A two-fluid model description of the Q-slope and Q-drop as observed in niobium superconducting accelerating cavities

Superconducting cavities made from niobium allow accelerating gradients of about 50 MV/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 London two fluid theory of RF superconductivity and compares the model with experimental data for a large variety of cavity tests.