Vacuum interface flashover with bipolar electric fields

High energy, compact, particle accelerators require accelerating cavities that have large gradients and operate with high efficiency. The bipolar electric fields required in these efficient accelerating cavities place severe requirements on the vacuum interface. Experimentally, we have found that the bipolar flashover field varies as t/sup -1/2/ for times out to 300 ns and then remains essentially constant at 33 kV/cm for longer duration waveforms, whereas materials subjected to unipolar electric fields follow a t/sup -1/6/ relationship. Furthermore, specific accelerating cavities offer enhancements that may be employed to achieve highly uniform electric fields across the vacuum interface. Using these results and the results of a previously developed theory of unipolar flashover, a new interface has been designed and 50 kV/cm bipolar flashover field achieved for a waveform train that lasted 1 ..mu..s. This paper will discuss the design of this vacuum interface and the evaluation of various materials that led to achieving bipolar flashover fields 50% greater than we had previously obtained for long duration waveforms. 10 refs., 6 figs.