Dark current spikes as an indicator of mobile dislocation dynamics under intense dc electric fields

Breakdown of metals subject to intense electric fields is a long-standing limiting factor in high-voltage applications. The mechanism leading to breakdown nucleation is not completely understood. Previously, it was suggested that breakdown can be nucleated by a critical transition in the population of mobile dislocations near the surface of electrodes. This was formulated in terms of a mean-field mobile dislocation density fluctuation (MDDF) model. Based on this model, it was proposed that prebreakdown fluctuations of the mobile dislocation density might be observed as spikes in the dark current between the electrodes. We constructed a setup in which these fluctuations were measured. The rate of fluctuations, as a function of the electric field between the electrodes, agrees with the predictions of the MDDF model, both in functional form and in absolute numerical rates. This numerical agreement was obtained using previously derived numerical parameters of the model. In addition, for each electric field, the distribution of times between current fluctuations was examined. The results indicate that each such prebreakdown fluctuation is the result of a two-step process. This characteristic, too, is in line with the MDDF model, which predicts that a characteristic prebreakdown current event is described as two separate steps in a Markov process, occurring in quick succession.