The science and technology of superconducting cavities for accelerators

Rapid advances in the performance of superconducting cavities have made RF superconductivity a key technology for accelerators that fulfil a variety o f physics needs: high-energy particle physics, nuclear physics, neutron spa llation sources and free-electron lasers. New applications are forthcoming for frontier high-energy physics accelerators, radioactive beams for nuclea r astrophysics, next-generation light sources, intense proton accelerators for neutron and muon sources. There are now nearly one kilometre of superco nducting cavities installed in accelerators around the world, providing mor e than 5 GV of acceleration. The most recent installation of 20 m for a fre e-electron laser realized an average gradient a factor of four higher than existing applications. improved understanding of the physics of RF supercon ductivity, together with advances in technology, are responsible for the sp ectacular increases in performance. RF superconductivity is a mature scienc e going well beyond technological know-how and trial-and-error approaches t o genuine understanding of the underlying physics. Research continues to pu sh performance levels towards the theoretical limit, which is another facto r of two higher than the levels yet achieved.