Impact of geometry on the magnetic flux trapping of superconducting accelerating cavities

Abstract

Controlling trapped magnetic flux in superconducting radio frequency (rf) cavities is of crucial importance in modern accelerator projects. In order to study flux trapping efficiency and sensitivity of surface resistance, dedicated experiments have been carried out on different types of low-β superconducting accelerating cavities. Even under almost full trapping conditions, we found that the measured magnetic sensitivities of these cavity geometries were significantly lower than the theoretical values predicted by commonly used models based on local material properties. This must be resolved by taking account of geometrical effects of flux trapping and flux oscillation under rf surface current in such cavity shape. In this paper, we propose a new approach to convolute the influence of geometries. We point out a puzzling contradiction between sample measurements and recent cavity experiments, which leads to two different hypotheses to simulate oscillating flux trapped in the cavity surface. A critical reconsideration of flux oscillation by the rf Lorentz force, compared with temperature mapping studies in elliptical cavities, favored the results of previous sample measurements, which suggested preferential flux trapping of normal component to the cavity inner surface. Based on this observation, we builded a new model to our experimental results and the discrepancy between old theory and data were resolved.