The effect of pressure on the prebreakdown events in toluene

Optical studies of the events leading to electrical breakdown in dielectric materials have suggested that the initiation of this process takes place at the metal dielectric interface [1,2]. From high-speed photographs of these prebreakdown events it has been suggested that at selected metal sites electrons are injected into the material or extracted from it depending on the local electrical field conditions [3]. Consequently, streamers are formed that propagate through the dielectric. The photographs obtained with image — converter cameras permit the study of the growth rate, but they indicate only the existence of a region of different refractive index near the injection sites and do not reveal the cause of that region. It has been suggested that these streamers represent regions of lower density than that of the surrounding material, possibly even a vapor phase or a void [2,4]. To check these possibilities studies were initiated of the effect of pressure on these processes. It was reasoned that if these regions were indeed of lower density and contained a vapor phase, their formation might be inhibited by increasing the ambient pressure. In a preceding paper [5], it was shown that, in n-hexane, pressure indeed caused the disappearance of these primary streamers. To check the broader validity of these observations a study was undertaken of the effect of pressure on the prebreakdown process in toluene which with its aromatic structure represents a quite different molecular species than the linear saturated hydrocarbon, n-hexane. The results of this study are presented in this paper.