Spectroscopic study of self-pulsing discharge with liquid electrode

The article reports and discusses the results of a thorough spectroscopic investigation of the self-pulsing electrical discharge in contact with aqueous media. The discharge occurs between a bare high voltage electrode positioned over the liquid and a grounded ring submerged by the liquid. It is supplied with DC high voltage, but it operates in pulse mode that is determined by the charging and discharging of a parallelly connected capacitor. This type of discharge has attracted our attention due to its complex physics, as well as to its high efficiency in inducing the degradation of highly inert hazardous pollutants present in the water, such as perfluoroalkyl substances. The generated discharge unites several types of plasma in a single discharge cell. It starts as a high temperature (2500 K), high electron density (10^16–17 cm⁻³) spark-like discharge close to the high voltage electrode and then branches into a large number of cooler leaders characterized by one order of magnitude lower electron density (10¹⁵ cm⁻³) in the middle of the gap and finally touches the liquid electrode through a dense network of low temperature (500 K) and low electron density streamers (10¹⁴ cm⁻³). The paper discusses the results of a parametric study that has provided temperature and electron density data in different regions of the discharge. The measurements were thus performed for discharges in ambient air, in synthetic air (an 80/20 N₂/O₂ mixture) and in argon with, as liquid electrode, either tap or ultrapure water containing a small amount of perfluorooctanoic acid.