Speaker
Description
Trapped vortices can contribute significantly to a residual surface resistance $R_i(H)$ of SRF cavities, but the behavior of an elastic vortex line driven by strong RF Meissner currents through a random pinning potential remains poorly understood. Here we report extensive numerical simulations of large-amplitude oscillations of a trapped vortex under a strong RF magnetic field parallel to a superconducting film with different pinning landscapes, including bulk, surface, and cluster pinning. The RF power dissipated by an oscillating vortex was calculated, taking into account the vortex line tension, vortex mass, the linear Bardeen-Stephen and a nonlinear Larkin-Ovchinnikov viscous drag coefficients. We show that the global surface resistance $\bar{R}_i(H)$ averaged over statistical realizations of a random pinning potential exhibits a linear increase at small fields. However, $R_i(H)$ at strong RF field becomes a nonmonotonic function of $H$ and decreases with $H$ at higher frequencies, which can contribute to the negative $Q(H)$ slope observed on SRF cavities. Overheating can mask the descending field dependence of $R_i(H)$ as frequency increases. Our simulations also show that the strong RF field can cause a jumpwise increase in the RF power.
