Tunable diode laser measurements on nitric oxide in a hypersonic wind tunnel

TLDR: In this paper, a tunable infrared diode laser was used in the F4 high-enthalpy hypersonic wind tunnel at ONERA to make line-of-sight measurements on nitric oxide (NO).

Abstract: A tunable infrared diode laser was used in the F4 high-enthalpy hypersonic wind tunnel at ONERA to make line-of-sight measurements on nitric oxide (NO). The NO is generated in the arc chamber and remains frozen in the hypersonic flow. We report the results of a medium enthalpy run. The Ω (14.5), Ω = 3/2 doublet was recorded simultaneously in two views : one normal to the flow and the other biased at 63-deg to the flow axis. Spectra were recorded at 2 ms intervals during a tunnel run, and 30 sets of spectra were analyzed by a least-squares fitting procedure. The mean of the velocity measurements inferred from the Doppler shift of the doublet was 2740 m/s (±5%). The velocity decayed about 10% over 60 ms. The temperature and NO column density determined by the least-squares fits were subject to much larger uncertainties. The static temperature of the freestream is estimated to be 168 K (±35%). The median inferred NO column density of 4 X 10 17 molecule/cm 2 corresponds to an NO number density of order 5 x 10 15 molecules/cm 3 in the freestream.