In-liquid nanosecond-pulsed plasma: comparison of water and liquid nitrogen, and estimations of electric fields

Here we report on observations of the development of nanosecond-pulsed plasma in liquids, comparing liquids with two drastically different properties: water and liquid nitrogen/argon. Here we compare discharge appearance using high speed imaging, followed by examination of the bubble formation using shadow imaging, as well as measure time-averaged optical emission spectra of these plasmas. We show that the discharge development appears not to be affected by the type of liquid. Optical emission, however, is strikingly different: in water we noted no black-body continuum, no atomic lines and only continuum emission in the UV region, while liquid nitrogen spectrum is populated by molecular and longer-wavelength broadband emission.

Using imaging and optical emission spectroscopy, we estimate neutrals temperatures and densities, as well as local electric field values, and the obtained results indicate that the discharge develops via streamer (“electronic”) mechanism. We show that mm-scale plasma propagates in liquid nitrogen with the velocities of ~500 km/s with corresponding required local electric fields as high as 25 MV/cm, while estimated local electric field in the “core” of the discharge are around 6-8 MV/cm (corresponding to reduced electric field values of 600-1000 Td). The neutral and electron densities in the “main body” of the discharge were done using broadened argon lines, indicating that the neutral densities in the near-electrode region are around 10²⁰ cm^-3 (tens of atmospheres), while maximum recorded temperature was just a few tens of degrees above the surrounding liquid. Electron densities were estimated to be ~10¹⁷ cm^-3, about two orders of magnitude lower that those measured for water discharge.