Superconducting Radiofrequency Cavities

Abstract

Superconducting radiofrequency (SRF) cavities are devices that store electromagnetic energy with very high efficiency. SRF cavities are a building block of modern particle accelerators as a charged particle beam travelling through the cavity axis absorbs the stored energy by interacting with the electric field and is therefore “accelerated”. Besides their use in particle accelerators, SRF cavities are being used in circuit quantum electrodynamics to achieve long life-time of photons in the resonator. The superconducting material which is most commonly used for the fabrication of SRF cavities is bulk niobium, because of its good formability, highest Tc among elemental superconductors and highest lower critical field. An alternative way to fabricate an SRF cavity is to deposit a thin film of Nb onto a high thermal conductivity substrate, typically oxygen-free high conductivity copper. Two well-known phenomena contribute to the residual resistance even for very high-quality Nb: trapped vortices and niobium hydride.