Metastable oxygen atom velocity and temperature in supersonic CO2 plasma expansions

Abstract

Radial and axial velocity distribution functions of metastable oxygen atoms were obtained by way of resonant laser induced fluorescence spectroscopy at 777.19 nm in rarefied supersonic plasma jets produced from pure CO2 and CO2–N2 gas mixtures. The N2 seeded fraction is 3% in volume in order to mimic the Martian atmosphere composition. The measured lineshapes allow computation of the mean radial velocity and the perpendicular temperature, with respect to the jet axis, as well as the mean axial velocity and the parallel temperature. With an arcjet plasma source input power of 6.5 kW, the metastable O(5S) atom axial velocity is around 4500 m s−1 and the atom perpendicular temperature reaches 4500 K at the nozzle outlet. Under identical voltage and current conditions, a small addition of N2 does not change the measured values, i.e. it does not modify the overall energy balance. Distribution of flow parameters along both the radial and the axial directions reveals the existence of a barrel shock wave and a stationary shock front, which originate in shock wave regular reflexion. Flow properties, shock wave structure, departure from thermal equilibrium and impact of the applied power are debated for the two gas mixtures. Finally, the capacity of supersonic plasma expansions to simulate interplanetary probe entry into the Martian atmosphere is discussed in the light of experimental outcomes.