Cross-comparison of diagnostic and 0D modeling of a micro-hollow cathode discharge in the stationary regime in an Ar/N₂ gas mixture

Abstract

A micro-hollow cathode discharge (MHCD) operated in Ar/N2 gas mixture, working in the normal regime, was studied both experimentally and with a 0D (volume-averaged) model in this work. This source provides high electron densities (up to 1015 cm−3) at low injected power (1 W). To understand the mechanisms leading to the production of N atoms, the densities of electrons, N atoms and argon metastable atoms (Ar*) were monitored over a wide range of experimental conditions. Electrons, N atoms and Ar* densities were probed by means of optical emission spectroscopy, vacuum ultra violet Fourier transform spectroscopy and tunable diode laser absorption spectroscopy, respectively. Measurements showed that using a smaller hole diameter enables to work with less injected power, while increasing the power density inside the hole and, subsequently, increasing the densities of excited species. Varying the percentage of N2 in the gas mixture highlighted that, up to 80%, the density of N atoms increases although the dissociation rate drops. Looking at the processes involved in the production of N atoms with the help of the 0D model, we found that at very low N2 fraction, N atoms are mostly produced through dissociative electron-ion recombination. However, adding more N2 decreases drastically the electron density. The density of N atoms does not drop thanks to the contribution of Ar* atoms, which are the main species dissociating N2 between 5% and 55% of N2 in the gas mixture. A reasonable agreement is found between the experiments and the model results. This study shows that, with this MHCD, it is possible to significantly modify the production of N atoms when modifying the physical parameters, making it particularly relevant for applications requiring a N atoms source, such as nitride deposition.

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

“…In this section, we compare the measurements of the N atoms density performed using TALIF (this work) and the high resolution Fourier Transform Spectrometer (FTS) in Vacuum Ultra Violet (VUV) of the DESIRS Beamline at SOLEIL synchrotron 40 . The latter results are reported by Remigy et al 19 . Both works refer to the performance of parametric studies of the N atom density versus the gas mixture and pressure.…”

“…In Fig. 7, TALIF measurements are done 4 mm away from the plasma (purple circle) to avoid reflections of the laser radiation on the MHCD surface, while the FTS VUV measurements were performed at the cathodic expansion (blue square) 19 .…”

“…Figure 8 shows the densities of N atoms as a function of the percentage of N 2 into the Ar/N 2 gas mixture, measured with the TALIF method. FTS VUV measurements from 19 are plotted as well for comparison. With both methods, we note a similar trend: the N atoms density increases with the percentage of N 2 , but not linearly as one would expect from the increase of N 2 concentration only.…”

“…At higher current (> 1 mA), however, the plasma becomes stable, with continuous voltage and current, and it expands on the cathodic surface (normal regime). This plasma source has been shown to produce high electron densities 17,18 (up to 10 22 m −3 ) and high density of N atoms (up to few 10 19 m −3 in the normal regime 19 ). Typically, the plasma volume generated by MHCD is confined in a micrometric hole.…”

“…Addition of small quantities of argon to the discharge may enhance the dissociation of N 2 and increases the lifespan of the MHCD device. The MHCD is first studied without a pressure differential, to compare with a previous study of the N atoms density performed using absorption spectroscopy 19 . We then introduce a pressure differential to increase the plasma volume, which is useful when targeting an application of the MHCD source in material deposition.…”