Ignition and propagation of nanosecond pulsed discharges in distilled water -- negative vs. positive polarity applied to a pin electrode

Abstract

Nanosecond plasmas in liquids are being used for water treatment, electrolysis or biomedical applications. The exact nature of these very dynamic plasmas and most important their ignition physics are strongly debated. The ignition itself may be explained by two competing hypothesis: (i) ignition via field effects or (ii) via electron multiplication in nanovoids. Both hypothesis are supported by theory, but experimental data are very sparse due to the difficulty to monitor the very fast processes in space and time. In this paper, we are using fast camera measurements and fast emission spectroscopy of nanosecond plasmas in water applying a positive and a negative polarity to a sharp tungsten electrode. It is shown that plasma ignition is dominated by field effects at the electrode-liquid interface either as field ionization for positive polarity or as field emission for negative polarity. This leads to a hot tungsten surface at a temperature of 7000 K for positive polarity, whereas the surface temperature is much lower for the negative polarity. At ignition, the electron density reaches 4 $\cdot$ 10$^{25}$ m$^{-3}$ for positive and only 2 $\cdot$ 10$^{25}$ m$^{-3}$ for the negative polarity. At the same time, the emission of the \Ha~light for the positive polarity is 4 times higher than that for the negative polarity. During plasma propagation, the electron densities are almost identical of the order of a 1 to 2 $\cdot$ 10$^{25}$ m$^{-3}$ and decay after the end of the pulse over 15 ns. It is concluded that plasma propagation is governed by field effects in a low density region that is created either by nanovoids or by density fluctuation in super critical water surrounding the electrode that is created by the pressure at the moment of plasma ignition.