We have developed a database for high magnetic field science and technology. Tsukuba Magnet Laboratory (TML) of the National Research Institute for Metals (NRIM) holds one of the world's leading high magnetic field facilities, incorporating external scientists from around the globe. We have established cooperative relationships with other high magnetic field laboratories worldwide. The NRIM is also a central organization of VAMAS activities for the standardization of measurement methods for superconductive materials. The database contains experimental results obtained at our facilities, standard references given by VAMAS and technological information provided by high magnetic field laboratories. However, from the user's point of view the database should be global, preferably including other fields of superconductivity and cryogenics. We will embody up-to-date results reported in symposiums, conferences and journals, and bring together existing data scattered throughout different sources into our database. We call it the “Engineering Database for Applied Superconductivity and Cryogenics”. Our system also includes “Reference Database” dealing with data sources, and “Technical Terms Database” as a thesaurus of the technical terms used in the field of superconductivity and cryogenics. The database can be accessed from around the world through the Internet using a web browser. This project will be conducted in cooperation with the Cryogenic Association of Japan. We would appreciate researchers' contributions to the engineering database.
We have measured the low temperature specific heats for Er (Ag1-xMx) (M=Al, Pd and Fe, x≤0.2) alloys over the temperature range of 1.5-25K to explore efficient regenerator materials suitable at low temperatures around 10K. In particular, the peak height of the volumetric specific heat of Er (Ag0.9Al0.1) alloy has reached 1.2J/(Kcm3) at 16.5K, which is 1.5 times higher than that of the ErAg compound previously reported. Its large specific heat is attributed to the 4-fold degeneracy in the ground state of the Er 4f-levels. Moreover, we have revealed that the addition of third elements M=Pd and Fe to the ErAg compound could lower the peak position of the specific heat to about 10K. We conclude that a series of the ternary Er (Ag1-xMx) (M=Al, Pd and Fe, x≤0.2) alloys can serve as good regenerator materials in the temperature range below 17K.
The mechanical properties of an (Nb, Ti)3Sn superconductor have been improved by using Ta as a reinforcer. The conductor was manufactured on an applicable continuous-piece length and showed 0.2% yield strength of 305MPa at 4.2K. Stress-strain characteristics could be explained by the mixture law with Ta itself and another part of the conductor. The dependence of critical current densities on plastic strain was studied based on the scaling law. The actual performances of the developed conductor exceeded all target values in the preliminary design of a 1GHz NMR magnet.
Nonuniform current distribution is generated in a conductor consisting of strands coated by a high resistive layer, such as chromium plating, as a result of superimposition of transport and induced circulation currents. The characteristics of the induced circulation current are analytically studied by using a distributed model circuit. The parameters mostly used in this calculation are those of US-DPC coil, which at first exhibited instability and so-called ramp rate limitation (RRL) because of current imbalance in the conductor consisting of chrome-plated strands. Thus the conductance among strands and the inductance of unit length loop and length of the conductor are mostly assumed to be 10kS/m, 0.5μH/m and 150m, respectively. The analysis results indicate that the induced circulation current can be classified into the boundary and interstrand-induced circulation currents, hereafter referred to as BICC and IICC. BICC is induced only across the joint at the ends of the conductor, resulting in a constant along the conductor axis, when the total leakage magnetic flux of the loop is not zero. Its decay time constant is quite long, more than a few hours. In contrast, when the leakage magnetic flux distributes along the conductor axis, IICC is induced among strands in the conductor to eliminate this flux. Since the leakage magnetic flux normally becomes largest where the magnetic field is highest, it becomes larger where the time variation of the magnetic field is larger. Its decay time constant is much less than that of BICC. If the leakage magnetic flux linearly changes along the US-DPC conductor, it is evaluated to be about 10s. This IICC therefore becomes dominant in a pulse charge, whose ramping time is less than 10s. Moreover, it is found that the variation of the leakage magnetic flux with the relatively long cycle, such as more than a few 10-meter lengths, causes IICC with a decay-time constant of more than several hundred milliseconds. Such an IICC can greatly enhance the total IICC in a short charge. The nonuniformity of the current distribution therefore becomes larger in the shorter charge where the magnetic field is high. Consequently, RRL can be explained qualitatively by these effects from IICC. Note that the decay time constant of IICC generated from magnetic flux as a result of cabling is too short to explain RRL of the US-DPC coil.
Heat transport characteristics of superfluid helium (He II) in channels segmented by different types of spacers having several holes have been investigated on the basis of both experimental and numerical analyses. The spacer is placed along the channel axis so that the internal flow path of the channel is divided into two rectangular portions of equal size. In the present study, the heat transport performance has been examined for the following four types of spacers: (a) FRP spacer with several holes of 2mm diameter, (b) FRP spacer with several holes of 4mm diameter, (c) FRP spacer with no holes, and (d) porous spacer made of fine powder of sintered Al2O3. The λ-transition heat-flux when spacers with holes of 2mm and 4mm in diameter are used is almost the same. This value is about 1.5 times larger than when a spacer without a hole is used. However, this value is smaller than that of the porous spacer, because the fountain effect is not induced in spacers with relatively larger holes. The temperature distribution in the He II channel and the λ-transition heat-flux, which is obtained on the basis of present two dimensional computation taking account only of the heat conduction of He II, shows satisfactory agreement with those obtained by the experiment. The results of the present study lead to the understanding that the introduction of some holes in the spacer is a very effective way to suppress a local temperature rise in the He II channel.