Squid based non-destructive testing instrument of dished Nb sheets for SRF cavities

The performance of superconducting RF cavities used in accelerators can be enhanced by detecting micro particles and inclusions which are the most serious source of performance degradation. These defects prevent the cavities from reaching the highest possible accelerating fields. We have developed a SQUID scanning system based on eddy current technique that allows the scanning of curved Nb samples. This SQUID scanning system successfully located Tantalum defects about 100 mum diameter in a flat Nb sample and was able to also locate the defects in a cylindrical surface sample. Most importantly, however, the system successfully located the defects on the backside of the flat sample and curved sample, both 3-mm thick. This system can be used for the inspection and detection of such defects during SRF cavity manufacturing.