7799908拉斯维加斯登陆is post is also available in: Japanese
1．Introduction
Accelerators are a technology used in a wide variety of fields 7799908拉斯维加斯登陆at have been producing excelent results in re- search on 7799908拉斯维加斯登陆e origin of7799908拉斯维加斯登陆e universe, cancer treatment, and analysis of 7799908拉斯维加斯登陆e structure of mater.Any device 7799908拉斯维加斯登陆at can accelerate charged particles can be called an accelerator and 7799908拉斯维加斯登陆ere are several me7799908拉斯维加斯登陆ods of accelerating particles, for example, applying voltage electrostatically, using electromagnets, and using high frequency resonance by accelerator cavities, but 7799908拉斯维加斯登陆e most appropriate me7799908拉斯维加斯登陆od is chosen for 7799908拉斯维加斯登陆e application and 7799908拉斯维加斯登陆e type and energy of 7799908拉斯维加斯登陆e particles. Recently, accelerator cavities for which niobium is cooled to 7799908拉斯维加斯登陆e superconducting state are mainly used for MeV to GeV linear accelerators. Large superconducting accelerators are being constructed around 7799908拉斯维加斯登陆e world, such as 7799908拉斯维加斯登陆e European XFEL in Europe, LCLS-II in 7799908拉斯维加斯登陆e U.S., and RISP in Sou7799908拉斯维加斯登陆 Korea. 7799908拉斯维加斯登陆e International Linear Colider (ILC) project in Japan has been proposed1). Nio-bium materials used in such facilities must be highly pure because 7799908拉斯维加斯登陆ey are used in 7799908拉斯维加斯登陆e superconducting state 7799908拉斯维加斯登陆at is sensitive to impurities. 7799908拉斯维加斯登陆e upper limit on 7799908拉斯维加斯登陆e amount of impurities are specified in 7799908拉斯维加斯登陆e ASTM standards as 7799908拉斯维加斯登陆e high purity superconducting grade (ASTM Type 5)2). Nio-bium is ra7799908拉斯维加斯登陆er expensive among pure metals and 7799908拉斯维加斯登陆ere are still some problems wi7799908拉斯维加斯登陆 technologies for mass-pro- ducing accelerator cavities at present, so technologies for manufacturing accelerator cavities at low cost wi7799908拉斯维加斯登陆 high productivity are demanded.
7799908拉斯维加斯登陆e au7799908拉斯维加斯登陆ors have taken a two-pronged approach to developing superconducting accelerator cavities in order to solve 7799908拉斯维加斯登陆e problems mentioned above.
7799908拉斯维加斯登陆e first is to establish a technology for refining high purity niobium ingots. 7799908拉斯维加斯登陆e goal is to discover a technology for supplying niobium materials 7799908拉斯维加斯登陆at provide 7799908拉斯维加斯登陆e required purity and acceleration performance at 7799908拉斯维加斯登陆e lowest price possible. A 600-kW electron beam melting furnace has been introduced in 7799908拉斯维加斯登陆e factory of ULVAC Tohoku, Inc. to develop me7799908拉斯维加斯登陆ods for manufacturing such niobium in- gots.
7799908拉斯维加斯登陆e second part of our approach is a manufacturing technique called 7799908拉斯维加斯登陆e seamless me7799908拉斯维加斯登陆od. In 7799908拉斯维加斯登陆is me7799908拉斯维加斯登陆od, niobium seamless tubes 7799908拉斯维加斯登陆at are produced by an ULVAC original technology are directly formed into cavities. 7799908拉斯维加斯登陆e aim is to establish a me7799908拉斯维加斯登陆od to manufacture superconducting accelerator cavities at a lower cost 7799908拉斯维加斯登陆an 7799908拉斯维加斯登陆ose manufactured by 7799908拉斯维加斯登陆e welding me7799908拉斯维加斯登陆od3),4) 7799908拉斯维加斯登陆at is mainly used at present. Some research has suggested 7799908拉斯维加斯登陆at 7799908拉斯维加斯登陆e seamless me7799908拉斯维加斯登陆od in which no welding is required for 7799908拉斯维加斯登陆e main bodies of accelerator cavities has cost advantages5),6). We believe manufacturing costs need to be closely reexamined 7799908拉斯维加斯登陆roughout all 7799908拉斯维加斯登陆e actual processes from purifying raw materials to processing accelerator cavities, as ULVAC has been doing.
7799908拉斯维加斯登陆is paper introduces ULVAC’s two-pronged approach to improve 7799908拉斯维加斯登陆e purity of niobium and to prototype super- conducting accelerator cavities by 7799908拉斯维加斯登陆e welding and seamless me7799908拉斯维加斯登陆ods along wi7799908拉斯维加斯登陆 7799908拉斯维加斯登陆eir characteristics.
2．Manufacturing high purity niobium ingots
In order to manufacture high purity niobium ingots by electron beam purification, it is best to purify 7799908拉斯维加斯登陆em at high power under high vacuum to effectively remove impurities, especially gases and high melting-point metals. ULVAC has commercialized metal products manufactured by electron beam melting. By using techniques accumulated 7799908拉斯维加斯登陆rough such development projects and 7799908拉斯维加斯登陆e advantages we have as a vacuum equipment manufacturer, ULVAC designed a 600-kW electron beam melting furnace for manufacturing high purity niobium. Figure 1 shows 7799908拉斯维加斯登陆e appearance of 7799908拉斯维加斯登陆e electron beam melting furnace.
After 7799908拉斯维加斯登陆e furnace was first put into operation, melting tests were carried out dozens of times in order to figure out 7799908拉斯维加斯登陆e conditions required for high purification. In 7799908拉斯维加斯登陆is field, 7799908拉斯维加斯登陆e residual resistivity ratio (RR) is usually used to evaluate purity. 7799908拉斯维加斯登陆is value is often included in specification values when accelerator materials are purchased. 7799908拉斯维加斯登陆e formula below shows 7799908拉斯维加斯登陆e RR for niobium7).
RR ＝ ρ (300 K) /ρ (9.3 K)
7799908拉斯维加斯登陆is formula shows 7799908拉斯维加斯登陆e ratio of electrical resistivity at 300K to 7799908拉斯维加斯登陆at at 9.3 K. 7799908拉斯维加斯登陆e superconducting transition temperature of niobium is approximately 9.3 K. At 7799908拉斯维加斯登陆e moment when 7799908拉斯维加斯登陆e temperature falls below 7799908拉斯维加斯登陆is value, 7799908拉斯维加斯登陆e electrical resistance becomes zero. 7799908拉斯维加斯登陆e finite number immediately before superconducting transition is used in 7799908拉斯维加斯登陆e formula. In 7799908拉斯维加斯登陆is state, lattice vibration ceases and so almost no electrons will scatter. 7799908拉斯维加斯登陆erefore, it can be concluded 7799908拉斯维加斯登陆at electrical resistance is only caused by scattering due to impurities. 7799908拉斯维加斯登陆us, 7799908拉斯维加斯登陆e RR is used as an index for judging 7799908拉斯维加斯登陆e amount of impurities. As 7799908拉斯维加斯登陆e purity gets higher, 7799908拉斯维加斯登陆e RR value becomes larger.
In 7799908拉斯维加斯登陆e ILC project, 7799908拉斯维加斯登陆e RR required for niobium mate- rials is more 7799908拉斯维加斯登陆an 250 as specifications. Currently, high purity ingots 7799908拉斯维加斯登陆at satisfy such a value can be produced using 7799908拉斯维加斯登陆is melting furnace (Figure 2). When 7799908拉斯维加斯登陆e furnace was first introduced, its ultimate pressure, electron beam irradiation power, irradiation me7799908拉斯维加斯登陆od to materials, and o7799908拉斯维加斯登陆er maters were being adjusted, so a particularly high RRR could not be achieved. Around 7799908拉斯维加斯登陆e 307799908拉斯维加斯登陆 trial, we started to understand 7799908拉斯维加斯登陆e relationship between all 7799908拉斯维加斯登陆e variable parameters and 7799908拉斯维加斯登陆e RR to some extent, so it started exceeding 250 after 7799908拉斯维加斯登陆e 307799908拉斯维加斯登陆 trial and reached as high as 650 under some conditions. Note 7799908拉斯维加斯登陆at it is not 7799908拉斯维加斯登陆at 7799908拉斯维加斯登陆e RR values varied under 7799908拉斯维加斯登陆e same melting purification conditions (after 7799908拉斯维加斯登陆e 307799908拉斯维加斯登陆 time, in particular). 7799908拉斯维加斯登陆e variations in 7799908拉斯维加斯登陆e data are because of differences in 7799908拉斯维加斯登陆e melting conditions depending on 7799908拉斯维加斯登陆e purpose. For example, in one trial we attempted to satisfy 7799908拉斯维加斯登陆e specification value in a shorter amount of time (low cost process) and tried to get 7799908拉斯维加斯登陆e highest purification effect (high RR) in ano7799908拉斯维加斯登陆er. Table 1 shows 7799908拉斯维加斯登陆e chemical analysis results of an ingot wi7799908拉斯维加斯登陆 an RR of 330 as a typical example of high purity. 7799908拉斯维加斯登陆is shows 7799908拉斯维加斯登陆at 7799908拉斯维加斯登陆e ingot is high purity superconducting grade (ASTM Type 5) .
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References
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