Ultracold neutrons are defined and their general physical properties are explained. Their relations with modern physics and how they can be produced in laboratories are discussed. Superthermal production in superfluid liquid helium, the most promising method, is presented, quoting the results of Mark 3000, a superthermal ultracold neutron generator.
After a brief history of the International System of Units (SI), the effects of a recent revision of the Japanese Measurement Law, in particular those of its policy of mandatory use of SI units, are described together with their backgrounds. Some of the conventional units for force and pressure such as kilogram-force and Torr shall be relinquished from the official use in trade and certification activities not later than September 30, 1999 so as to harmonize with the trend in international certification systems. Major problems are explained and some countermeasures are suggested.
In multiply-twisted superconducting cables exposed to an external AC transverse magnetic field, a component of the magnetic field parallel to the strand axis brings about an additional AC loss (longitudinal AC loss) in the strands. We have experimentally evaluated the longitudinal AC loss in two types of triply-twisted superconducting cables composed of insulated strands with different diameters, by eliminating the hysteresis loss and the transverse coupling-current loss from the total loss in the strands. The evaluated longitudinal AC losses were quantitatively explained by a theoretical expression obtained so far, under consideration of the effects of the longitudinal component of the applied field to the strand. The present results suggest that the longitudinal loss can be dominant in AC magnetic fields with a relatively large amplitude. To minimize the total loss of the strand in multiply-twisted cables, we discuss an optimum condition for the twist pitch of the strand.
Superconductively jointed silver-sheathed Bi-2212 tapes have been newly fabricated. Two types of methods, which are a side joint and a center joint, were employed for the fabrication of superconducting joint. Side joint and the center joint refer to the techniques by which two separate tapes were connected to each other at both terminal sides and the center of an additional intermediate tape, respectively. The center jointing technique is more promising than the side jointing one. However, reduction of the distance between the two tapes is required to realize a high critical current through the jointed part even in the case of the center joint. The resulting jointed tapes proved that the critical current through the jointed part is higher than in the jointless part.
This paper describes a novel suspension technique using high-Tc superconductors and magnetic materials. For a field-cooled superconductor and an adjacent magnetic material, it has been found that the usual inverse relationship between the attractive magnetic force and gap distance switches to a direct relationship for small gap lengths under certain conditions. This phenomenon was found for even soft magnetic materials possessing a minimal coercive force. Thus, stable, noncontact suspension is possible within a certain gap range. When compared to a conventional superconductor-permanent magnet combination, both reduced cost and increased mechanical robustness can be expected since permanent magnets are not needed. Since remarkable switching in the force-gap relationship has been observed for magnetic materials with diameters much smaller than that of the effective pinned area, it was considered that the pinned flux is subject to a “gathering effect” in the magnetic material. Stable contactless suspension of a 180-g carbon steel weight under a superconductor has been performed previously. In continuing work, suspension of a 1.4-kg superconductor vehicle under a 1.8-m silicon steel rail was achieved by an extension of this technique.