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We report radio-frequency measurements of quality factors and temperature mapping of a nitrogen doped Nb superconducting RF cavity. Cavity cutouts of hot and cold spots were studied with low temperature scanning tunneling microscopy and spectroscopy, X-ray photoelectron spectroscopy and secondary electron microscopy. Temperature mapping revealed a substantial reduction of the residual resistance upon cooling the cavity with a greater temperature gradient and hysteretic losses at the quench location, pointing to trapped vortices as the dominant source of residual surface resistance. Analysis of the tunneling spectra in the framework of a proximity effect theory shows that hot spots have a reduced pair potential and a wider distribution of the contact resistance between the Nb and the top Nb oxide. Alone, these degraded superconducting properties account for a much weaker excess dissipation as compared with the vortex contribution. Based on the correlation between the quasiparticle density of states and temperature mapping, we suggest that degraded superconducting properties may facilitate vortex nucleation or settling of trapped flux during cooling the cavity through the critical temperature. 

Superconducting Radio Frequency (SRF) cavities used in particle accelerators are typically formed from or coated with superconducting materials. Currently, high purity niobium is the material of choice for SRF cavities that have been optimized to operate near their theoretical field limits. This brings about the need for significant R & D efforts to develop next generation superconducting materials that could outperform Nb and keep up with the demands of new accelerator facilities. To achieve high quality factors and accelerating gradients, the cavity material should be able to remain in the superconducting Meissner state under a high RF magnetic field without penetration of quantized magnetic vortices through the cavity wall. Therefore, the magnetic field at which vortices penetrate a superconductor is one of the key parameters of merit of SRF cavities. Techniques to measure the onset of magnetic field penetration on thin film samples need to be developed to mitigate the issues with the conventional magnetometry measurements that are strongly influenced by the film orientation and shape and edge effects. In this work, we report the development of an experimental setup to measure the field of full flux penetration through films and multi-layered superconductors. Our system combines a small superconducting solenoid that can generate a magnetic field of up to 500 mT at the sample surface and three Hall probes to detect the full flux penetration through the superconductor. This setup can be used to study alternative materials that could potentially outperform niobium, as well as superconductor–insulator–superconductor (SIS) multilayer coatings on niobium. 

The SIS structure which consists of alternative thin lay- ers of superconductors and insulators on a bulk niobium has been proposed to shield niobium cavity surface from high magnetic field and hence increase the accelerating gradient. The study of the behavior of multilayer supercon- ductors in an external magnetic field is essential to opti- mize their SRF performance. In this work we report the de- velopment of a simple and efficient technique to measure penetration of magnetic field into bulk, thin film and mul- tilayer superconductors. Experimental setup contains a small superconducting solenoid which can produce a par- allel surface magnetic field up to 0.5 T and Hall probes to detect penetrated magnetic field across the superconduct- ing sample. This system was calibrated and used to study the effect of niobium sample thickness on the field of full magnetic flux penetration. We determined the optimum thickness of the niobium substrate to fabricate the multi- layer structure for the measurements in our setup. This technique was used to measure penetration fields of Nb 3 Sn thin films and Nb 3 Sn/Al 2 O 3 multilayers deposited on Al 2 O 3 wafers. The system was optimized to mitigate thermo- magnetic flux jumps at low temperatures. 

The magnetic field at which first flux penetrates is a fundamental parameter characterizing superconducting materials for SRF cavities. Therefore, an accurate technique is needed to measure the penetration of the magnetic field directly. The conventional magnetometers are inconvenient for thin superconducting film measurements because these measurements are strongly influenced by orientation, edge and shape effects. In order to measure the onset of field penetration in bulk, thin films and multi-layered superconductors, we have designed, built and calibrated a system combining a small superconducting solenoid capable of generating surface magnetic field higher than 500 mT and Hall probe to detect the first entry of vortices. This setup can be used to study various promising alternative materials to Nb, especially SIS multilayer coatings on Nb that have been recently proposed to delay the vortex penetration in Nb surface. In this paper, the system will be described, and calibration will be presented. 

Trapped vortices can contribute significantly to a residual surface resistance of superconducting radio frequency (SRF) cavities but the nonlinear dynamics of flexible vortex lines driven by strong rf currents has not been yet investigated. Here we report extensive numerical simulations of large- amplitude oscillations of a trapped vortex line under the strong rf magnetic field. The rf power dissipated by an oscil- lating vortex segment driven by the rf Meissner currents was calculated by taking into account the nonlinear vortex line tension, vortex mass and a nonlinear Larkin-Ovchinnikov viscous drag force. We calculated the field dependence of the residual surface resistance R i and showed that at low frequencies R i (H) increases with H but as the frequency increases, R i (H) becomes a nonmonotonic function of H which decreases with H at higher fields. These results sug- gest that trapped vortices can contribute to the extended Q(H) rise observed on the SRF cavities.