Development of low cost superconducting cavity at ULVAC

Niobium (Nb) refining and cavity fabrication process are investigated for low cost superconducting cavity. In order to carry out Nb purification, 600 kW electron beam furnace was introduced in our company. It makes possible the stable refining for cavity-quality RRR > 250 grade by optimization of melting condition. We also obtained RRR > 300 ingot from commercial grade raw material which includes 2200 ppm tantalum. This suggests that cavity production cost might be decrease by using cheap raw materials. We performed the trial manufacture of two 1.3 GHz single-cell cavities are made from our high purity Nb ingot (RRR > 300). One is adopted the half cell conjugation with electron beam welding. We confirmed that an accelerating gradient of this cavity was achieved 41 MV/m at vertical test in KEK. Another cavity is fabricated by hydroforming from seamless tube, i.e. direct cavity-shape forming without electron beam welding process at cell equator. Seamless cavity was successfully formed. We consider that key point of successful forming is homogenization of crystal grain size all over the seamless tube. (author)