Experimental Study on a Glow Discharge Driven by a Flash Evaporation-Induced Density Gradient From Porous Conductive Liquid Electrodes

The high density of admixture neutral particles beyond air is crucial for regulating the density and species of active products in plasmas. However, high density always leads to breakdown streamers. We engineered a glow discharge plasma source driven by a flash evaporation-induced density gradient from a porous anode or cathode with NaCl solution in a vacuum. The controlled groups with the metal anode and cathode were also conducted. The discharges with the liquid electrodes and controlled groups were carefully analyzed using optical spectrum and imaging methods. For both the discharges with the liquid anode and cathode, the spatial–temporal morphology measurements using intensified charge-coupled device (ICCD) and discharge current monitoring revealed the generation of glow discharge. As the gas density increases from the low-density region near the pumping outlet to the high-density region near the liquid electrode, the light intensity and avalanche diameter decrease smoothly without sharp saltation, presenting a density dependence and independence from the liquid electrode polarity. Furthermore, the spectra of discharges at various axial positions are dominated by the hydrogen Balmer series, OH(A–X) and O I (777.4 nm), resulting from collision reactions of evaporated water molecules. The rotational temperature of OH(A–X) ranges from 573 to 1135 K, lower than the rotational temperature of N2 (C–B) in discharges without liquid. This result reveals the heat dissipation effect of the liquid electrode. The electron excitation temperature indicates the creation of nonthermal equilibrium plasmas with an electron density ranging from
$0.15\times 10^{14}$
to
$2.83\times 10^{14}$
cm−3.