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We outline a model of breakdown in RF cavities. In this model, the breakdown trigger is the injection of ions, atoms and clusters into cavities by either of two mechanisms. One mechanism is some combination of fracture and field evaporation of ions from solid surfaces caused by locally high electric fields. The second mechanism, driven by high local current densities, is localized Ohmic heating at grain boundaries and defects. Field evaporation and fracturing are similar processes, both driven by the high tensile stresses in the electric field that occur at local electric fields of ∼10 10 V/m, which have been measured in a number of experimental environments. We also outline how ions can be injected into cavities in the presence of large RF electric fields. The model can explain most of the behavior seen in a variety of cavities at different frequencies without assuming that melting or gas emission occurs at breakdown sites. This model may also be relevant to DC vacuum breakdown. |
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