A Two-Color Heterodyne Interferometry Sensor for Direct, High-Speed Electron Density Measurements

Planetary entry and atmospheric reentry from deep space missions involve very high velocities and a significant source of vehicle heating is due to radiation from the high temperature, shock-heated plasmas formed in front of the vehicle. The next generation of reentry vehicles supporting the Artemis program and Mars exploration (e.g., the Mars Retrieval Lander) will reenter at higher velocities and experience much higher peak heat flux than previous missions. The radiative environment corresponding to different planetary entry conditions is simulated in the NASA Ames Electric Arc Shock Tube (EAST) facility and is used to obtain shock radiation data for the validation of phenomenological models for nonequilibrium radiation transport and improved understanding of the flow physics. There is a need for developing nonintrusive, accurate, spatially and temporally resolved diagnostics of electron density beyond currently available spectroscopic techniques and also help improve the physics of radiative transport prediction models. To aid this, we plan to develop a Two-Color Heterodyne Interferometry (TCHI) diagnostic for electron number density at the requisite temporal resolution gt; 1 MHz and a spatial resolution of less than 5 mm. The deliverables from the Phase I effort will include an initial demonstration and quantitative measurement of the TCHI sensitivity as well as an initial assessment of feasibility and integration with the NASA EAST facility. The end-deliverables of a potential follow-on Phase II program will be a validated and well-characterized instrument with lasers and detectors that are capable of measurements of electron number density profiles to a high precision and accuracy. Given sufficient resources, the follow-on efforts may also include integration of a prototype TCHI system into the NASA EAST facility to obtain preliminary measurements and benchmarking against the existing Stark broadening diagnostic system.