Process parameters for electron beam welding of various parts of a 650 MHz five cell SCRF cavities

RRCAT is working on technology development for manufacturing of SCRF cavities. Large number of high beta 650 MHz multi-cell SCRF cavities are required to accelerate beam from 500 MeV to 1GeV for the proposed SC proton Linac. A 15 KW EB welding machine has been in installed and successfully commissioned at RRCAT for fabrication of SCRF cavities. Using in-house EBW machine, initially a prototype single cell 650 MHz beta 0.92 SCRF cavity was fabricated to qualify the manufacturing process, tooling required for machining, and electron beam welding. Based on this experience, a 650 MHz five cell SCRF cavity has also been fabricated. The five cell cavity consist of dumbbells, Main coupler (MC) end group, Field probe (FP) end group and Titanium end cap. The paper presents complete manufacturing process sequence plan, weld joint design, covering weld parameters development, design and fabrication of welding, machining and dumbbell tuning fixtures. The paper describe half cells, fabrication of dumbbells, endgroups and their RF and Mechanical qualifications at intermediate stages and final equator welding. A proper weld shrinkage estimation and suitable allowance in various parts of SCRF cavity is essential to maintain target length and RF frequency at 300K. The paper also presents fabrication challenges like crucial equator weld, dissimilar welding between Niobium to Niobium-Titanium (Nb-Ti) and Nb-Ti to Titanium. The completed five cell SCRF cavity has been pre-qualified for dimensional measurement, vacuum leak testing and RF testing at 300K. (author)