Shielding Superconductors with Thin Films as Applied to rf Cavities for Particle Accelerators

Superconducting rf cavities are used to transfer energy to charged particle beams in accelerators. To shield such cavities from ever-higher magnetic fields, millions of dollars have been spent in attempting to fabricate high-quality superconductor-insulator-superconductor (SIS') films---to little avail, as the underlying physics has not been fully developed. The authors present analytical and numerical analyses of SIS' structures based on practical materials, to illustrate the ultimate limitations of these films.