Temperature measurement in a CW HF chemical laser plenum

The stagnation temperature of the SF6-He gas mixture of a continuous-wave HF supersonic diffusion laser was measured by observing the emission spectra of excited HF (v) molecules when trace amounts of H2 were injected into the hot plenum gas. Since the plenum pressure was high (1-3 atm), the mean free path was very short and collision frequency high, so that rotational equilibrium was expected to be obtained. Hence, rotational temperatures deduced from the spectra should represent the gas translational temperature well. When compared with the temperatures predicted from plenum pressure and mass flow measurements, the measured temperatures were 10-36% higher. The difference is concluded to be the result of heat loss to the wall of plenum and throat, a factor not considered in the predictions.