Since the discovery of high temperature superconducting oxides, much study has been done to understand the electronic structure of the oxides and to examine various aspects of the superconducting properties. Recent progress in the theoretical study of the high temperature superconducting oxides is reviewed.

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An overview of the physical properties of high-T_c superconductors is given. Present status of experimental studies on electronic properties in the normal state, superconducting properties such as isotope effect, energy gap, penetration depth and coherence length, and static and dynamic properties of the flux line are presented.

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A 40A/mm²-24kA high-rigidity large forced-cooled (NbTi)₃Sn superconducting coil named DPC-TJ was developed under the collaboration between Japan Atomic Energy Research Institute and Toshiba Corporation. The purpose of the DPC-TJ program is to demonstrate the realizability of the high average-current-density with a large coil wound by a large Nb₃Sn superconductor which meets the requirements of the conductor for the next thermonuclear fusion machine. The DPC-TJ coil was fabricated as a first coil of the preformed-armor type, which was thought out in this program as a fabrication method that enables the high-rigidity and the high-current-density simultaneously without difficulty. In this paper, the outline of the DPC-TJ program is described laying stress on the development of the DPC-TJ coil.

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The electrical resistance measurement provides us with information on the most fundamental and important quantities in low temperature physics and engineering, such as, temperature and materials characteristics. A typical example may be the measurement on superconductors. The present paper discusses the general aspect of the electrical resistance measurement.

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A forced-cooled superconducting coil (DPC-TJ) was wound with a double walled cable-in-conduit named as “Preformed Armor Type CICC.” In this paper, we describe the fabrication of the conductor for the DPC-TJ coil with the first conduit which plays a role of sealing the supercritical helium from the vacuum. Because the DPC-TJ coil has high average current density of 40A/mm² (12T, 4.2K) in winding area, the (NbTi)₃Sn strand by Nb tube method was selected, which has a very high critical current density in non Cu area. As a result of development work, the strand could be fabricated which meet the following requirements; the critical current density of more than 600A/mm² at 12T and the RRR (Residual Resistivity Ratio) value of higher than 50. The fabrication line of the cables-in-conduit conductors by roll forming method was also developed. Through the fabrication of the conductor for the DPC-TJ coil, it was demonstrated that large and long conductors for future fusion reactor could be fabricated.

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The DPC-TJ coil experiment took place successfully at the Japan Atomic Energy Research Institute (JAERI) in 1991. This coil was developed by Toshiba and JAERI in collaboration to demonstrate the realizability of superconducting coils for fusion with high average current density of 40A/mm². The DPC-TJ was charged up to its rated current of 24kA (40A/mm²) at 7.6T without quench in the DPC test facility. Thereafter the electromagnetic performances were tested by measuring I_c and T_cs, and the critical current at 12T was estimated 41kA. Many electromagnetic results were obtained, which are necessary for the design of the next fusion machines like International Thermonuclear Experimental Reactor (ITER).

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In this paper stability test results of the DPC-TJ coil are summarized. The DPC-TJ coil is a cable-in-conduit forced-flow type large superconducting coil and is cooled by supercritical helium. Stability test was performed with an inductive heating method, and the dependencies of the stability margins on the operating current and on the heating duration were investigated. The relation between the heating power and the take-off time, defined as the time from the start of heating till the appearance of the normalcy on the conductor, was also studied. Major results are as follows: The so-called limiting currents were not obvious within the present test conditions. The influence of the heating duration on the stability margin was little. And the heating power was inversely proportional to the square root of the take-off time up to about 20ms.

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Thermal and hydraulic test results of the DPC-TJ coil, a 24kA-40A/mm² forced-cooled Nb₃Sn coil, are presented in this paper. The DPC-TJ coil was installed and tested between the DPC-U1 and U2 coils. The weight of the DPC-TJ coil is 2.8 tons and the total cooldown weight as the coil system is 23 tons. It took about 180h to cool the coils from room temperature to 20K. The DPC-TJ coil's heat load was 20W at zero transport current and 40W at 24kA, the rated charging state. The pressure drop in the DPC-TJ coil was measured and agreed well with the empirical formula which was at the design of the DPC-TJ coil. AC losses and inductive by heat input cause the quick decrease of the helium flow rate at the coil inlet position. This phenomenon was analyzed, and one of the design standard was derived.

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Mechanical characteristics of the coil was measured during the charge-up test by strain gauges and displacement gauges attached directly to the coil. To estimate the winding stiffness of the coil, the expansion test was performed on the test winding at both 300K and 77K after the fabrication of the test winding. The test results obtained in current operation were compared with the calculation by FEM analysis and expansion test results. As a result of comparison, equivalent stiffness calculated by FEM was good agreement with one measured by the charge-up test of the DPC-TJ, which had higher rigidity than the test winding. As a result of the charge-up test, it was demonstrated that the DPC-TJ coil had high rigidity as was designed.

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For earth observations and for astronomical observations in space, there is a need to extend observation periods and improve detector performance. Development of mechanical cryocoolers with longer lifetimes and improved cooling capacity is anticipated. The free piston-type Stirling cryocooler, which applies 1) the Stirling cycle for refrigeration, 2) linear motors for driving, 3) magnetic bearings for support systems, and 4) fine gaps for clearance seals, is the most suitable design for a mechanical cryocooler utilized in a space environment. Development of this type of cryocooler began in 1987 with Bread Board Model 1. Its major components, the linear motors and magnetic bearings, were studied in order to identify the technical problems. In the present model, Bread Board Model 2, design criteria which enhance operating life, compactness, and light weight are being studied. The thrust force of moving magnet-type oscillatory linear motors satisfies the required specifications. Radial vibration amplitudes were reduced to less than 25μm with the use of PID control for magnetic bearing control. Further, the total amplitude of axial vibration was decreased to only 17μm when a dynamic damper was utilized to reduce cryocooler vibration. A cooldown time of about 17min, to a surface temperature of 70K, and an ultimate surface temperature of about 50K were achieved. The cooling power at a temperature of 70K is 2W, which is not sufficient compared with the design specification of 5W. It is supposed that stroke shortage in a piston is the primary reason for this. It was also confirmed that the stroke of a piston can be significantly increased by decreasing the clearance between the piston and the cylinder, as well as by decreasing the passage resistance of a connecting section. Further investigation of cooling power improvement to satisfy the design specifications and durability tests are planned on Bread Board Model 2, which is redesigned and now being manufactured based on the forementioned results.

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He refrigeration systems for the TRISTAN detector magnets were constructed for the Superconducting solenoid magnets that are used in high energy particle physics experiments at the TRISTAN e⁺e^- collider. Three superconducting magnets and the He refrigeration systems (VENUS, TOPAZ and AMY) are situated at different locations around the 1km diameter TRISTAN ring. Therefore, the He refrigeration systems were designed such that they can be operated remotely from a central control station. Test operations of the He refrigeration system and magnetic field measurements were carried out between 1985 and 1986. By the end of July 1991, the total of cumulative operation hours reached 79, 000hrs (VENUS 28, 000hrs, TOPAZ 26, 000hrs and AMY 25, 000hrs). The failures for this long running period were analyzed and examined about the reliability of He refrigeration systems. The result of analysis was MTBF 909hrs and MTTR 6.7hrs. The availability became 0.993 by using those values. This shows that the He refrigeration systems have a high reliability.

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