Proximity effects of niobium nano-hydrides on state of the art superconducting radio frequency Nb cavity

Nb hydrides are responsible for performance degradation at high accelerating field regime in a superconducting radio frequency cavity for the linear particle accelerator, known as high field Q slope (HFQS). Normal conductive Nb hydride precipitates at the cavity operation temperature , ~2K, and leads to breakdown of surface superconductivity due to proximity effect under RF magnetic field, from which significant power dissipation occurs on the cavity surface. In this study, precipitation of nano-meter scale Nb hydrides on a SRF Nb cavity surface is first directly observed using cryogenic atomic force microscopy, and statistical morphological features of Nb hydrides are compared with HFQS behaviors of state of the art SRF Nb cavities that have shown high accelerating gradient close to the theoretical limit, 50MV/m. Additionally, RF magnetic field enhancement due to Nb hydride topological properties are discussed.