DEVELOPMENT OF PULSED DIELECTRIC-BREAKDOWN IN LIQUIDS

A model is presented for the development of breakdown in liquids subje cted to uniform high amplitude electric field stresses with duration o f microseconds or less. In this model, breakdown proceeds in four stag es: (i) formation of a low density site (nucleation) in the liquid nea r an electrode, (ii) growth and expansion of this site until the local density is reduced below a critical density for electron impact ioniz ation to take place, (iii) growth of an electron avalanche and its tra nsformation into an ionizing front, and (iv) propagation of the ioniza tion front via a sequence of processes occuring in the region ahead of the front; namely, heating by electron injection, lowering of the liq uid density and avalanche growth and retardation. Expressions for the duration of each of these stages are derived and their behaviour with pressure and field strength analysed for cathode initiated breakdown. From this, a critical energy criterion for breakdown is obtained. Comp arison is made with experimental results for water, salt solutions, an d the liquid noble gases and for pulse durations in the microsecond an d sub-microsecond time scales. This model serves to elucidate the dyna mics of pulsed breakdown of liquids.