Current runaway in insulators affected by impact ionization and drift

Current runaway in insulators as affected by impact ionization and drift was treated rigorously by O’Dwyer. The theory did not result in explicit relations for the current runaway field Fr, and approximate relations derived later were based on rather arbitrary assumptions. To avoid these difficulties, in this report an approximate theory is developed which results in simple relations for Fr and for times to breakdown. The Fr values calculated are only a few percent smaller than those obtained with the rigorous theory. The approximate theory is presented for small and large rates of ionization, for the cases of trapping and field ionization, and for steady states and transients. To help with the identification of the breakdown mechanism on the basis of experimental observations, the properties of various breakdown models are compared and tabulated.