Nanosecond pre-breakdown and breakdown phenomena in water: influence of pressure, conductivity, and ionic sheath formation

The influence of hydrostatic pressure (⩽400 atm), liquid conductivity, and ionic sheath formation on the dielectric breakdown of water (distilled and ultrasonically treated, or non-distilled) and sodium chloride solutions (0.001-1.0 molar) subjected to sub-microsecond electric fields has been investigated. The prebreakdown current-voltage relationship is found to be nearly linear for the fields considered and therefore described by a resistance, R. For constant concentration, R does not extrapolate to zero as the gap width, d, goes to zero, suggesting the presence of a high resistivity sheath near the electrodes. Estimates for the sheath parameters (electric field, E_s , width, d_l, formation time, t_l, and ion density, n_s) are obtained. The time lag to breakdown, τ _bd, increases with increasing pressure and is independent of liquid conductivity. For the parameters investigated, the input power is primarily due to electron field emission currents whose magnitude is a function of the field in the ionic sheath. These findings have relevance to the ongoing discussion concerning `thermal´ vs. `electronic´ mechanisms for dielectric breakdown in liquids