Microwave Microscopy of Materials Limitations of Superconducting RF Cavities*

There are considerable efforts world-wide to improve particle accelerators by using Superconducting Radio Frequency (SRF) cavities which are limited by surface defects which lead to cavity breakdown at high accelerating gradients. A novel near-field magnetic microwave microscope that can study these defects was successfully built using a magnetic writer from a conventional magnetic recording hard-disk drive. We study the 3rd harmonic response-V_{3f}(T, H_{rf}) because it is far more sensitive to rf field amplitude H_{rf} and temperature (T) than linear response measurements.In our experiments on Nb surfaces we observed 2 different classes of nonlinearity depending on the location of the probe, which we call Low-field and Periodic. In the low-field case we observe that V_{3f} increases uniformly as a function of applied rf field amplitude, reaches a peak value and decreases to back to 0. In the periodic case V_{3f} has periodic dips at H_{rf} = H_1(T), H_2(T), H_3(T)… The periodic case nonlinear response can be linked to Josephson effect at or near the surface and is in good agreement with the nonlinear response expected from rf-current-biased Resistively and Capacitively Shunted Junction (RCSJ) model.