Nanosecond Pulsed Plasma Discharge in Oxygen–Water Mixture: Effect of Discharge Parameters on Pre- and Post-Breakdown Characteristics

Underwater discharge plasmas are widely acclaimed for their potential applications in wastewater treatment, biomedical sciences, and industry. This article reports on pre- and post-breakdown characteristics of nanosecond pulsed plasma discharge generated in oxygen–water mixture flowing through a quartz capillary tube. In pre-breakdown region of applied voltage range of 4–4.5 kV, medium capacitance varies between 20 and 26 pF. For applied voltage range of 5–9 kV, a plasma discharge was observed during which the prominent current and voltage peaks show 5–10-ns phase difference. Electron number density was estimated using current density and Stark broadening techniques. It was noted that electron number density estimated from current density values ranges from 0.08
$\times \,\,10^{\mathrm {23 {}}}\,\,\text{m}^{-3}$
to 0.18
$\times \,\,10^{23}\,\,\text{m}^{-3}$
, while the electron number density estimated from Stark broadening of Balmer series hydrogen
$H_{\alpha }$
and
$H_{\beta }{}$
lines was found between 1.26
$\times \,\,10^{23}\,\,\text{m}^{-3}$
–2.69
$\times \,\,10^{23}\,\,\text{m}^{-3}$
and 1.25
$\times \,\,10^{23}\,\,\text{m}^{-3}$
–2.52
$\times \,\,10^{23}\,\,\text{m}^{-3}$
, respectively. The estimated values of plasma coupling parameter were in the range of 0.031–0.21, indicating the generated plasma to be on the margins of noncoupling ideal plasma to nonideal weakly coupled plasma. Effect of oxygen–water mixture flow direction on discharge stability and other plasma parameters was also monitored. It was observed that plasma discharge was more stable with higher electron number density in the direction of fluid flow.