Hydroforming of N[lc]bC[lc]u clad cavities at DESY

Abstract

Several bimetallic NbCu single cell cavities of the TESLA shape have been fabricated at DESY. The cavities have been hydroformed by expansion of seamless tubes with internal water pressure while simultaneously swaging them axially. Tube radius and axial displacement are being computer controlled according to results of FEM simulations and experimentally obtained strain-stress curve of tube material. The tube ends were necked before hydroforming. The seamless tubes have been produced by explosion bonding and flow forming. The thickness of the Nb and Cu layers in the tube is about 1 mm and 3 mm, respectively. Surface treatments such as buffered chemical polishing (BCP), high pressure ultra pure water rinsing (HPR), annealing at 800 C and baking at ca. 150 C have been applied. The RF performance of NbCu clad cavities is similar to that of bulk Nb cavities. The highest accelerating gradient achieved was E(sub acc) = 40 MV/m with a Q(sub 0)-value of approximately 10(exp 10) after ca. 180 micrometer BCP, annealing at 800 C and baking at 140 C for 30 hours. The degradation of the quality factor Q(sub 0) after quenches of the cavity is moderate; after ca. 150 quenches it reaches the saturation point of Q(sub 0) = 1.4 x 10(exp 10) at low field. This indicates that on the basis of RF performance and material costs the combination of hydroforming with tube cladding is a very promising option.