Theory of Q-degradation and nonlinear effects in Nb-coated superconducting cavities

Abstract

Amplitude-dependent absorption of RF power in superconducting cavity with a wall of normal metal (Cu ) covered by a film of superconductor (Nb ) is determined by three factors: (1)depairing effect of high-current density on superconducting energy gap; (2)increase of electromagnetic power penetrating to normal metal and Drude-absorbed in it, at higher RF amplitude; (3)heating of both superconducting coating and normal substrate resulting in increase of quasiparticle excitation density at higher RF power (P). Cavity Q-factor is calculated as a hnction of RF amplitude A = fi and is shown to follow a relationship 1n(Q/ Q,) = -const-P" with a=l at low temperature T < T* and a=1/2 at T > T* where T* is characteristic temperature T* X TcS I d, S is the London penetration depth and d the thickness of superconducting coating.