Electrical and spectroscopic characterization of underwater plasma discharge inside rising gas bubbles

The generation of high-energetic species in plasma by discharge in liquids makes it suitable for applications such as water treatment. Effective reduction of input power and an increase in the process efficiency can be achieved by externally generated gas bubbles introduced in the liquid. Pulsed discharge in nitrogen, helium and argon bubbles in between a pin-to-plate electrode system submerged in a water solution has been investigated by electrical characterization and emission spectroscopy. A dynamic model for a bubble discharge, based on previously reported experiments and our measurements, is reported. Two types of bubble discharge are observed: a direct bubble discharge, with an immediate onset of a spark discharge inside the bubble, and a delayed bubble discharge, where spark discharge occurs after a delay time of typically several microseconds. The width of the Hα Balmer line in the emitted spectra is dominated by Stark broadening and implies electron densities in the order of 1024 to 1025 m-3, which is relatively high in comparison with other low-temperature atmospherical plasmas. Also, the emitted spectra contain a continuum, originating from black-body radiation of the heated bubble gas, with black-body temperatures from 6000 to 9000 K. The dependence of the measured plasma characteristics on the experimental parameters is in agreement with our proposed model. (paper)