Swarm parameters along gas-solid dielectric interfaces

In order to understand the surface flashover mechanisms in a spacer-bridged, compressed-gas-insulated system, it is essential to have a detailed knowledge of the associated physical parameters, these being dominantly the first Townsend ionization coefficient (α) and the electron drift velocity (Ve). As the addition of a solid spacer to a compressed gas insulated system inevitably reduces the breakdown strength it is important to know how α and Ve are modified or if these are modified at all. These parameters can be determinated through the measurement of electron avalanches using the pulsed Townsend technique [1,2]. Jaksts and Cross attempted such measurements, however, the relatively long pulse width (∼10ns) of the laser used for photo-initiation of the electron avalanche and the poor bandwidth of the electron current amplifier, coupled with noise from the laser, prevented accurate measurements [3].