Emission and raman spectroscopy measurements in hypersonic nitrogen flows

The self-luminosity of nitrogen hypersonic flows in the free-piston shock tunnel TCM2 at Marseille is analyzed for different enthalpy conditions. To perform vibrational population measurements in these flows by spontaneous Raman spectroscopy (SRS), the intensity of this emission is tentatively reduced by acting on various technical parameters and flow conditions. Thus a few low-enthalpy conditions are defined for which the SRS method can be used without being disturbed too much by the flow emission at the nozzle exit and behind the bow shock of a blunt body. For these conditions the vibrational populations of the first several vibrational levels of nitrogen are determined during the steady phase of the flow: The vibrational temperatures defined from the population of these levels correspond to the values given by classical computations assuming Landau-Teller models, that is, the expected frozen value at the nozzle exit and a value close to equilibrium behind a bow shock. However, an overpopulation of the highest levels cannot also be excluded.