DC induced streamers in n-hexane

摘要

The use of high speed photography has facilitated the capture of events that immediately precede electrical breakdown in insulating liquids. The events have been rendered visible by schlieren, shadowgraph, and scattered light systems [1–3]. In these studies, an impulse voltage was first applied, and then the camera or light source was triggered: thus enabling the event to be temporally located between voltage onset and the possible ensuing breakdown. For a continuously stressed dielectric it is difficult to predict when the breakdown process will begin. For such cases, prebreakdown studies are normally limited to the measurement of conduction current, light, and acoustic emissions [4–6]. If needle electrodes are used, a spraying process can sometimes be observed in the high field region. The spray has been identified as a field of bubbles streaming away from the electrode tip [4]. It has been postulated that the bubbles originate in a plasma, which acts as an extension of the needle tip, and that an instability can occur on its surface resulting in the development of a breakdown channel [7]. A recent study in this laboratory has revealed the dynamic nature of this process [8]: for a negative point polarity, the region has been identified as the formation and decay of discrete prebreakdown streamers. The work described here presents results for the positive point case, with additional negative point data to enable polarity effects to be delineated.