Understanding RF Losses in Nb Cavities

Magnetic vortices pinned near the inner surface of SRF Nb cavities are a possible source of RF hotspots, frequently observed by temperature mapping of the cavities outer surface at RF surface magnetic fields of about 100 mT. Theoretically, we expect that the thermal gradient provided by a high-power green laser shining on the inner cavity surface at the RF hotspot locations can move pinned vortices to different pinning locations. We built an apparatus to send the beam onto the inner surface of a photoinjector-type large-grain Nb cavity. Preliminary results confirm the possibility of breaking and moving some hotspots with the laser beam.