Achim von Keudell1
Ruhr University Bochum1
Plasmas inside and in contact with liquids are a promising field for material synthesis and electrolysis. The understanding of the physics of the plasma ignition and sustainment is, however, still in its infancy due to the complexity of the phenomenon. For example, in the case of nanosecond high voltage pulses applied to a sharp tungsten electrode, electrostriction is very large so that rupture of the liquid occurs at a negative pressure of at least 24 MPa. Cavitation voids are formed and ignition inside these voids may occur. However, also field effects inside the liquid or a the tungsten liquid interface may cause ignition. Ignition converts the liquid spontaneously in the plasma state, which then expands this initial void to form a bubble. This creates a pressure pulse of the order of 10s of GPa depending on the HV voltage and the emission of acoustic waves that can be observed via shadowgraphy. The plasmas are observed with emission spectroscopy using a temporal resolution of 2 ns. This reveals a broad continuum consisting of black body radiation and line radiation of the hydrogen Balmer series. The hydrogen emission lines are significantly broadened from Stark broadening and affected by self absorption. It is shown that the plasma behave very similar for positive and negative polarity applied to the tungsten electrode. The temporal evolution of the electron density does not significantly depend on the polarity. This indicates that plasma generation follows similar mechanisms for both polarities. It is proposed that field ionization of water molecules or field emission at the internal surfaces of nanopores drives ionization and plasma propagation. Examples for the use of such plasma for electrolysis are given.