Plasmas sustained in bubbles in water: optical emission and excitation mechanisms

Plasmas in bubbles in water are being investigated for their ability to produce chemically reactive species for water purification and medical treatment. The gas forming the bubble is potentially a design parameter for water purification as the type and rate of production of active species may be controllable by the type of gas in the bubble. In this paper, we report on a computational investigation of the dynamics of plasmas in bubbles in water sustained in different gases. Images, optical spectra and plasma properties are discussed for plasmas in bubbles of N2, Ar and He in water, and compared to experiments. The differences in plasma dynamics and spatial distribution of the plasma (e.g., volume discharge or surface hugging) when using different gases depend in large part on the electron energy relaxation length, and the rate of diffusion of water vapour into the interior of the bubble. Electron impact dissociative excitation of water vapour, electron impact excitation of dissociation products and excitation transfer from the plasma excited injected bubble gases to water vapour all contribute to plasma emission. Variations in the contributions of these processes are responsible for differences in the observed optical spectra and differences in radical production.