Field-dependent surface resistance for superconducting niobium accelerating cavities

Abstract

Superconducting cavities made from niobium allow accelerating gradients of about 50MV/m close to the theoretical limit. Quite often, however, the rf losses increase with the gradient faster than quadratic. This observation is equivalent with a decrease of the quality factor Q with the gradient, called "Q slope" for intermediate gradients, and "Q drop" for larger ones. The paper provides an explanation by an elementary model based on the two-fluid theory of rf superconductivity and applies it to experimental data for a large variety of cavity tests. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.