Recent progress in a Josephson computer technology is reviewed. A Josephson computer is very attractive because Josephson digital circuits have the potential for realizing LSI circuits with extremely high performance in both circuit speed and power dissipation. Recently, various Josephson LSIs, such as processors and memories, have been made successfully with Nb or NbN junctions, particularly in Japan. These junctions exhibit high reliability for thermal cycling and long term storage, and have a good uniformity of the Josephson tunnelling current due to the tri-layer junction sandwich process. On the basis of such progress in Josephson IC technology, research, on Josephson computers in which Josephson memories are installed with a processor, is now under way.
A superconducting magnet, not fully stabilized, has instability that a normal zone, induced by a certain local disturbance, may lead to normal transition of the magent as a whole. Approximate evaluation of propagation of normal zones in superconducting wires is outlined and some remarks are made for anisotropic propagation in magnets. As a typical example, an analytical expression proposed by Wilson is presented and is compared with experimental results in bare and insulated wires. Basic ideas of magnet protection from mechanical and thermal damages in quench process are explained in two types of protection systems by an external resistor and “quenchback.” New aspects of magnet protection are also discussed in prospective applications of superconducting magnet.
The mechanism of the so-called fast quench, i.e., an abnormal quench process with a high propagation velocity of normal zone in the multi-strand cables is discussed. Firstly, a set of basic equations were proposed so as to describe precisely the electromagnetic coupling among strands, propagation of normal zone and thermal interaction among strands. A good agreement of the theoretical value of transport current, at which the fast quench began to appear, with our observed data for a 2-strand cable supported strongly the availability of the proposed equations for a description of the fast quench. Then numerical calculations were also carried out for the commutation of transport current and temperature distribution inside a 2-strand cable when one strand was locally quenched. With the aid of the obtained results, a detailed mechanism for the occurrence of the fast quench was proposed.
Fast quench processes in a 6-strand cable composed of multifilamentary NbTi wires with Cu-Ni matrices and insulation layers are numerically simulated by equations whose applicability was confirmed by good agreement between the theory and experiments for a 2-strand cable, as shown in the previous paper. The simulated results indicate in detail how the transport current commutates among strands and how flux-flow and normal zones grow up in each strand during the process of the fast quench in the 6-strand cable. With the aid of the obtained results, the conditions for occurence of the fast quench and the normal zone length, which is generated by fast quench, are also discussed.
The accuracy and the reliability of quench detection method of superconducting coil is the most important for the safety of the superconducting coils. If the superconducting coil is high current density and has large stored energy, the accuracy and reliability of the quench detection method dominates the limitation of these capacity. The improved potential method and the fluid dynamic method for the quench detection are developed for the poloidal coil of the Demonstration Poloidal Coil (DPC). These methods have high accuracy and high reliability from the experimental results.
Low resistivity contacts to the bulk specimens of YBa2Cu3O7-y with Ag have been developed. In order to obtain homogeneous mixtures of YBa2Cu3O7-y and Ag we used Ag2O powder, The contact resistivity of 0.19μΩ·cm2 was obtained by annealing the specimen at 1, 100°C. The current versus voltage characteristic of the contact was found to be ohmic. The contact was made to the specimen which is exposed to air for 110 days; the contact resistivity after this period had elapsed was almost the same as the contact made to the fresh specimen. Using physical and chemical analysis it was found that all the Ag2O in YBa2Cu3O7-y was separated to Ag and that Ag existed in all voids.