Pulsed Discharges in Liquids: Generation and Applications*

Abstract

Plasmas that are generated submerged in a liquid offer different possibilities for material processing, material synthesis and decontamination. Moreover fundamentals on generation and plasma properties are of great scientific interest for almost a century. In particular the mechanisms for the generation of discharges with pulsed high voltages is discussed controversial. The possibility for a breakdown development that is not mediated by an initial gaseous phase is disputed especially for the application of very short high voltage pulses of only a few nanoseconds or less. Associated with this specific excitation scheme are differences in plasma development, plasma parameters and reaction mechanisms. We have compared discharges in a point-to-plane geometry that were generated with 50-us, 100-ns, 10-ns or 1-ns high voltage pulses. Time-resolved shadowgraphy and spectroscopy were performed to evaluate discharge structures, plasma parameter and reactive species that were formed in water (in some cases ethanol). Different propagation modes, with velocities from 50 m/s to 6.7 km/s were observed. Optical emission spectroscopy has shown the formation of molecular bands of nitrogen, as well as strongly broadened atomic hydrogen and oxygen lines. Although for the very short pulses an initial bubble might not be observed, further studies are underway to verify this conclusion.