Title: Improving the performance of SRF cavities by pairbreaking mechanisms

Speaker: Alex Gurevich (Old Dominion University)

I discuss possibilities of improving the performance of SRF cavities by engineering an optimum density of states of quasiparticles at the surface, the thickness of a suboxide metallic layer, and surface nanostructuring. The surface resistance R_s(T) in the Meissner state can be optimized and even reduced below its BCS value for an ideal surface by optimizing the quasiparticle the density of states at the suface using pairbreaking mechanisms, for instance, by incorporating a small density of magnetic impurities or by tuning the thickness and conductivity of the normal suboxide layer and its contact resistance.

I also discuss recent numerical simulations of vortices driven by a strong rf field in a film with randomly distributed pinning centers which revealed complex dependencies of R_s(ω, H) on frequency and the field amplitude H, particularly, a non-monotonic dependence of R_s(H). The ways of engineering an optimal density of states by surface nano-structuring and impurity management to reduce losses in SRF cavities for particle accelerators are discussed.