Nonlinear Surface Impedance of HTS in High Magnetic Fields

Recently envisaged high-temperature superconductor (HTS) applications rely on their radio-frequency properties in very high magnetic fields. As an example, coating of surfaces facing particle beams with HTS for reducing the beam coupling impedance is being considered for the next generation of hadron colliders, such as the FCC-hh at CERN, where HTS would be exposed to the high-frequency wakefields generated by the particle bunches and to the strong field (>16 T) of the steering magnets. Other considered applications are radio-frequency cavities for axion detection as well as capture cavities for muon colliders. In these applications, it is important to keep under control the level of the nonlinear effects, always present using superconductors. In this frame, we developed a simple model for the calculation of the radio-frequency behavior of HTS exposed to a strong external magnetic field, which takes into account the nonlinearity of the pinning potential of the Abrikosov vortices. In particular, we calculate the surface resistance as a function of the impressed radio-frequency current and the harmonic content of the electric field produced by vortex oscillation as a function of the nonlinearity level of the assumed potential. The calculations are compared with recent experimental data on YBCO tapes' nonlinear behavior.