A novel method to identify and correct asymmetry of dumbbells in a multi-cell elliptical superconducting cavity

Raja Ramanna Centre for Advanced Technology (RRCAT) has an ongoing program to develop 650 MHz, 5-cell elliptical superconducting RF (SCRF) cavities under the Indian Institutes and Fermilab Collaboration. The elliptical multi-cell SCRF cavity fabrication process involves forming of half-cells and their precise machining and joining by electron beam welding to form end groups and dumbbells, which are then joined to make the final cavity. To ensure that the final welded cavity achieves physical lengths and resonant frequencies within design tolerance and has good field flatness, the measurement and correction of resonant frequency are carried out for dumbbells and end groups. A novel method to identify the frequency of individual half-cells in a dumbbell cavity and a dedicated tuning fixture to correct them had been developed. The paper details the RF characterization and correction procedure employed during fabrication of the first six 650 MHz cavities at RRCAT.