Locally-resolved temperature and electron number density measurements in the VKI inductively-coupled plasma wind tunnel

We perform locally resolved Optical Emission Spectroscopy (OES) from the UV to the NIR wavelengths in the air plasma jet produced by the 160 mm diameter VKI Plasmatron ICP torch, at a pressure of 100 mbar. A 750 mm focal length Czerny-Turner spectrograph, featuring a 1200 grooves/mm grating and an intensified CCD camera is coupled to a double mirror imaging system to provide spatially resolved measurements across the jet axis. Absolute calibration and Abel inversion allow to retrieve locally resolved emission intensities. Temperatures are determined either from LTE absolute line fitting, Boltzmann plot method or by rotational/vibrational fitting of molecular transitions. Oxygen and nitrogen lines provide consistent temperature measurements up to 50 mm from the jet-axis, while spectral fitting shows a fair agreement with LTE spectra. A mixture of synthetic air and 2% H2 is used to provide enough line strength of the H-beta line at 486 nm. Electron density profiles are being determined from the line shape fitting, accounting for the broadening mechanisms. OES analysis provides useful information on the thermodynamic state of the plasma, such as enthalpy profiles and local thermodynamic equilibrium, for ground test facilities aiming at replication of atmospheric entry conditions.