The influence of pressure and conductivity on the pulsed breakdown of water

The influence of hydrostatic pressure and liquid conductivity on the dielectric breakdown of water solutions subjected to high amplitude electric fields of sub-microsecond duration has been investigated. Well-defined pulses (80 kV, 3 ns risetime, 100 ns duration) have been applied to a gap (0.4 to 2.1 mm), between Rogowski profile electrodes (thus ensuring a uniform electric field), containing de-ionized water (non-distilled, and distilled and ultrasonically treated), sodium chloride solutions (0.001 to 1.0 molar), or magnesium sulfate solutions (0.01 to 0.1 molar). Breakdown in these liquids has been studied at pressures from atmospheric up to 40 MPa. The inter-electrode potential and the current response were measured indicating the time lag to breakdown, breakdown voltage, and temporal characteristics of the breakdown process. The breakdown time lag increases with increasing pressure, and is insensitive to the liquid conductivity. These findings have relevance to the ongoing discussion concerning `thermal´ vs. `electronic´ mechanisms for dielectric breakdown in liquids. In particular, the results suggest that breakdown evolves via `bubble´ formation by field emitted currents near asperities on the cathode, and that the time for the change in liquid conductivity as a result of breakdown is limited by processes other than ionization growth (due to electron impact ionization of molecules in the `bubble´) of prebreakdown electron currents