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The mechanism of the coupling loss is examined by measuring the trajectories of strands of sample CIC conductor. The studies have shown that probable loop lengths of long conductors are calculated by contact probabilities of two strands at each contactable point of two sub cables by newly developed function of twisting pitches.

The R&D coil is developed for Large Helical Device (LHD) pool-cooled superconducting magnets and the stability tests are performed in saturated sub-cooled helium. The current propagation becomes higher at the lower temperature and it exceeds 11.7 kA in subcooled helium of 3.5 K as a temperature inside the R&D coil.

The sub-cooled He I is used as a coolant for superconducting magnets of the LHD (Large Helical Device) coils and their stability is tested. The helium temperature distribution in the cooling channels of the R & D coil is measured under steady state conditions and the transient helium temperature change in the channels is observed in stability tests.

A Large Helical Device (LHD) coil and sub-cooled helium test facility is designed and tested. Studies reveal that the saturated liquid helium could be cooled from 4.4 K to 3.0 K in eight hours and maintained at 3.0 K.

Research involving AC losses in a large superconducting coil wound with Cable-in-Conduit (CIC) conductor is described and discussed.

Areas of research involving high temperature superconductors (HTS) that are being investigated at Japan's National Institute for Fusion Science (NIFS) are identified and discussed. For example, a superconducting magnetic-levitation coil is being developed.

High temperature superconducting current feeders were developed for a large-scale superconducting experimental fusion system.

A 20 kA current lead based on Ag/Au stabilized Bi-2223 tapes was examined and test results reported.

The superconducting magnet system for the Large Helical Device is examined and results of tests are reported.

The first excitation of helical coils of the Large Helical Device has successfully taken place.

The Large Hadron Collider cryogenic system was observed during cooling and excitation tests.

A report on the completion and subsequent trial operation of the superconducting coils of the Large Helical Device (LHD) is presented. The LHD, a device intended for plasma physics experiments, is composed of two superconducting helical coils and three pairs of poloidal coils. The device, whose assembly and other attachments were finished in Jan 1998, completed trial operations and the initial plasma ignition on schedule in Mar 1998.

A study was conducted on the influence of current redistribution on the stability of Nb3Sn compacted-strand cables with chrome coating and without any coating. Minimum quench energy (MQE) measurements were derived under different external magnetic fields and transport currents. Results show that the cable with the chrome coating has a slightly higher MQE than the cable without any coating.

The performance of a single inner vertical coil for the Large Helical Device (LHD) was analyzed through excitation experiments. The coil, which is made up of a forced-flow NbTi cable-in-conduit conductor, is among the poloidal field coils of the LHD. The coil's superconducting transition was verified after 250 hours of cooldown. Coil energization was achieved up to the specific current 20.8 kA.

We applied a new solid state bonding technique to the joints of the forced-cooled superconducting poloidal coils for LHD. The NbTi/Cu wires of the cable-in-conduit(CIC) conductors were joined superconductively by this technique to realize the low electrical resistance and compactness. We make several joint samples and study the joint condition among the NbTi filaments. By the Scanning Electron Microscope (SEM) we make sure that the filaments are joined with very narrow gaps. We measure the magnetization of the joint using Superconducting Quantum Interference Device and estimate the effective diameter of the filaments to be about 90[[micro]meter]. This value shows that the joint is magnetically stable by the adiabatic theory.

In order to elucidate the dependence of the internal electromagnetic-forces on the inter. strand coupling loss in superconducting cable-in-conduit conductors, we develop a new measuring system. This system enables us to measure the coupling loss of large scale conductors with transport current of 20kA and the transverse magnetic field of 3.22T bias in the wide frequency range of the super-posed ripple field of 0.1-300Hz. The data of the preliminary measurements show that the change of the coupling loss is only a little in the low frequency range to the extent of the electromagnetic-force of 0.44MPa in the case of IS-conductor of the poloidal field coil for LHD.

The Large Helical Device (LHD) under construction at NIFS is a nuclear fusion experimental device consisting of two superconducting helical coils and three pairs of poloidal coils. A pair of inner vertical (IV) coils with a combined stored energy of 161 MJ have been delivered to NIFS. They were the 4 m outside diameter coils wound from cable-in-conduit conductors composed of 486 strands with a copper matrix. We conducted supercritical helium cooling and excitation tests to confirm the cooling and superconducting characteristics of one of the IV coils before its assembly into the LHD cryostat in 1997. It has been assembled into the SHe test facility at the Toki site of NIFS, and was cooled down and energized to 2.2 kA in February and March 1995. The test facility will be repaired in part, and the coil will be tested to confirm the characteristics at the rated current of 20.8 kA within the year.

A single cooldown test of an Inner Vertical (IV) field coil for the Large Helical Device (LHD) was started on February 1 of 1995, and the superconducting transition of the coil was confirmed on February 23. The coil is a forced-flow type using a cable-in-conduit (CIC) conductor and weighing about 16 tons. The total cooldown time was about 250 hours, not including suspended time. Pressure drop characteristics were measured during the cooldown, and the same results were obtained as the R&D coil previously tested. The cooldown time could be reduced by indirect cooling using stainless steel sleeves with cooling pipes. The indirect cooling is effective for the CIC conductor which has no subchannel because otherwise an adequate mass flow cannot be obtained due to a high pressure drop in the high temperature region. The temperature distribution in the vertical direction was also examined during the indirect cooling.

A solid-state bonding method for fabricating cable-in-conduit conductor joints has been developed with respect to the specifications of the poloidal coil of the Large Helical Device. The technique, which has been applied to niobium-tin filaments, reduces the quench current in a joint to one-third of the conductor critical current. It also reduces the space requirement and magnetic field errors.

Simulations and measurements of the Hall effect on the inter-strand coupling losses in aluminum-stabilized superconductors were carried out. The Hall effect, which is defined as the relation between the inter-strand coupling current and the external dc magnetic field, was derived for a niobium titanium conductor, KISO-33, under transverse and longitudinal magnetic fields. The results showed that the coupling loss caused by the ac transverse field is by the longitudinal dc field.

The stability and limiting current of a NbTi forced-cooled superconducting coil have been investigated experimentally and analytically at the subcooled condition below 4.5 K. The test coil was wound in a one layer solenoid from a cable-in-conduit type conductor of critical current of 5.1 kA at 4.2 K and 6.5 T, whose total length was 10 m. It was installed in the bore of a back-up superconducting magnet that provided field 8 T at the test coil. The stability margin of the coil as a function of normalized current (transport current/critical current) were measured. The results are compared with the formula proposed by Dresner and reported to predictions of stability margin and limiting current at subcooled condition below 4.5 K.

Stability and quench experiments of a Nb-Ti cable-in-conduit superconductor with bare strands have been carried out to determine the performance of the poloidal coil for the Large Helical Device (LHD). The conductor was formed into a double-pancake coil named IV-S and was mounted on TOKI-PF - a previously tested R&D coil. In excitation tests at 7.4-8.3K, the IV-S coil reached the critical currents without premature quenches. Stability tests at 7.5K indicated that the limiting current exceeds 20kA, which is the nominal operating current of the LHD poloidal coils. These results demonstrated that the stability of the chosen conductor with bare strands is high enough for the LHD.

The TOKI-MC is a solenoidal test coil that was constructed for simulating the operating conditions of the helical coils in the large helical device, a fusion experimental machine. Built of aluminum-stabilized superconductors, the pool-cooled coil is designed for 20-kA operation but quenched at 17 kA during excitation experiments. To determine its stability, tests were carried out to obtain its recovery current and explain its degradation.

The large helical device is a fusion experimental machine containing several helical coils. To simulate the operating conditions of the helical coils, the pool-cooled TOKI-MC test coil was constructed from aluminum-stabilized superconducting wires. It was then subjected to excitation experiments to study its quenching point. This was determined to be 17 kA and caused by mechanical disturbances, as borne out be acoustic emission signals.

An experimental study on the stability of cable-in-conduit superconductors developed for the large helical device's poloidal field coils is presented. The conductor under investigation is the result of a new design that was made after tests revealed problems related to strand motion, current distribution and inter-filamentary current transition. Designed for operation at 20.8 kA and 6.5 T, it has a stability margin of 5x10(super 5) j/m3.

The frequency-loss properties of an niobium titanium cable-in-conduit conductor were determined. The measurements facilitated an evaluation of its applicability for the poloidal coils of the large helical device currently under construction in Japan. The tests, which were conducted from 0.1 Hz to 200 Hz, demonstrated a negligible rise in coupling loss when the copper niobium-matrix strands were displaced by copper-matrix filaments.

Work on the Large Helical Device (LHD) currently being built at Japan's National Institute for Fusion Science is reported. A nuclear fusion structure, the LHD consists of superconducting magnets which are configured as helical coils, poloidal coils and vertical coils. The various coils are described along with the cryostat and cryogenic supports. Ongoing work include the design of additional poloidal coils.

Fuzzy theorem is employed to integrate main detection items in a monitoring system for superconducting magnets (SM). The approach, which quantifies the SM's operating conditions as a performance gauge, facilitates early detection of quench initiation in SMs. Test results of the proposed system's implementation on the TOKI-MC module coil of Japan's National Institute of Fusion Science are also discussed.

An aluminum-stabilized superconductor with a high current capacity is often used under the coexistence of the transverse and the parallel magnetic fields. A changing transverse field produces a path of inter-strand coupling currents in cross sections of the aluminum region. The magnetic interaction between the current and the parallel field causes what is called the Hall effect, so that the coupling loss feature is different from that of the ordinary conductor. The aluminum-stabilized conductor must generally be designed so as to avoid the increment in the resistivity of the aluminum due to the Hall effect at quench, but the low loss point of view should also simultaneously be taken into account in such a conductor design.