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TY - UNPB AU - Kubo, T. AU - Gurevich, A.V. ED - Michel, Peter ED - Arnold, André ED - Schaa, Volker RW TI - Field-Dependent Nonlinear Surface Resistance and Its Optimization by Surface Nano-Structuring of the SRF Cavities J2 - Proc. of SRF2019, Dresden, Germany, 30 June-05 July 2019 CY - Dresden, Germany T2 - International Conference on RF Superconductivity T3 - 19 LA - english AB - We propose a theory of nonlinear surface resistance of a dirty superconductor in a strong RF field (H⁰), taking into account magnetic and nonmagnetic impurities, finite quasiparticle lifetimes, and a thin proximity-coupled normal layer characteristic of the oxide surface of many materials. It is shown that the interplay of the broadening of the quasiparticle density of states (DOS) peaks and a decrease of a quasiparticle gap caused by the RF currents produces a minimum in Rs(H⁰) and an extended rise of the quality factor Q(H⁰) with the RF field. Paramagnetic impurities shift the minimum in Rs(H⁰) to lower fields and can reduce Rs(H⁰) in a wide range of H0. Subgap states in the DOS can give rise to a residual surface resistance while reducing Rs at higher temperatures. A proximity-coupled normal layer can shift the minimum to either low and high fields and can reduce Rs below that of an ideal surface. The field dependence of Q(H⁰) can be very sensitive to the materials processing. The nonlinear RF losses can be minimized by tuning pairbreaking effects using impurity management or surface nanostructuring. PB - JACoW Publishing CP - Geneva, Switzerland ER -