High-spatial resolution measurements of NO density and temperature by Mid-IR QCLAS in open-air confined plasmas
Résumé
Radial distributions of absolute nitric oxide (NO) density and gas temperature are measured in atmospheric confined plasmas using Mid-IR quantum cascade laser absorption spectroscopy (QCLAS). Two ro-vibrational transitions of the fundamental band are probed in the electronic ground state NO(X) at 1900.076 cm−1 and 1900.517 cm−1, respectively. Plasmas are generated using nanosecond repetitively pulsed (NRP) discharges in air at atmospheric pressure. The spatial measurements are radially performed halfway between the discharge electrodes with a resolution down to 0.3 mm by the Abel inversion technique. The gas temperature is determined using two methods: (i) based on the ratio of the two ro-vibrational absorption lines and (ii) based on the collisional broadening line shape. The local NO density is obtained from local absorption coefficients and temperature dependent line strengths. The results were found in good agreement. The time averaged gas temperature and density at the discharge center are found at 800(±100) K and 2(±0.2) × 1015 cm−3, respectively. The FWHM of the NO density radial profile is found at 6–7 mm. This is large compared to the discharge channel width, i.e. typically below 0.5 mm, which is explained mainly by hot reactive jets induced in the post-discharge.