Simulation of the triple junction effects in vacuum devices

It is currently believed that surface breakdown is initiated at the triple point and different geometry´s of the insulator-electrode junction have been tried to increase the electric strength. It is also still widely believed that the breakdown can be described by the secondary electron cascade model. However, some authors have reported that surface flashover also exists when negative charges are formed rather than positive charges and this result cannot be explained via this model. Solving the charge transport equation in a high field and taking into account the energetic exchanges that occur during trapping and detrapping, have allowed to predict where the breakdown will occur. It is expected at the cathode in pulse mode whereas it will occur at the anode under dc bias. An experiment has been set in order to verify this theory. It will be shown that cathode breakdown occurs at the triple junction only when the injected charge dose rate reaches a critical level, otherwise the breakdown occurs at the anode. This result is coherent with those obtained by others