  The performance of Niobium-based Superconducting Radio Frequency (SRF)
particle accelerator cavities can be sensitive to localized defects that give
rise to quenches at high accelerating gradients. In order to identify these
material defects on bulk Nb surfaces at their operating frequency and
temperature, a wide bandwidth microwave microscope with localized and strong RF
magnetic fields is developed by integrating a magnetic write head into the
near-field microwave microscope to enable mapping of the local electrodynamic
response in the multi-GHz frequency regime at cryogenic temperatures. This
magnetic writer demonstrates a localized and strong RF magnetic field on bulk
Nb surface with $B_{surface}$ > 102 mT and sub-micron resolution. By measuring
the nonlinear response of the superconductor, nonlinearity coming from the
nano-scale weak link Josephson junctions due to the contaminated surface in the
cavity fabrication process is demonstrated.
