Thermally induced nonlinear behaviour of HTS films at high microwave power

The resonance curve shape of a thin film parallel plate HTS resonator was observed as a function of delay time from 0.3 to 12 s at each frequency step across the resonance at various microwave surface magnetic field levels up to 28 mT. Skewed resonance curves and a pronounced shift of resonance frequency were encountered as the microwave input power was increased. A simple phenomenological model was set up in order to explore whether the skewed shape and frequency shift observed could originate from a global temperature rise. The simulation produces resonance curve shapes that agree well with the experimental observation whether the frequency is swept up or down. However, the model fails to produce a sufficiently large resonance frequency shift unless input powers two orders of magnitude larger than in experiment or unrealistically small values for the thermal conductivity are used. The delay time experiments in combination with the simulation suggests that the films fail due to a complex combination of mechanisms.