Loss minimization in superconducting radio frequency cavities by artificial vortex pinning

AC magnetic fields in a superconducting radio frequency (SRF) cavity may lead to parasitic penetration of magnetic flux into the cavity material. This effect depends on the frequency of the AC filed, the quality of the cavity surface, and the distribution and size of (nanometer-sized) impurities near the surface. We present a numerical study of the energy dissipation near the surface of the superconductor in the external AC field concentrating on the spatial distribution of non-superconducting defects. We employ 3D large-scale time-dependent Ginzburg-Landau approach to simulate the vortex dynamics and consequent energy dissipation. We discuss in details the influence of two generally opposite effects: (i) pinning of frozen and induced vortices by defects in the vicinity of the surface and (ii) reduction of the surface pinning potential by means of the same defects. Our findings reveal the details of flux penetration in the presence of the external AC field and provide a novel recipe for the design of next-generation SRF cavities with high quality factor.