Non-equilibrium analysis of emission spectroscopy data taken in the freestream of the NASA IHF arc jet facility

To support the determination of total specific enthalpy of the plasma flow of the 60 MW Interaction Heating Facility at NASA Ames Research Center, spectroscopic measurements of non-equilibrium emission were performed in the free stream. In the visible-near infrared wavelength region, the most prominent emission was from molecular N₂, and in the ultra violet region, the spectra were dominated by emission from molecular NO. The only atomic lines observed were those of copper (an erosion product of the electrodes). The bands of the 1^st Positive system of N₂ (B➜A) differed significantly from spectra computed spectra assuming only thermal excitation, suggesting overpopulation of the high vibrational states of the B state of N₂. Populations of these high vibrational levels (peaking at v_upper=13) of the N₂ B state were determined by scaling spectra simulated for each upper vibrational state separately. The initially manual scaling process has been replaced with an automated multiparameter scaling to determine the population of the vibrationally excited N₂ states. The overpopulation of the high vibrational levels is assigned to inverse pre-dissociation of neutral atoms into the N₂ B state at v_upper>10 via level crossing processes with the N₂ A’⁵∑_g⁺ state. The v=13 emission has been used to determine N atom densities which agree very well between the different experiment campaigns both with manual and automated fitting and also with the CFD simulation of these flows.