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Performance tests are carried out on the subcooling system for the Large Helical Device (LHD) helical coils. The studies have shown that the coil temperature is around 3.6 K under a steady heat load of 100 W from outside of the coil case in the system, which has maintained the above performance.

The cryogenic stability of the Large Helical Device (LHD) helical coils is improved by installing an additional cooler like subcooled helium at 3.0 K. The temperature rise by excitation is analyzed as it is required for maintaining low temperatures for better heat transfer.

The synthesis and various electromagnetic properties of different NbTi/Cu multifilamentary films with high aspect-ratio cross-sections are described. These films are found to have strong anisotropy of properties like ac losses and critical current density.

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.

A study is conducted to investigate the performance of large helical device in pool-cooled superconducting magnet over last six years. The experimental results show that degradation is not observed in the coil performance and the stable operation is demonstrated.

The behavior of conductor motions in the helical coils of the Large Helical Device 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.

Helical coils for the Large Helical Device were studied to determine excitation properties and cryogenic stability.

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.

The Large Helical Device helical conductor was studied and temperature and electric field distribution measurements taken.

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.

An evaluation of the time constant of current diffusion in the superconductor stabilizer was conducted using computer simulations. This peculiar property, which results in the generation of excess joule heat in a small area near the superconducting filaments, was studied with regard to the quench process applied to the helical device of the Large Helical Device (LHD) of the National Institute for Fusion Science in Japan. The results of the simulations were verified against stability tests from short samples of the LHD conductor.

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.

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.

On-site winding of two helical coils of LHD started at the beginning of 1995, using a numerically controlled winding machine. They are large-scale pool-cooled superconducting coils which are designed to satisfy fully stabilized criterion. The conductor consists of Nb-Ti/Cu strands with a pure aluminium stabilizer and copper sheath. The main demanded values for winding the coils are 2.0 mm and 65 [[micro]meter] for the accuracy of coil and the average gap between layers, respectively. A winding method was established by trial of winding.

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.

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 transverse alternating current (AC) losses in flat superconducting cables were investigated numerically at varying frequencies. Mathematical formulas were developed to take into account the different factors that may contribute to transverse AC losses, including the cable thickness and length. Investigations were carried out for both finite and infinite cables. Results indicate that finite cables exhibit higher loses than infinite cables at the high frequency range. It was also found that such losses are not dependent on the cable length.

The frequency characteristics of ac losses in aluminum-stabilized superconducting conductors are investigated in order to elucidate their loss mechanism under changing transverse magnetic field. AC loss measurement is carried out for the short and straight sample conductor for the helical coil system of the Large Helical Device. The sample of about 50 cm in length is subjected to an ac ripple field with a frequency range from 0.05 to 337 Hz superimposed on the bias field. The experimental results show that the loss feature much depends both on direction of the transverse magnetic field and location of the stabilizer relative to the strand bundle. These results are quantitatively explained by the theoretical analysis.

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.

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.

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.