A simplified numerical model has been developed to simulate nonlinear superconducting radiofrequency (SRF) losses on Nb surfaces. This study focuses exclusively on excessive surface resistance ($R_s$) losses due to the microscopic topographical magnetic field enhancements. When the enhanced local surface magnetic field exceeds the superconducting critical transition magnetic field $H_c$, small volumes of surface material may become normal conducting and increase the effective surface resistance without inducing a quench. We seek to build an improved quantitative characterization of this qualitative model. Using topographic data from typical buffered chemical polish (BCP)- and electropolish (EP)-treated fine grain niobium, we have estimated the resulting field-dependent losses and extrapolated this model to the implications for cavity performance. The model predictions correspond well to the characteristic BCP versus EP high field $Q_0$ performance differences for fine grain niobium. We describe the algorithm of the model, its limitations, and the effects of this nonlinear loss contribution on SRF cavity performance.

• Received 10 June 2014

DOI:https://doi.org/10.1103/PhysRevAccelBeams.19.033501

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Published by the American Physical Society