CLASSE, Ithaca, New York
High field Q-slope in niobium cavities is presently not understood. In this contribution we present experimental data that point toward the possible role of crystalline lattice defects (i.e. dislocations) in the HFQS. In addition, TEM and EELS investigations on samples cut from electropolished HFQS-limited cavities are presented.
CLASSE, Ithaca, New York
Cornell University’s superconducting cavity development program is one contributor to the global collaborative effort on critical SRF R&D for the ILC. We conduct R&D in support of the baseline cavity development as well as several alternate cavity development paths. For the baseline program we are preparing and testing ILC cavities. We have developed a new quench detection system and successfully applied it to ILC 9-cell and 1-cell cavities to find quench producing defects, which were characterized with subsequent optical examination. We have successfully repaired a 9-cell cavity using tumbling to raise the accelerating gradient from 15 to above 30 MV/m. We have identified quench producing defects in single-cell cavities using our large-scale thermometry system and subsequently extracted and inspected the defect region with an SEM. For the alternate R&D, we are developing reentrant cavity shapes with 70 mm and 60 mm apertures, and a simpler, potentially faster and less expensive electropolishing method called vertical electropolishing. We are also assisting in developing new cavity vendors by rapidly testing single-cell cavities they produced to qualify their fabrication methods.
CLASSE, Ithaca, New York
Pit-like structures are defect candidates that cause cavity quenches. Thermometry and SEM examination results of two such pit candidates are presented. The observed and simulated correlations between defects size and pre-heating temperature near the defect region at helium side can provide useful information about the effective defect size and resistance. Calculations based on a disk-type defect model suggest that the observed pit is much larger than the actual normal conducting region responsible for initiating the quench. This finding is consistent with the sharp edge segments of the pit as the possible regions responsible.