• J. Halbritter
  FZ Karlsruhe, Karlsruhe

In rf cavities dielectric surface losses are usually neglected as compared to magnetic rf shielding current losses where the latter become very small well below the superconducting critical temperature. But the high density of localized states n(z)<10²1/cm³ of Nb2O5 adjacent to the high conduction electron density nc=6·10²²/cm³ of Nb yield measurable dielectric surface impedances Z by strong interface tunnel exchange (ITE) between n(z) and nc. For high fields the energy gain ezE(t) lifts n(z) states above the energy gap ITE-coupled to empty nc states acting as source term being BCS amplified dominating the rf absorption as loss term of high field Q drop (HFQ). The new ITE theory with tunnel properties of Nb2O5 yields a quantification of HFQ, i.e. the field emission free, exponential R(E)- and dT(E)-increases for E>10MV/m with minimal error bars, by one fit constant b increasing with ordering of Nb-oxide interfaces whereby HFQ hot spots shift towards the equator and to higher fields above 160mT. By baking b decreases, like by Al coatings on Nb.