Abstract
The microscopic physical mechanisms of micro-discharges produced in liquid waters by nanosecond high-voltage pulses are quite complex phenomena, and relevant coherent experimentally supported theoretical descriptions are yet to be provided. In this study, by combining a long-distance microscope with a four-channel image splitter fitted with four synchronised intensified charge-coupled device detectors, we obtained and analysed sequences of microscopic discharge images acquired with sub-nanosecond temporal resolution during a single event. We tracked luminous filaments either through monochromatic images at two specific wavelengths (532 and 656 nm) or through broadband integrated UV–vis–near infrared (NIR) discharge emission. An analysis of the sequences of images capturing discharge filaments in subsequent time windows facilitated the tracking of movement of the luminous fronts during their expansion. The velocity of expansion progressively decreased from the maximum of ~2.3 × 105 m/s observed close to the anode pin until the propagation stopped due to the drop in the anode potential. We demonstrate the basic features characterising the development of the luminous discharge filaments. Our study provides an important insight into the dynamics of micro-discharges during the primary and successive reflected high-voltage pulses in de-ionised water.
Highlights
The initiation and formation of micro-discharges in liquid water driven by nanosecond high-voltage (HV) pulses are conditioned by the ultrafast response of the structure of the H-bonded matrix to very high (~GV/m) non-uniform transient electric fields
From an experimental point of view, such a lack of description is conditioned by the sub-micrometre and sub-nanosecond scales associated with the events and the stochastic and irreproducible nature of filamentary discharge structures often studied under insufficient spatio-temporal resolutions
This disadvantage can be eliminated after splitting a single microscopic image of the region of interest into four equivalent images produced at the splitter output, with each output image channel sampled by one intensified charge-coupled device (ICCD) detector in the preselected time-window
Summary
The initiation and formation of micro-discharges in liquid water driven by nanosecond high-voltage (HV) pulses are conditioned by the ultrafast (fs/ps) response of the structure of the H-bonded matrix to very high (~GV/m) non-uniform transient electric fields. Experiments performed using extremely short pulses (~100 ps) have shown that discharges in liquid water can be formed in the absence of formation of gas bubbles and with minimal thermal effects [3,11]. A relevant coherent theoretical description supported by the unambiguous and undisputable experimental demonstration is still missing. From an experimental point of view, such a lack of description is conditioned by the sub-micrometre and sub-nanosecond scales associated with the events and the stochastic and irreproducible nature of filamentary discharge structures often studied under insufficient spatio-temporal resolutions. The poor event-to-event spatial reproducibility of the bush-like/tree-like dark/luminous discharge structures makes the use of diagnostic techniques challenging and requires the acquisition of data from a large number of events to draw conclusions based on the statistical averaging of parameters evaluated from different events
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