Ultraviolet-induced flashover of highly-angled polymeric insulators in vacuum.

The flashover of electrically stressed polymeric insulators in vacuum induced by exposure to intense ultraviolet radiation was investigated. Two geometries, positive and negative 45 degrees, were studied at electric field stresses of up to 80 kV/cm. Insulating materials tested were polyethylene, polystyrene, acrylic, nylon-6, acetal, PVC, and teflon. A krypton fluoride excimer laser at a wavelength of 248 nm provided the ultraviolet illumination. Ultraviolet-induced flashover over polymeric insulators in vacuum depends on the ultraviolet fluence (energy density) incident on the insulator surface. The negative-angle (unconventional) configuration exhibits superior ultraviolet tolerance compared to the positive-angle (conventional) configuration by approximately a factor of two in fluence. Insulating materials with high dielectric constants and low secondary electron emission coefficients exhibit superior ultraviolet tolerance. A model of ultraviolet-induced insulator flashover based on induced charging of the insulator surface is sufficient to explain the observed phenomena. The ultraviolet fluences required to initiate flashover are sufficiently low so that the contribution of neutral particles to the initiation of flashover may be disregarded, except perhaps at very low field stresses where the critical flashover fluences are correspondingly higher. Overall, of the materials tested, nylon exhibited the best performance (that is, the highest critical fluence in both polarities) because of its high dielectric constant and low secondary electron emission.Ph.D.Plasma physicsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128148/2/8812890.pd