Luminous phase of nanosecond discharge in deionized water: morphology, propagation velocity and optical emission

Abstract

We employed the techniques of time-resolved intensified charge-coupled device (ICCD) microscopy and spectroscopy to register basic morphologic and emission fingerprints of micro-discharges produced in deionized water. Fast rise-time positive high-voltage pulses (full width at half maximum of ∼7 ns and amplitude of ∼100 kV) in a point-to-plane electrode geometry produced micro-discharges, either periodically or in a single-pulse regime with the energy of ∼0.1 J dissipated during a single discharge event. Time resolved ICCD images evidence typical streamer-like branched filamentary morphology. Luminous discharge filaments show very fast and approximately linear initial expansion of the length with propagation velocity of ∼2 × 105 m s−1. When the HV pulse reaches its maximum value, the length of the primary luminous filaments reaches ∼1.3 mm. After initial expansion, the length of luminous filaments collapses and can be characterised by velocity of ∼1.9 × 104 m s−1. The first collapse is followed by a second slightly slower expansion, which is driven by the arrival of a reflected HV pulse, and which can be roughly approximated by propagation velocity of ∼1.5 × 105 m s−1. The second collapse (occurring after second expansion) proceeds at a nearly identical velocity compared with the first one. By combining two ICCD based techniques, we have been able to associate, for the first time ever, characteristic emission spectra with the most important phases of the micro-discharge development. The UV–vis-NIR emission spectra show a broad-band continuum evolving during the first expansion and collapse, followed by the well-known HI/OI atomic lines occurring together with continuum emission during the second expansion and collapse. We conclude that bound–free and free–free radiative transitions are basic emission characteristics of the nanosecond discharge initiation mechanism in liquid water which does not involve the formation of vapour bubbles.