Systematic investigation of the effect of N2 admixture ratio on barrier discharge in helium

Abstract

The influence of the N2 admixture ratio on a He–N2 uniform atmospheric pressure glow discharge (APGD) generated using an asymmetric 20 µs, 10 kHz bipolar applied voltage pulse is systematically investigated, using electrical and optical emission spectroscopic measurements as diagnostic tools. We focused our investigation on understanding the discharge characteristics of the plasma ignited after an off-voltage period of 80 µs duration. The measured breakdown voltages for various N2 admixture ratios of up to 3% are compared with a theoretically calculated breakdown voltage, taking into account the secondary electron emission coefficient of the dielectric surface. The results indicate that the value of the secondary emission coefficient should increase from 0.1 to 0.5 to realize the experimentally obtained breakdown voltage variation within a 3% N2 admixture ratio. Using experimentally obtained values of the breakdown voltage, discharge current densities for different N2 admixture ratios, and 66 reactions among electrons, helium and N2, the variations of the N2 (A), N2 (C) and (B) state densities with N2 admixture ratio are calculated. The N2 admixture ratio dependence of the calculated N2 (C) and (B) state densities are similar to the N2 admixture ratio dependences of the emission intensities of the 337 nm band of the N2 second positive system (SPS) and the 391.4 nm band of the first negative system, respectively. Experimental observation and numerical investigation indicates that below a 1% N2 admixture ratio, the He (23S) metastable state has a strong influence on the discharge characteristics, and above a 2% N2 admixture ratio, the N2 (A) metastable state influences the discharge behavior. The N2 (A) metastable state density is estimated experimentally using the emission intensity ratio of the 337 nm band of N2 SPS and the 247 nm band of a NO-γ system. The experimentally estimated and theoretically calculated value of N2 (A) metastable state density is of the same order of 1013 cm−3. The estimated N2 (A) metastable state density shows a rapid increase above a 2% N2 admixture ratio, which supports the idea that the secondary emission coefficient increases due to an increase in N2 (A) flux.