Ionic sheath formation in water and aqueous solutions and its influence on pulsed dielectric breakdown

The current response of water and sodium chloride solutions subjected to high amplitude electric fields of sub-microsecond duration has been investigated. The prebreakdown current-voltage relationship for these liquids 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 in the sheath, E/sub s/, width, d/sub l/, formation time, t/sub l/, and ion density, n/sub s/) are obtained from these measurements. The contribution of electronic and ionic currents to the breakdown probability has been assessed from the power input to the liquid during the pre-breakdown phase. For applied fields with sub-/spl mu/s duration, the input power is primarily due to electron field emission currents whose magnitude is a function of the field in the ionic sheath. For the parameters investigated, ionic currents are found to play a secondary role in these time scales. These results extend our investigations (1993-5) into the physical processes at play during pulsed dielectric breakdown of liquids.