Optical Emission Spectroscopy of Underwater Spark Generated by Pulse High-Voltage Discharge with Gas Bubble Assistant

Abstract

This paper is aimed at the investigation of the acoustic and spectral characteristics of underwater electric sparks generated between two plate electrodes, using synchronized gas bubble injection. There are two purposes served by discharge initiation in the bubble. Firstly, it creates a favorable condition for electrical breakdown. Secondly, the gas bubble provides an opportunity for the direct spectroscopy of plasma light emission, avoiding water absorption. The effect of water absorption on captured spectra was studied. It was observed that the emission intensity of the Ha line and a shockwave amplitude generated by discharge strongly depend on the size of the gas bubble in the moment of the discharge initiation. It was found that the plasma in the underwater spark channel does not correspond to a source of black-body radiation. This study can be also very useful for understanding the difference between discharges produced directly in a liquid and discharges produced in gas/vapor bubbles surrounded by a liquid.

Pressure in underwater spark discharge initiated with the help of bubble injection and its evaluation based on H-alpha line broadening

“…Therefore, for a large triggering bubble, we find a well-defined H α spectral line in the visible spectrum. With a reduction in the dimensions of the triggering bubble, the width of this spectral line increased with a simultaneous decrease in its amplitude [9]. It is clear that the smaller the triggering bubble, the longer the streamers must be built in the water, the larger volume of water must be evaporated, and the smaller the volume (volume of the bubble) into which the evaporated water can expand; therefore, the higher the pressure in the entire discharge channel.…”

“…Our previous article [9] demonstrates (1) experimental arrangement that enables to collect visible spectra without water absorption (through ground electrode), (2) that the underwater spark in its early stage is not a black body radiator (it has a dominant hill-like region in the range from 350 to 580 nm {probably a result of substantially broadened line radiation}, and clearly pronounced H α line that broadens {undoubtedly due to increased pressure} with the gas bubble diminution), (3) that the increased pressure with gas bubble diminution is confirmed by a pressure probe that measures amplitude of the pressure wave propagating from the discharge channel. Our present paper proceeds from the same experimental data published in [9], but their interpretation is based on in-depth analysis of H α line profile (line width) that not only leads to determination of the pressure in the discharge channel, but also casts new light on other processes taking place in the early stage of underwater spark discharge initiated with gas bubble assistance.…”

“…However, a moderately long underwater gap (a few millimetres in length), in which reasonably large amounts of energy are dissipated, is difficult to breakdown. For this reason, a spark gap with a gas bubble injection has been patented [7] and is being investigated [8,9]. Unfortunately, the dimensions of the bubble influences not only the breakdown voltage and breakdown delay (following the voltage application) [9], but also the pressure in the discharge channel.…”

“…For this reason, a spark gap with a gas bubble injection has been patented [7] and is being investigated [8,9]. Unfortunately, the dimensions of the bubble influences not only the breakdown voltage and breakdown delay (following the voltage application) [9], but also the pressure in the discharge channel. Therefore, for a large triggering bubble, we find a well-defined H α spectral line in the visible spectrum.…”

“…The aim of this study was to use optical emission spectroscopy for underwater spark-channel diagnostics. To the best of our knowledge, besides our recent paper [9], only two such papers have been presented [10,11], in contrast to the numerous others that have investigated the light emission of streamer or corona discharges {see e.g. [12,13]}.…”