Processing and testing of single cell 650 MHz superconducting RF cavity

RRCAT, Indore has taken up a program on R and D activities for 1 GeV high intensity superconducting proton linac for Indian Spallation Neutron Source (ISNS). Five-cell 650 MHz cavities will be used for acceleration of protons/H^- in medium to high energy range. Few single-cell 650 MHz cavities have been fabricated as first step towards development of multi-cell cavities. The superconducting RF (SCRF) cavities require a very high quality interior surface without any defects or impurities embedded on it. A high quality surface prevents electrical breakdown at high accelerating electric fields in the cavity, which helps in enhancing the efficiency of RF power transfer from the cavity to the beam. Defects at the level of a few microns can lead the cavity to lose its superconductivity (referred to as cavity quenching). Hence the cavity after fabrication needs to go through several steps of processing to conform to the desired surface quality. SCRF cavity fabrication and processing procedure have to be qualified for their performance at a temperature of 2 K. The cavities are tested at 2 K in the Vertical Test Stand (VTS). Cavities, qualified in VTS are then dressed with their auxiliary equipments, like helium vessel, HOM couplers, cold tuner and main coupler. Facility for processing and testing of 650 MHz SCRF cavities have been set up. This include electropolishing setup (EP), high pressure rinsing (HPR) setup, high temperature annealing facility, low temperature baking facility and vertical test facility. To develop the cavity processing recipe and generate required experience, a 650 MHz single cell SCRF cavity fabricated indigenously was processed and tested for the first time using these facilities. The cavity was inspected on an optical inspection bench suitably modified to accommodate up to five-cell 650 MHz cavity, electropolished, high pressure rinsed with ultra-pure water, high temperature annealed and tested at 2 K. The paper reports the description of the setups developed for cavity processing and steps followed for processing and testing of single-cell 650 MHz SCRF cavity at RRCAT. (author)