Study of Field-Limiting Defects in Superconducting RF Cavities for Electron-Accelerators

Superconducting radio-frequency resonators made from niobium are an integral part ofmany accelerator projects. Their main advantage are the low ohmic losses resulting inthe possibility for a long pulse structure and high duty cycles up to continous wave (cw)operation. The European X-Ray Free-Electron Laser (XFEL) and the International LinearCollider (ILC) are based on this technology. In some cases the resonators reach acceleratingelectric fields close to the theoretical limit of bulk niobium. Yet most resonatorsare limited at lower fields and mass production for large scale accelerator projects suffersfrom the spread in the achievable gradient per resonator. The main limitations arefield emission and the breakdown of superconductivity (quench). While field emissionis mostly attributed to the overall surface cleanliness of the resonator, quench is usuallyassociated with local defects.Optical inspection of the inner surface of the resonators with unprecedented resolution,accuracy and a special illumination has been established at DESY and used to study suchlocal surface defects. More than 30 resonators have been inspected. Distinctive featuresfrom these inspections have been catalogued and assessed for their potential risk for theperformance of the resonator. Several confirmed quenching defects could be extracted forfurther analysis and could be traced back to likely origins in the production process.A new, automated set-up for optical inspection of large series of resonators, namedOBACHT, has been developed and successfully commissioned. Its design includes theminimal need for operator interference, reproducibility, robustness and versatility, in orderto fit the requirements for application both in a laboratory and in a production environment.To facilitate the comparison of the results obtained during the global R&D effort on resonatorsfor the ILC, the ILC global yield database has been established. The yield andselection rules for the resonators have been defined and the ILC yield database has servedas a tool to monitor the progress of the ILC R&D program