The purity of niobium for a superconducting cavity refined using an electron-beam melting method was explained from RRR measurement, gas analysis and GDMS analysis. Metallic purity notation generally refers to the value obtained by subtracting the total of metal impurities other than gas components from100%. For example, it is 3N at 99.9% and 4N at 99.99%. In the case of niobium, since tantalum containing tens to hundreds of ppm is the majority of the impurities, it is 3N or 2N8 in a normal purity notation. However, with the agreement of customers, when tantalum, tungsten and molybdenum are excluded from the target elements, purity corresponding to 6N - 7N is obtained. Tantalum is not removed by electron-beam melting, so we select and purchase less tantalum at the raw materials stage.
High-purity Nb as a superconducting cavity material for accelerators is investigated. In order to carry out Nb purification, a 600-kW electron beam furnace procured by our company. This has made stable refining for cavity applicationgrade material possible by optimizing the melting conditions. The change in hardness achieved by processing in high-purity Nb is definitely different from that of low-purity Nb. High-purity Nb requires considerably high processing power to induce sufficient strain hardening. We believe that the key point of fine forming in high-purity Nb is the homogenization of crystal grain size using a strong process. Trial manufacturing of two single-cell cavities using our high-purity Nb ingot was performed. An accelerating gradient of 35 M/m was achieved using these cavities. Additionally, we succeeded in the fabrication of a seamless tube for a three-cell cavity in a scale-up study. Since the average grain size in the tube for a three-cell cavity is smaller than that for a singlecell cavity, it is expected that a smoother surface will obtained after the hydroforming process.
We have been studying the characteristics of ice as a dielectric or electrical insulation material at cryogenic temperature. IceXI, ferroelectric ice, exists at cryogenic temperature. It is very difficult to make iceXI. We applied direct-currentvoltage to ordinary ice at 253 K and cooled it to 77 K to obtain a polarized ice similar to iceXI. In this procedure, the protons move towards the cathode side under an electric field. The protons were stopped on the cathode side by cooling to 77 K, resulting in ice polarization. This polarized ice is called “ice electret”. We have previously reported the amount of electrical charge calculated from an integral of the ice electret depolarization current observed. The amount of charge increased as voltage and application time increased. In this paper, the effect of voltage, application time and temperature on depolarization