  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".
