Two-photon laser induced fluorescence spectroscopy performed on free nitrogen plasma jets

Abstract

The properties of free nitrogen plasma jets are examined by studying the ground-state nitrogen atom flow characteristics. The plasma is created by a cascaded arc and subsequently it expands freely into a low pressure vessel. In such a way supersonic flows with high Mach number are achieved. N(4S) atoms are locally probed by means of two-photon absorption laser induced fluorescence spectroscopy. Axial and radial N(4S) atom temperature and velocity profiles present the shape predicted by the neutral gas supersonic expansion theory. The adiabatic exponent γ is equal to 1.45 in the supersonic domain. A Mach number M of 4.4 is measured ahead of the normal shock wave. In contrast, density profiles reveal a departure from the classical expansion picture. Too small density jumps over the shock region indicate a non-conservation of the N(4S) atom forward flux. Moreover the partial N(4S) atom static pressure decreases in the subsonic domain. Loss of nitrogen atoms during the plasma expansion is a direct consequence of plasma–wall interactions. However, losses are limited because of the relatively high N atom mass and because of a low surface recombination probability of N atoms. The dissociation degree at the arc exit is around 40%. Under such circumstances N2(A) molecules cannot survive in the jet. The local electron density is estimated from a measure of the radiative lifetime of the nitrogen atom excited state.