Development and study of a Rayleigh scattering technique for microwave plasma measurements taken in vacuum

This work focuses on the study and development of a Rayleigh scattering technique to be used to measure velocity and temperature in the plume of a microwave plasma thruster. Tests were conducted from atmospheric pressure down to 1 torr. A detailed uncertainty analysis was conducted along with an extended analysis of the thermal drift of a Fabry-Perot interferometer. The uncertainty analysis run on a Rayleigh scattering code, for atmospheric pressure, zero velocity, and room temperature images, resulted in uncertainties that were +/- 38.8 m/s for velocity and +/- 45.5 K for temperature. The thermal drift was seen to be most stable using a commercially available thermal enclosure that provided a half-hour steady state window when set at 33°C. Rayleigh scattered light from an injection-seeded, frequency doubled Nd:YAG laser was analyzed using a planar mirror Fabry-Perot interferometer operating in the static imaging mode. Images were taken with an ICCD camera and digitally stored for later evaluation using a least squares curve fit. Preliminary data taken at 1 torr shows the Rayleigh signal dominated by scattered laser light coming from windows and metal surfaces. Methods of reducing this scatter along with the theory of Rayleigh scattering are discussed.