Study of RF flux penetration on Nb for SRF Applications

Superconducting Radio Frequency (SRF) cavities are being widely used in new generations of particle accelerators. A novel near-field magnetic field microwave microscope was successfully built to identify defects on Nb surfaces that limit the accelerating gradient. Our microscope can measure local nonlinear response (3^rd harmonic voltage V_3f) from Nb bulk and thin film superconductor surfaces under conditions approaching B_surface ~ 600mT and frequencies in the multi-GHz range. Using our probe, we performed local microwave measurements of V_3f(T,H_rf) and its dependence on temperature and rf input power. We observed that below a temperature dependent onset rf field amplitude H₀(T), V_3f(T,H) is below our noise floor. For fields H_rf > H ₀(T) there exists relatively high nonlinear response signal with periodic dips at H_rf = H₁(T), H₂(T), H₃(T)… Similar behavior is observed in both bulk Nb and thin film Nb samples. The origins of this response is being investigated with the most likely explanation being nonlinearity generated by Josephson Junctions on the surface where such periodicity would be expected. As temperature approaches T_c, H₀(T) goes to zero, thus providing a tool for local measurement of T_c.