NONLINEAR SURFACE IMPEDANCE IN LOW AND HIGH T-C SUPERCONDUCTORS

The degradation of the surface impedance Z=R + iX with rf power is a p rincipal limit for measurements and applications. Such nonlinearities occur for normal conducting cavities by heating or electron loading. B ut superconductors show a much richer spectrum of causes for nonlinear ities. The nonlinearities in the surface resistance delta R proportion al to gamma H-n and surface reactance delta X proportional to alpha H- n can be classified by the ratio r=delta X/delta R. This ratio differs in value, temperature, or frequency dependence for the different mech anisms, allowing a unique identification of those mechanisms. This ide ntification is a prerequisite for an improved rf cavity design and imp rovement of superconducting materials.