Electron dynamics, emission continua and ionisation potential depression in nanosecond plasma generated in deionised water

In this study, we investigated time- and space-resolved plasma emissions generated by reflected high-voltage (HV) pulses in deionised water to determine the electron density. HV pulses with a 3.5 ns rise time and 160 kV amplitude were applied in a point-to-plane electrode geometry under single-shot conditions. The primary pulse, delivered to the chamber through a coaxial cable, produced a series of reflected pulses owing to load mismatch. Optical emission characteristics were recorded and resolved spatio-temporally across visible and near-infrared spectral regions. High-speed images showed that the primary discharge was diffuse, whereas the reflected pulses produced filamentary plasma structures. Imaging spectrometry revealed that the primary pulse generated a continuous spectrum with higher intensity away from the anode apex, while the reflected pulses exhibited broadened hydrogen and oxygen atomic lines superimposed on the continuum. These reflected pulses were analysed to estimate electron density and temperature under both space- and time-resolved conditions. The continuum was first removed by fitting its profile, and the resulting residuals were used to extract plasma parameters. Electron density ( ne) was determined from the broadened spectral lines, and electron temperature ( Te) was obtained from Boltzmann plots. The density decayed over time (from 2.5 to 0.5×1019 cm−3) while remaining roughly constant along the anode axis, whereas the electron temperature remained stable at 2–3 eV across space and time. The emergence of additional Balmer-series lines at higher reflections signalled ionisation threshold depression during the early reflections. Overall, this study provides the first consistent experimental evidence of the extreme conditions generated during nanosecond discharges in liquid water.