Electrical breakdown in vacuum is a complicated phenomenon, since there are no gas particles to become carriers of breakdown currents. The breakdown characteristics are, therefore, fully influenced by surface conditions of electrodes and insulators. This article presents fundamental mechanisms of electrical breakdown in vacuum. It is pointed out that characterization of electrode surface condition is principally important and thus in situ surface analysis is necessary to reveal the electrical breakdown mechanism of a vacuum gap. An experimental system is introduced to carry out this in situ surface analysis and breakdown experiments. Some examples of surface precautions improving breakdown strength of a vacuum gap, such as in situ sputter cleaning, diamond turning to mirror finish, in situ heat treatment are given.
