Near-Field Microwave Magnetic Nanoscopy of Superconducting Radio Frequency Cavity Materials

A localized measurement of the RF critical field on superconducting radio frequency (SRF) cavity materials is a key step to identify specific defects that produce quenches of SRF cavities. Two new measurements are performed to demonstrate these capabilities with a novel near-field scanning probe microwave microscope. The first is a third harmonic nonlinear measurement on a high Residual- Resistance-Ratio bulk Nb sample showing strong localized nonlinear response for the first time, with surface RF magnetic field $B_{surface} \sim 10^{2}$ $mT$. The second is a raster scanned harmonic response image on a high quality $MgB_{2}$ thin film demonstrating a quench defect-free surface over large areas.