Laser Absorption Sensor Targeting Potassium for Hypersonic Velocity, Temperature, and Enthalpy Measurements

Abstract

A laser absorption-based sensor for hypersonic gas flows was developed, targeting the D1 spectroscopic transition of atomic potassium near 770 nm. The sensor applies rapid-scanning tunable diode laser absorption spectroscopy to measure velocity from the Doppler shift and to infer temperature from the hyperfine-split transition lineshape. This sensor measured velocities and temperatures across three distinct conditions and six shots in the Hypervelocity Expansion Tube at the California Institute of Technology. Velocity and temperature were sampled at 5  μs intervals, and temperature measurements were validated with a supplementary laser absorption-based sensor targeting carbon dioxide transitions near 4.2  μm. Measured velocities across the three conditions ranged from 3.3 to 4.4 km/s, and measured temperatures ranged from 900 to 1600 K. The combined measurements were used to infer the freestream specific total enthalpy, which ranged from 7 to 10 MJ/kg. Because atomic potassium naturally forms in the test gas of many hypersonic impulse facilities, similar sensors may be widely applicable to facility characterization.

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