Absolute N-atom density measurement in an Ar/N2 micro-hollow cathode discharge jet by means of ns-two-photon absorption laser-induced fluorescence

Abstract

In this work, nanosecond two-photon absorption laser-induced fluorescence (TALIF) is used to probe the absolute density of nitrogen atoms in a plasma generated using a micro-hollow cathode discharge (MHCD). The MHCD is operated in the normal regime, and the plasma is ignited in an Ar/N2 gas mixture. First, we study a MHCD configuration having the same pressure (50 mbar) on both sides of the electrodes. A good agreement is found between the density of N atoms measured using TALIF in this work and previous measurements using vacuum ultraviolet Fourier transform absorption spectroscopy. Then, we introduce a pressure differential between the two electrodes of the MHCD, creating a plasma jet. The influence of the discharge current, the percentage of N2 in the gas mixture, and pressures on both sides of the MHCD is studied. The current has a small impact on the N-atom density. Furthermore, an optimal N-atom density is found at around 95% of N2 in the discharge. Finally, we demonstrate that the pressure has a different impact depending on the side of the MHCD: the density of N atoms is much more sensitive to the change of the pressure in the low-pressure side when compared to the pressure change in the high-pressure side. This could be due to several competing phenomena: gas residence time in the cathodic region, recirculation, or recombination of the N atoms at the wall. This study contributes to the optimization of MHCD as an efficient N-atom source for material deposition applications.