High-Frequency Nonlinear Response of Superconducting Cavity-Grade Surfaces

Superconducting Radio Frequency (SRF) cavities are used to accelerate charged particles to high energies, but their performance is 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} as a function of rf field amplitude (H_{rf}) and temperature (T). In our experiments on bulk and thin-film Nb surfaces, we observed 2 different classes of nonlinearity, which we call Low-field and Periodic. The low-field response is the intrinsic response of the sample due to the dynamics of vortex semiloops created by the magnetic writer (arXiv:1909.02714). In the periodic case there exists a relatively strong onset nonlinear response signal with periodic dips at H_{rf} = H_1(T), H_2(T), H_3(T)… This response can be linked to the Josephson effect at or near the surface and is in good agreement with the nonlinear response expected from rf-current-biased RCSJ model (PRApplied 11, 064030 (2019)). Efforts to image third harmonic response across Nb surfaces will be presented.