Magnetic Shielding Larger Than the Lower Critical Field of Niobium in Multilayers

The current technology in superconducting radio frequency linear accelerators is based on the use of bulk niobium cavities. However, optimization of bulk technology is approaching the accelerating gradient limit set by the thermodynamic critical field of niobium, H_C = 200 mT . In order to surpass niobium´s ultimately achievable accelerating gradient, it has been proposed to use multilayer coatings to shield bulk niobium from higher fields. These multilayer coatings involve alternating superconducting and insulating layers. The superconductor used in this multilayer structure must have a higher H_C than that of niobium. NbN is one such superconductor that has potential application in these multilayer coatings. Recently, it has been shown that NbN can sufficiently shield an underlying niobium layer. However, this reported shielding has never been shown to be above the lower critical field of niobium. In this work, we present NbN multilayers that for the first time are shown to be capable of shielding an underlying niobium layer beyond the lower critical field of bulk niobium.