Susumu Shimamoto

AC loss measurement of 46 kA-13T Nb/sub 3/Sn conductor for ITER

AC losses of Nb/sub 3/Sn conductor samples with various void fractions for the ITER Central Solenoid Model Coil (CSMC) were measured by using calorimetric and magnetization techniques. The CSMC is designed to generate the magnetic field of 13 T at the operating current of 46 kA. The conductor consists of the multi-stage cable, having 1152 Nb/sub 3/Sn strands, and Incoloy 908 square jacket with circular hole. The strands are coated by chrome plating with 2 /spl mu/m layer. The last sub-cables are wrapped with Inconel tape, having high electric resistivity, to reduce the coupling current loss. The optimum void fraction for pulse coils is obtained from the relation between the coupling time constant and the void fraction. It is indicated that the sub-cable wrapping is very effective in limiting the coupling current between the sub-cables, as expected. The AC losses of the CS Insert were measured in various operating modes. From these obtained results, the validity of conductor design is demonstrated.

Thermal stability of oxide superconductor at various temperatures

Quench properties of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8//AgMg tape were experimentally investigated under a cryocooled condition in high magnetic fields. The critical current (I/sub c/), quench current (I/sub q/), and minimum quench energy (MQE) were measured at various initial temperatures of 4.2 K, 10 K, and 20 K and at magnetic fields of 7 T and 9 T. MQE was the minimum disturbance energy enough to make thermal quench and was evaluated from the input energy by a resistive heater. It was found that MQE exhibited the minimum value around 20 K. The temperature margin, difference between the initial temperature and the thermal runaway temperature, decreased monotonously with increase at the operating temperature. Since the temperature dependence of MQE did not agree with that of temperature margin, we focused on the internal energy calculated by integrating specific heat of the sample. The temperature dependence of the internal energy was in agreement with that of MQE qualitatively.

Three-directional analysis of thermally-induced strains for Nb/sub 3/Sn and oxide composite superconductors

Composite superconductors like Cu/Nb/sub 3/Sn and Ag/Bi-oxides are subjected to thermally-induced residual strain by other component materials due to the around 1000 K temperature difference between the high temperatures where the superconductor is formed and the cryogenic temperatures where they are operated. To clarify especially the radial (r) and (/spl theta/) tangential strain behaviors, we analyzed elastic-plastically two models, the concentric core model (single-core model) and multi-core model and used two analysis methods, calculation of the force balance equation for the former model and computing of FEM for the former and the latter models. Strains in r and /spl theta/ directions varied with combination of component materials having large and small thermal expansion coefficients; the superconductor sandwiched by high thermal expansion materials is subjected to larger tensile r-strain and larger compressive /spl theta/-strain, as compared with superconducting core only embedded in the matrix. Furthermore it was found that there was a strain distribution by the core location at the inner or the outer. Finally, a better combination of the superconductor with other materials was obtained.

Stability of Nb/sub 3/Sn wires with CuNb reinforcing stabilizer on cryocooled superconducting magnet

The stability of advanced Nb/sub 3/Sn wires with CuNb reinforcing stabilizer cooled by refrigerator is studied in order to obtain data base for the future compact design of cryocooled superconducting magnets. The experiments on the critical currents, the minimum quench energy and the normal zone propagation velocity for a cryocooled sample-coil of the CuNb/Nb/sub 3/Sn wires are carried out under the condition of the magnetic field up to 15 T at temperatures ranging from 4 K to 10 K. Added to this, we make a comparison between the stability of CuNb/Nb/sub 3/Sn and Cu/Nb/sub 3/Sn wires in the cryocooling case. It is concluded from this experimental result there are the slight differences between two kinds of wires, which have to be considered for cryocooled magnet design.

Study of plasma current start-up of the middle-sized tokamak with force-balanced coils

Abstract For a high field tokamak device, we have developed force-balanced coils (FBCs) which have nonuniform-pitch multi-pole helical-configuration to reduce centering electromagnetic forces without using cancellation coils. As a superconductivity magnet, the FBC configuration is favorable to the achievement of high critical current densities because of a Jc‖B effect. In the tokamak operation with the FBCs, the rise of the toroidal field synchronizes with the plasma current ramp up because the FBCs function both as toroidal field coils (TFCs) and as primary coils for ohmic heating. The torsional force exerted on the FBCs is comparable and the net centering force is reduced to 20% compared with those on conventional TFCs of the same dimensions.

Two-dimensional quench simulation of composite CuNb/Nb3Sn conductors

Abstract In order to clarify effect of utilizing a Nb rich CuNb reinforment on superconducting stability, r – z two-dimensional time-dependent numerical simulations on composite CuNb/Nb 3 Sn wires are conducted. The time variations of temperature and current density distributions, minimum quench energy (MQE), and normal zone propagation velocity ( v p ) of a Cu–17vol%Nb/Nb 3 Sn wire, a Cu–63vol%Nb/Nb 3 Sn wire, and a conventional Cu/Nb 3 Sn wire are investigated. The increase of the volume fraction of an outermost Cu stabilizer provides high MQE but decreases the total current density. Although the v p is not significantly influenced by the Nb fraction, the Nb rich CuNb reinforcement sacrifices the MQE for its high tensile strength. It is important for magnet design to control the volume fraction of the Cu stabilizer and Nb fraction in the CuNb reinforcement to balance the desired current density, tensile strength, and superconducting stability.

AC loss time constant measurements on Nb/sub 3/Al multifilamentary superconductors

AC loss time constants have been measured by an inductive method developed at the Japan Atomic Energy Research Institute. Measurements were performed on four multifilamentary Nb/sub 3/Al test wires, manufactured by the jelly-roll method and one multifilamentary NbTi test wire. The theoretical AC loss time constants were calculated and compared with the measured values. The time constants of the Nb/sub 3/Al wires were found to vary as the square of the twist pitch as predicted. The Nb/sub 3/Al wires have time constants that are higher than the lower limit predicted by theory, suggesting that the contact resistance between the copper matrix and superconducting filaments is lower than that in the NbTi wire.<<ETX>>

Analysis of current after normal transition in a cable-in-conduit conductor

When some strands carry a large current as a result of current imbalance, they may initially become normal and the current is transferred to other superconducting strands. A one-dimensional simulation code for stability and quench of a cable-in-conduit conductor is improved to study this phenomenon using an infinitely long distributed circuit model, in which the current can easily be calculated. The results show the normal zone propagation plays a significant role in the current decay. Also, they are indicated that the conductor is more stable for higher conductance but the stability of the conductor seems more important to prevent instability due to the current imbalance.

Evolution of Cryogenic Structural Material Development for Superconducting Coils in Fusion Reactors

Synopsis: Cryogenic structural materials are an essential element to fulfill the performance of superconducting coils for fusion reactors. In this field, there was no specialized steel and mechanical properties data at 4 K until 30 years ago when the Japan Atomic Energy Agency (JAEA) started the development of the Tokomak fusion reactor. JAEA set a target of mechanical characteristics which could satisfy requirements for the coil structure at 4 K and equipped evaluation facilities at 4 K for tensile tests, fatigue tests and so on. At the same time, JAEA initiated collaboration with the steel industries in order to realize new cryogenic structural materials and carried out mechanical evaluation tests at 4 K on numerous samples which were supplied from the industries. There are two areas: a coil case and a conductor jacket for forced flow conductor. The former requires large forging products and thick hot-rolled plates for the coil case. The latter is influenced by heat treatment at approximately 650 °C and more than one-hundred hours to produce Nb3Sn superconducting material at the final stage. This is a unique issue for the jacket. Finally, both the case and the jacket material for construction of the ITER are now being supplied from the Japanese industries through JAEA. In addition, JAEA contributed to the standardization of these testing methods at 4 K in Japanese industrial standards (JIS). Furthermore, JAEA supported the establishment of a design code for the structure of superconducting coil for a fusion facility at the Japan Society of Mechanical Engineers (JSME). This paper describes the more than 30-year history of the development of cryogenic structural material.

350-liter/hour, 1.2-kW Helium Liquefier/Refrigerator System

This paper describes thermal and system design of the 350l/h helium cryogenic system which was constructed for testing the Japanese LCT coil at the Japan Atomic Energy Research Institute (JAERI). The cryogenic system, which is a second step to develop a large and reliable helium cryogenic system for fusion, was designed to have a maximum refrigeration capacity of 1, 000W or a maximum liquefaction capacity of 300l/h at 4.5K; this permits to cool the LCT coil down to 4K in 120 hours. Themodynamic cycle and control system were specified in accordance with technical informations obtained from the Cluster Test Facility. The design and construction work of the system were started in May, 1980 and the first test to check the performances was carried out from June 12 to 16 in 1981 at the JAERI. The measured performances are described in this paper.