When and why are streamers attracted to dielectric surfaces?

Summary form only given. Solid insulation surfaces in gas insulated high voltage (HV) equipment can be advantageous or dangerous with respect to dielectric breakdown by a discharge in the gas insulation, depending on whether the surface blocks the discharge (perpendicular, or dielectric barrier, configuration) or allows the discharge to creep along it (tangential configuration). Although discharge propagation along a surface is an old problem, there is still room for improved understanding of the fundamental physics and for the development of knowledge-based design rules for HV equipment. We thus investigate experimentally the initial (streamer) phase interacting with a dielectric surface. We studied streamers inside a gas-filled vessel using ICCD imaging, both stroboscopically as well as with single-shots. Inside the vessel, HV was applied to a needle located 10-15 cm above a grounded cathode. A dielectric sample was placed in the discharge gap. We varied several experimental parameters, such as pressure, gas composition, relative permittivity, pulse voltage and various geometrical parameters to study their effect on the discharge's affinity to prefer the dielectric surface instead of propagating through the bulk gas. Our experimental results provide us with the necessary information to start an in-depth discussion about the important mechanisms governing discharge propagation on surfaces. We show that the local availability of free electrons and the local electric field together determine the behavior of the discharge and explain how several parameters influence this behavior.