Frequency and Pulse Length Scaling of Rf Breakdown in Accelerator Structures∗

∗ Work supported by Department of Energy contract DE-AC03-76SF0051. Abstract The plasma spot model predicts that small areas of plasma on the order of 10 microns in diameter form in regions of intense rf electric field near the iris tips of accelerator structures. These plasma spots act as a current source, injecting electrons and ions into the rf field. The model suggests that an observable breakdown event will occur when electrons, extracted from the spot and accelerated, absorb energy faster than it can be supplied by the rf source. This field collapse, which can occur during processing, is not an ultimate limit on accelerating gradient. It is conjectured that surface melting, due to electron and ion back-bombardment at the emitting site, leads to a pseudospark-like discharge which limits the gradient and can produce massive surface damage. Using a statistical model, the dependence on frequency and pulse length of the probability of triggering a plasma spot is examined.