  In this study, we present new insights on 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 interstitial impurities layers with diffusion length of the order to tens
of nanometers can mitigate high-field Q-slope.
