A model to explain the Q-increase by moderate-temperature treatment in Nb SRF cavities

Abstract

It is well known that moderate temperature baking of niobium cavities can improve the surface resistance. Presently, it is believed that the diffusion of oxygen into the bulk, resulting in interstitial defects, is responsible for the change. In this note we propose that the damaged surface layer remaining after dissolution of the thin niobium pentoxide may in fact be the dominant contributor to the improved the cavity quality factor by strongly pinning trapped flux lines. We propose some of experiments to test this theory.