Laser Scattering Diagnostic for Shock Front Arrival and Electron Number Density

Three diagnostic methods are proposed for measuring properties of interest in the post-shock regions of a hypersonic bow shock wave that is used for studying planetary entry and earth reentry flows. Shock location is measured using an imaging approach by laser Rayleigh scattering from molecules, shock velocity is measured by beam deflection via schlieren effects, and electron number density is measured by Thomson scattering. The Rayleigh and Thomson scattering methods are complimentary to each other and can use the same pulsed laser. The schlieren deflection is accomplished with a continuous wave laser and can be used to generate a precise arrival time and provide triggering for the pulsed laser. Thomson and Rayleigh scattering imaging may be extended to MHz rates with pulse burst laser technology, providing a capability to time-resolve the motion of the shock wave as it passes through the test section. The electron density measurement is a direct technique with the potential for high accuracy time and space-resolved measurements. The Phase I effort will demonstrate all three techniques in laboratory environments at relevant conditions.

The proposed diagnostics will support the development of spacecraft for entering planetary atmospheres, such as Mars and Venus, as well as reentry to Earth?s atmosphere. Radiative heat transfer to the vehicle during atmospheric entry can be severe, yet predictive methods are hampered by a lack of data for validating models. The proposed tools will provide electron number density and electron temperature thus yielding key insight into radiative properties of the plasma formed post-shock during atmospheric entry and help improve the fidelity of current measurement techniques. Information obtained from these diagnostics should aid in the design of advanced space exploration vehicles, and in the improvement of prediction models that simulate radiative heat transfer used in the design of thermal protection systems (TPS). This can help in reducing the design margins of TPS and thus result in increased mission payload capability.

The ability to accurately measure shock location, shock velocity, and electron number density would be attractive to research on hypersonic vehicles, and should find use in research facilities employed in the development of high-speed missiles and aircraft. In particular, the Air Force has programs to develop hypersonic vehicles that would benefit from these diagnostics. Other Air Force programs that would benefit include those on the development of Hall thrusters for satellite propulsion, which have a need for accurate, time-resolved electron number density measurements.

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