understanding of the plasma dynamics inside the channel and exhaust plume of the thruster are required for progression of the technology. The development of CETS directly supports this development by presenting a new non-intrusive highly sensitive diagnostic technique. Additionally, development of this technique will be of benefit to the plasma research community, as it will also allow measurements in other weakly ionized plasmas (e.g. processing plasmas). Over the course of the grant, the CETS instrument progressed from an idea on paper to a fully functional diagnostic technique capable of generating an 11.7 kW intra-cavity beam. To date, CETS has been used to measure electron properties in the plume of a barium oxide (BaO) and a lanthanum hexaboride (LaB6) hollow cathode. Based on a measured signal-to-noise ratio (SNR) of 1100 for electron densities of ~3 x 1012 cm-3 a lower electron detection limit of ~3 x 109 -3 x 1010 cm-3 corresponding to a SNR = 1-10 can be estimated. The estimated detection limit is lower than what is readily achievable with current LTS techniques and supports the viability of using CETS for measurements in electric propulsion devices. In addition to the bench-top instrument development, a fully fiber optic coupled version of the CETS technique was designed and fabricated for implementation on a large vacuum facility at the Jet Propulsion Laboratory. Due to limited optical access and the variability of vacuum facilities from one lab to another, the CETS instrument was packaged such that the high-power excitation beam and the scattered Thomson signal could be fiber optic coupled. The fiber instrument made use of special coreless endcapped fiber patch cables capable of handling the several watts of CW 1064 nm light used to generate the intra-cavity beam. The scattered Thomson signal was collected using in-chamber optics and Page 2 – Revised 02/2018 a fiber optic bundle. The instrument was set up on the “Big Green” large vacuum facility and diagnostics were performed on a hollow cathode. The demonstration was successful as a proof of concept and served to further disseminate the CETS technique to the EP community and, more specifically, to NASA. Current work is underway to further advance the technique and to demonstrate temporal resolution by measuring periodic phenomena, such as the Hall effect thruster breathing or spoke mode.