Plasma discharges inside gas or vapour bubbles in water have been proposed in the last decade as a new and effective method of water treatment. However, the fundamental nature of bubble discharge is still poorly understood. In the present paper, the mechanism and properties of bubble discharge are investigated by application of a high voltage pulse to a single free gas bubble in between a pin-to-plate electrode configuration submerged in de-ionised water. A spark gap pulse generator is used to create triggered negative voltage pulses with rise times below 10 ns. The peak voltage is varied from 12 kV up to 18 kV. In the present research, the metal pin electrode is consequently chosen as the cathode. The low conductivity of the de-ionised water is maintained on the order of 10 µS/m for all experiments. The gas bubbles are formed on the tip of a capillary positioned underneath the electrodes such that every bubble passes in between the pin and plate electrode. The He gas flow rate through the capillary is kept as low as 30 sccm in order to avoid discharge in successive bubbles by a single voltage pulse. In our first setup, voltage pulses are not triggered according to the bubble flow rate. Therefore, vertical bubble position at the moment of discharge is not made reproducible in successive measurements. The bubbles are estimated to be slightly smaller than the distance between the electrodes, i.e. 2 mm.
Time dependence of voltage and current during discharge are measured in both cases with and without gas bubbling. Comparison shows that two types of bubble discharge are observed. One is spark discharge in a bubble and the other one is a delayed bubble discharge that starts the same way as discharge without bubbles. Spectra of the different types of discharge are measured in a wavelength range from 200 nm up to 900 nm. Electron temperature and density are determined from emission lines. Plasma parameters are determined for peak voltages of -12 kV, -15 kV and -18 kV respectively. Influence of the bubbling gas is discussed by comparison of measurements with helium and argon
