Submicrosecond breakdown and prebreakdown phenomena in water: influence of pressure, conductivity, and gap separation

The electrical breakdown of highly pressurized water and salt solutions subjected to high amplitude electric fields of sub-μs duration has been investigated. Well defined pulses (80-kV, 3 ns-risetime, 100 ns duration) have been applied to a gap (0.04-0.21 cm), between Rogowski profile electrodes (r=1.0 cm), containing distilled water, non-distilled water, sodium chloride solutions (0.001-1.0 molar), or magnesium sulfate solutions (0.01-0.1 molar). Breakdown in these liquids has been studied at pressures up to 400 atm. Calibrated voltage dividers situated on the source and load sides of the test cell permitted measurement of the inter-electrode potential and the current response. From these measurements, the time lag to breakdown, τ_bd; breakdown voltage, power input to the liquid, and temporal characteristics of the discharge have been determined. The time lag increases with increasing pressure and gap width, and decreases with increasing field. There is no dependence of τ_bd on liquid conductivity. A model has been developed to explain these results. In this model, field emission currents heat the liquid and create regions with density, n, below the critical density, n_c, necessary for the development of electron avalanches. An ionizing wave front subsequently develops and propagates via a sequence of processes occurring in the region ahead of the front; namely, heating by electron injection, lowering of the liquid density, and avalanche growth and retardation. The bubble formation time has been experimentally determined and is in good agreement with theory