Depth Distribution of Losses in Superconducting Niobium Cavities

In order to optimize performances of superconducting niobium cavities it is crucial to understand the structure of near-surface few tens of nanometers of the material. In particular, superconducting properties of niobium, which depend on the p resence of impurities and/or defects, may be non-uniform in the magnetic field penetration depth. A few cavity experiments based on oxypolishing* and anodizing**,*** provided some insight into the problem, but the definitive understanding is not developed yet. In this contribution we report on the depth profiling of the near-surface RF layer using an alternative technique based on the hydrofluoric acid (HF) rinsing. Tumbled, electropolished and buffered chemical polished cavities have been investigated an d tentative nanostructural interpretation is discussed.