High-field Q-slope mitigation due to impurity profile in superconducting radio-frequency cavities

Abstract

In this study, we present the recent insights into the origin of the high-field Q-slope in superconducting radio frequency cavities. Consequent hydrofluoric acid rinses are used to probe the radio frequency performance as a function of the material removal of two superconducting bulk niobium cavities prepared with low temperature nitrogen infusion. The study reveals that nitrogen infusion affects only the first few tens of nanometers below the native oxide layer. The typical high-field Q-slope behavior of electropolished cavities is indeed completely recovered after a dozen hydrofluoric acid rinses. The reappearance of the high-field Q-slope as a function of material removal was modeled by means of London's local description of screening currents in the superconductor, returning good fitting of the experimental data and suggesting that a layer of interstitial impurities with diffusion length on the order of tens of nanometers can mitigate high-field Q-slope.