Creeping sparks - stroboscopic imaging of surface streamers

In order to gain a better understanding of how to prevent sparks creeping along dielectric surfaces the most common cause of failure in high voltage devices we have studied the propagation of the preceding streamers along dielectric surfaces. We applied a voltage pulse to a needle a small distance above a dielectric sample a thin vertically positioned plate in low-pressure air, and imaged the resulting streamers using a stroboscopic ICCD camera which was positioned so that it imaged the thin side of the sample. We have investigated the chance of at least one streamer branch propagating along the sample surface, as a function of a number of variables: air pressure, sample positioning, voltage pulse amplitude and sample properties, notably relative permittivity. We have found that an increased pressure, more asymmetrical sample placement, smaller voltage pulse, and higher sample relative permittivity all increase the chance of a surface streamer, when other variables are kept constant. We have also investigated the velocity and diameter of surface and gas branches, and found that interactions with the sample cause surface branches to propagate twice as fast, with a diameter thrice as small. We have also investigated two cases of correlation. In the first case we found that the chance of a streamer containing a surface branch increases by about 10% when the previous streamer also contained a surface branch. In the second case we found that when we use a symmetrical geometry with the sample directly beneath the needle, a very strong (99%) preference for discharges on one side of the sample sets in after just a few discharges.