High Surface Fields in Superconducting rf Cavities. In Proceedings of SRF

Field emission in cavities requires electric fields that can be high enough to damage materials. We outline a model of breakdown and field emission from local asperities, and show how electric field effects, and field emission of electrons, field evaporation of ions, ultimately fracture, can limit accelerating fields. Although based on data from copper cavities and preliminary results from atom probe tomography experiments, the model seems to be generally valid for DC to 30 GHz, 10-11 to 105 Torr, different materials, temperatures, conditioning status, secondary emitters, strong magnetic fields, atom probe data, and the variety of surfaces encountered during copper cavity conditioning. We believe studies of normal materials can help to understand mechanisms operating in SCRF, such as fracture, field emission and evaporation effects, control of the surface, and surface metallurgy