Non-destructive measuring method to locate inhomogeneities on a contact surface of a vacuum circuit breaker

Abstract

One of the most important pre-breakdown phenomena is the field emission current. It can lead to a warming of a certain area of the cathode and the anode as well. Emission sites on the cathode’s surface are material inhomogeneities, for example microprotrusions, which cause microscopic electric field enhancements. Thus electrons are emitted and accelerated by the field applied between the electrodes. At the cathode surface these electrons cause a high local current density, which may result in a vaporisation of a microprotrusion due to joule heating. In addition, the impact of accelerated electrons at the anode surface causes secondary effects, which may also contribute to a reduction of the dielectric breakdown strength. To decrease the probability of field emission based breakdown, microscopic protrusions have to be reduced. This can be achieved by improving the contact surface. Therefore, a more detailed knowledge of the surface condition is required. In order to locate microprotrusions it is necessary to open the vacuum bottle and to analyse the surface with an electron microscope. This is a destructive investigation and, therefore, practically not applicable. Until now, a non-destructive measurement, which allows the interior of the vacuum tube to stay intact, has not been introduced. In this contribution, a non-destructive method will be presented, which allows the location of emission sites in commercial vacuum tubes by use of time variant external magnetic fields, which have effect on the internal electron trajectories.