  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.

Superconductivity