A physical model which can adequately describe the effects associated with breakdown at high field strengths in rf cavities and accelerating structures has been the subject of extensive theoretical and experimental investigation. Although a completely satisfactory model, supported by experimental evidence, has not yet been developed, a good start in this direction has been made by Knobloch(1). He has shown that a key component of breakdown is the rapid formation of a dense plasma at the breakdown site. In this paper we extend the plasma model to include additional information obtained from observations and calculations associated with dc vacuum arcs. One result is that such a surface plasma is capable of removing material at a rate on the order of 100 micrograms per Coulomb of ion bombardment current. This rate is sufficient to explain the erosion of iris material observed in some accelerating structures tested at high power in the NLCTA facility at SLAC(2).
(1) Jens Knobloch, 'Advanced Thermometry Studies of Superconducting Radio-Frequency Cavities', Ph.D. Thesis, Cornell University (August 1997). (2) C. Adolphsen, paper presented at this conference.
*Work supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00515
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