Evolution of cathode initiated pulsed dielectric breakdown in polar and nonpolar liquids

A model is presented for the development of breakdown in liquids subjected to uniform high amplitude electric field stresses with ⩽μs duration. In this model, breakdown proceeds in four stages: (a) formation of a nucleation site near the cathode, (b) growth and expansion of this site until the local density is sufficiently reduced for electron impact ionization to take place, (c) growth of an electron avalanche and its transformation into an ionizing front, and (d) propagation of the front 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. Expressions for the duration of each of these stages are derived and their behavior with pressure and field strength analyzed. From this, a critical energy criterion for breakdown is obtained. Comparison is made with experimental results for water, salt solutions, and the liquid noble gases