Effect of different coning angles on flashover characteristics across machinable ceramic in vacuum

Surface flashover in vacuum is a kind of complicated surface and interface phenomena, which seriously restricts the development of high voltage electro-vacuum devices. Introducing a kind of machinable ceramic with excellent machinable performance and good surface electrical capability into vacuum insulation system, and considering the commonly used truncated cone structure, machinable ceramic samples with different coning angles are machined. Their surface flashover voltage are investigated under pulsed voltage in vacuum, and the electric field distributions and surface charge density of different coning angles are numerically simulated. The experimental results indicate that, the flashover voltage of cone insulators is higher than that of cylindrical insulators, the flashover voltage of truncated cone samples with positive coning angle is much higher than that of with negative coning angles, and the flashover voltage with ~45° conical frustum is highest. It is considered that, the coning angle plays an important role on the electric field of cathode triple junction (CTJ) and surface charge distribution. When the coning angle is positive, the electric field of CTJ and surface charge distribution is smaller, and when the coning angle is negative, these values are bigger, so its higher CTJ electric field and surface charge result in a lower flashover voltage than that with positive angle, and also the short free path of electrons and decreased energy of electrons colliding with insulator surface lead to a higher flashover voltage than that of cylindrical insulators.