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TY - UNPB AU - Gurevich, A.V. TI - Microwave Suppression of Nonlinear Surface Resistance and the Extended Q(B) Rise in Alloyed Nb Cavities J2 - Proc. of International Conference on RF Superconductivity (SRF2015), Whistler, BC, Canada, Sept. 13-18, 2015 AB - The talk gives a theoretical overview of recent progress in our understanding of the field dependence of the SRF surface resistance Rs(H). A theory of microwave suppression of Rs(H) in the Meissner state of alloyed superconductors under a low-frequency, strong RF field at low temperatures is presented*. Solving equations for Rs(H) derived from a non-equilibrium BCS theory in the dirty limit shows that Rs(H) has a minimum as a function of H, mostly due to field-induced temporal oscillations of the quasiparticle density of states. The calculated field dependence Rs(H) is in good agreement with recent experiments on Ti and N alloyed Nb resonator cavities. The theory suggests that the extended Q(B) rise can be further enhanced by reducing subgap density of states by materials modifications. Applying superimposed dc and ac fields can be used to reduce the rf dissipation by tuning Rs(H⁰) with the dc field and also to probe the mechanism of microwave suppression of Rs(H) under equilibrium conditions. Ways of increasing the SRF performance by an optimized dirty layer at the Nb surface, which can withstand magnetic field exceeding the superheating field of Nb is discussed **. PB - JACoW CY - Geneva, Switzerland ER -