Takataro Hamajima

Test results of the 100 kWh SMES model coil – AC loss performance

Abstract In order to establish a technology needed for a small-scale 100 kWh SMES device, an SMES model coil was fabricated. Performance tests were carried out at the Japan Atomic Energy Research Institute (JAERI) in 1996. After that, the coil was installed in the Lawrence Livermore National Laboratory (LLNL) facility and tested in 1998, in collaboration between Japan and the United States. The AC losses measured at LLNL were in good agreement with those measured at JAERI. It was reconfirmed that the coupling loss of the coil could be expressed in two components: one with a short and another with a long coupling time constant. We found out from the Hall probe signals that the loop currents with long decay times were induced in the CIC conductor by varying magnetic field. These currents resulted in additional AC loss in the coil. To develop a concept of CIC with low AC loss, we made a sub-scale CIC conductor of strands coated with CuNi. We fabricated a small coil out of this conductor and measured the AC loss. The measured AC loss in this coil was about 1/6 of that in the SMES model coil conductor per strand volume. Thus, the CuNi coating of the strands was demonstrated to be effective to reduce the AC loss in the coil.

Analysis of current distribution in a large superconductor

An imbalanced current distribution which is often observed in cable-in-conduit (CIC) superconductors composed of multistaged, triplet type sub-cables, can deteriorate the performance of the coils. It is, hence very important to analyze the current distribution in a superconductor and find out methods to realize a homogeneous current distribution in the conductor. We apply magnetic flux conservation in a loop contoured by electric center lines of filaments in two arbitrary strands located on adjacent layers in a coaxial multilayer superconductor, and thereby analyze the current distribution in the conductor. A generalized formula governing the current distribution can be described as explicit functions of the superconductor construction parameters, such as twist pitch, twist direction and radius of individual layer. We numerically analyze a homogeneous current distribution as a function of the twist pitches of layers, using the fundamental formula. Moreover, it is demonstrated that we can control current distribution in the coaxial superconductor.

Long time constants of irregular AC coupling losses in a large superconducting coil

A large superconducting coil wound with Cable-in-Conduit (CIC) conductor caused an irregular AC loss that cannot be estimated from short conductor sample test results. It was confirmed that the irregular AC loss was generated by long current loops in the CIC conductor. We proposed a mechanism forming the long loops. The CIC conductor is composed of several staged sub-cables. If one strand on the surface of a sub-cable contacts with the other strand on the surface of the adjacent sub-cable, the two strands must encounter each other again at LCM (Least Common Multiplier) distance of all staged cable pitches and thereby result in forming a pair of a long loop. We measured cross over point contact resistance between two strands making the long loop. The calculated time constants of the long loops were shorter than the observed ones. We orderly labeled all strands in a real CIC conductor, disassembling carefully the cable after peeling the conduit. It was found that the strands in a triplet were widely displaced from their original positions, and thereby their contacting lengths became longer than cross over ones to form line contacts. This fact can make the time constant. of the loop longer and hence can explain the observed long time constants.

Optical microstructure and superconducting properties in jelly-roll Nb/sub 3/Al multifilamentary wire by rapid heating

To form the stoichiometric composition in the A15-phase, a rapid quench from the stable region at high temperatures is needed. Jelly-roll Nb/sub 3/Al wire with a Nb matrix is ohmically-heated up to 2000 /spl deg/C during 0.1 sec in a vacuum. An optimization of rapid heating conditions is needed to an improve critical current density at high field because very short heating times and high temperature close to melting points of the Nb/sub 3/Al filaments and Nb matrix are used. The magnetic field dependence of critical current density, J/sub c/, and the critical temperature, T/sub c/, were studied as a function of the maximum temperature, T/sub m/, using short samples that were heat-treated systematically. The cross-sections of these typical samples are analyzed by optical microscopy on various rapid heating conditions. In the short heating time of a range of 0.32-0.46 sec, the filament regions heated up to 2000 /spl deg/C would be uniform and have relatively homogeneous superconducting properties. In the samples with long heating time, the filaments reacted with around matrix. The filament regions heated up to 2300 /spl deg/C are inhomogeneous and some filaments melted.

Lift-to-weight ratio dependence of lift and stability in an active-Maglev system

In an active-Maglev system composed of YBCO bulk and electromagnets, lift-to-weight ratio dependence of lift and stability was investigated experimentally and theoretically. A bulk was levitated by an electromagnet after a field-cooling process for magnetization. It has been shown that lift and stability of the bulk are closely related to both the magnitude of trapped magnetic field and the magnetic field distribution generated by the electromagnet. Minimum field-cooling current for stable levitation, the operating current of electromagnet at initial levitation, and maximum stable-levitation height were measured as a function of the load on the top surface of the bulk. It was observed that the minimum trapped field for stable levitation increased with the load. Maximum permissible displacement in the radial direction was also investigated experimentally as a function of the load and levitation height. The maximum permissible displacement decreases with levitation height and stable-levitation range decreases with weight of load. Numerical analysis based on the three-dimensional finite element method was performed to investigate electromagnetic behaviors within bulk, especially stability at initial levitation. The stability depends on the ratio of operating current at initial levitation to field-cooling current.