Investigation of a millimeter nanosecond spark discharge at the interface of two immiscible liquids by time-resolved emission spectroscopy and ICCD imaging

Nanosecond pulsed spark discharges in liquid are transient stochastic plasma channels connecting two electrodes. The breakdown is preceded by a supersonic-speed streamer propagating toward the other electrode. Typically, a 500 ns pulse of 20 kV produces a 200-μm-length spark. Such a small gap distance makes the diagnostic of the spark by time-resolved techniques a challenge. Recently, we utilized two immiscible liquids to intensify the E-field at the tip of a pin placed close to the interface. Such an enhancement allows the establishment of spark discharges for a distance of a few millimeters with the 20 kV voltage pulse. With such a long distance, it becomes feasible to follow the propagation of the discharge with nanosecond time-resolved optical emission spectroscopy and ICCD imaging technique. The acquired spectra revealed broadened hydrogen and oxygen lines during streamer propagation, preceding a strong continuum emission (Blackbody and/or Bremsstrahlung) during breakdown. As the discharge fades, the continuum decays and the atomic lines reappear. Detail analysis of the spectra allows the determination of the temporal evolution of the electron density, as well as the gas and electron temperature, providing unprecedented insight into this type of in-liquid discharge.