On superconducting niobium accelerating cavities fired under N2-gas   exposure
  The dependence of the Q-value on the RF field (Q-slope) is actively studied in various accelerator laboratories. Although remedies against this dependence have been found, the physical cause still remains obscure. A rather straightforward two-fluid model description of the Q-slope in the low and high field domains is presented with emphasis on the recently experimentally identified improvement of the Q-value by so-called "N-doping". 
Ralf Eichhorn, Daniel Gonnella, Georg Hoffstaetter, Matthias Liepe and   Wolfgang Weingarten
physics.acc-ph