On the Dependence of the Q-Value on the Accelerating Gradient for Superconducting Cavities

The performance of niobium superconducting cavities for accelerator applications has improved considerably over the last decade. Individual cavities reach accelerating gradients close to the theoretical limit (about 50 MV/m), however sometimes at the expense of a re-treatment (baking, electro-polishing, rinsing) needed to eliminate an undesired decrease of the Q-value with the field gradient (Q-slope, Q-drop). Cures have been developed, but a generally accepted physical explanation is still missing. Furthermore, for successful research work on materials other than niobium, it is of utmost importance to understand better the limitations in gradient and Q-value of superconducting cavities. The paper presents an alternative explanation for the Q-slope and confronts it with experimental results for 352 and 1500 MHz cavities.