Electron density and electron temperature measurements in nanosecond pulse discharges over liquid water surface

Abstract

Time-resolved electron density, electron temperature, and gas temperature in nanosecond pulse discharges in helium and O 2-He mixtures near liquid water surface are measured using Thomson/pure rotational Raman scattering, in two different geometries, (a) 'diffuse filament' discharge between a spherical high-voltage electrode and a grounded pin electrode placed in a reservoir filled with distilled water, with the tip exposed, and (b) dielectric barrier discharge between the high-voltage electrode and the liquid water surface. A diffuse plasma filament generated between the electrodes in helium during the primary discharge pulse exhibits noticeable constriction during the secondary discharge pulse several hundred ns later. Adding oxygen to the mixture reduces the plasma filament diameter and enhances constriction during the secondary pulse. In the dielectric barrier discharge, diffuse volumetric plasma occupies nearly the entire space between the high voltage electrode and the liquid surface, and extends radially along the surface. In the filament discharge in helium, adding water to the container results in considerable reduction of plasma lifetime compared to the discharge in dry helium, by about an order of magnitude, indicating rapid electron recombination with water cluster ions. Peak electron density during the pulse is also reduced, by about a factor of two, likely due to dissociative attachment to water vapor during the discharge pulse. These trends become more pronounced as oxygen is added to the mixture, which increases net rate of dissociative attachment. Gas temperature during the primary discharge pulse remains near room temperature, after which it increases up to T ~ 500 K over 5 µs and decays back to near room temperature before the next discharge pulse several tens of ms later. As expected, electron density and electron temperature in diffuse DBD plasmas are considerably lower compared to peak values in the filament discharge. Use of Thomson scattering for measurements of electron density and electron temperature in near-surface plasmas is hindered by strong light scattering off the surface.

Plasma‐enhanced evaporation and its impact on plasma properties and gaseous chemistry in a pin‐to‐water pulsed discharge

“…This drop in electron density with water concentrations higher than 1000 ppm has also been reported in water‐contacting discharges by simulation and experiment [ 52,53 ] . The mechanism behind n e reduction in the solution surface is supposed to be as a result of the enhanced process of electron–ion recombination [ 52,54 ] . Water cluster ions such as H 2 O 3 + and H 3 O + (H 2 O) n are formed abundantly under a high vapor population condition [ 55 ] .…”

“…and experiment [52,53] . The mechanism behind n e reduction in the solution surface is supposed to be as a result of the enhanced process of electron-ion recombination [52,54] . Water cluster ions such as H 2 O 3 + and H 3 O + (H 2 O) n are formed abundantly under a high vapor population condition [55] .…”

Low-temperature plasmas in contact with liquids—a review of recent progress and challenges

“…The discharge in helium between bare electrodes without water generates a peak electron density of 1 × 10 15 cm −3 , while the addition of the water to the container, to the level below the peak of the grounded electrode, which practically acts as an addition of the water vapor to helium, reduces peak electron density twice as well as lifetime of plasma. This effect is most likely due to the dissociative attachment of electrons to water molecules, which reduces the net electron impact ionization rate at the same electric field [192]. In the case of diffuse DBD where plasma is in direct contact with water, the electron density is one order of magnitude lower than in the case of plasma between bare electrodes and reaches 6 × 10 13 cm −3 .…”

“…The Thomson scattering technique is successfully applied for the diagnostics of a bulk plasma in contact with liquid in several configurations [137,189,191,192]. Thomson scattering measurements in near-liquid surface plasma are extremely challenging due to the elastic laser scattering on the surface and due to a Raman scattering on liquid water [193].…”

“…Simeni studied nanosecond pulsed discharge ignited in two electrode geometries at reduced pressure in pure helium and helium-oxygen mixtures [192]. They investigated discharge between two bare metallic electrodes in the vicinity of water and the discharge between a metallic electrode and the grounded water, using Thomson scattering.…”