Applicability of 1 GW class hybrid energy transfer line of hydrogen and electricity is investigated. Target distance of hybrid energy transfer line is 1000 km. Hydrogen refrigeration station is placed on every 10 km of the unit section. The rated current and withstand voltage of the dc power line are 10 kA and 100 kV, respectively. Capacity of the liquid hydrogen transportation is 100 tons per day. Transfer line consists of the superconducting (SC) cable, space for liquid hydrogen, electrical insulation layer, vacuum space for thermal insulation, and cryogenic envelopes. High Jc performance in a liquid hydrogen temperature requires for the SC cable. The MgB2 wire is one of the potential candidates for this system as well as BSCCO or YBCO tapes. To keep the liquid state of hydrogen anywhere in the unit section, the temperature and pressure of the inlet point were selected to 17 K and 0.4 MPa, respectively. When the heat leak into the liquid hydrogen was 1.0 W/m (expected value), the temperature at the outlet became 18.1 K. Total power consumption of the 10 kW class refrigerator is estimated to 660 kW. The total power consumption for the hybrid energy transfer line of 1000 km length becomes 132 MW. This value is equivalent to 13.2 % to the transport capacity of 1GW.

A special active filter, which can eliminate the variable harmonic currents in continuity, was investigated. A variable-frequency band-pass filter was developed for the extraction of current component of fundamental frequency. Dynamic simulations for the active filter, motor generator with a flywheel (FW-MG) and power supplies of heating devices for the experimental fusion device of LHD have been carried. We found that the harmonic currents with the amplitudes of 20% have been reduced to less than 2% in the operational frequency range from 95Hz to 55Hz by using this active filter.

A magnetically levitated superconducting coil device, Mini-RT, has been constructed using a high temperature superconductor for the purpose of examining a new magnetic confinement scheme of high-beta plasmas. The floating coil is wound with Bi-2223 Ag-sheathed tapes, and it is operated in the temperature range 20-40 K. The excitation tests of the coil were carried out and the rated current was successfully achieved by overcoming many difficulties. The first magnetic levitation was realized for one hour and the plasma production was initiated.

The plasma experiment apparatus S-RT (Superconducting Ring Trap) is planned for the purpose of high beta plasma confinement research in the University of Tokyo. As a preceding step, Mini-RT, which is the size reduction version of S-RT, has been constructed as a joint research of NIFS, the University of Tokyo, and Kyushu University. In this experiment a magnetic-levitation coil (floating coil) operated in persistent current mode has to levitate for 8 hours in the plasma vacuum vessel. The HTS floating coil wound with a Bi-2223 tape has a diameter of 300 mm and an electromotive force of 50 kA. Since any refrigerant cannot be fed to the coil during the plasma experiment, the coil is designed so that the temperature rise after 8 hours of levitation is less than 40 K with the specific heat of the coil and radiation shield. At the end of the daily plasma experiment, the coil will be drawn down to the maintenance location at the bottom of the plasma vacuum vessel, and it will be re-cooled to 20 K.

A 60-kA high-temperature-superconductor (HTS) current lead has been fabricated and tested aiming at the application to a fusion magnet system. The HTS current lead consists of a forced flow cooled copper part and a conduction cooled HTS part. The HTS part is composed of 288 Ag-alloy sheathed Bi-2223 tapes, which are cylindrically arrayed on a stainless steel tube. For the Ag alloy, Ag–10at.%Au is used to obtain low thermal conductivity. Operation of a 60-kA current, which is the world record, was successfully achieved at a cooling condition of 20 K, 3.2 g/s for the copper part, and a low heat leak of 4.8 W at 4.2 K was demonstrated. This result indicates that the electric power of a refrigerator to cool the current lead can be reduced by 1/3 of that in a conventional current lead. In conclusion, technology of a large HTS current lead for fusion application is established.

The LHD is an SC experimental fusion device of heliotron type. Eight sets of the helium compressors with total electric power of 3.5 MW are installed in the cryogenic system. The analytical studies of the SMES-UPS for the compressors under the deep voltage sag are reported in this paper. The amplitude and frequency of the voltage decrease gradually by the regenerating effect of the induction motors. The SMES-UPS system proposed in this report has the following functions; (1) variable frequency control, (2) regulations by ACR and AVR, and (3) rapid isolation and synchronous reconnection from the loads to grid line. We have demonstrated that SMES was useful for the large-scaled cryogenic system of the experimental fusion device.

A magnetically levitated superconducting coil device, Mini-RT, is being developed using high temperature superconductors for the purpose of examining a new magnetic confinement scheme of high-beta plasmas. The floating coil has Bi-2223 Ag-sheathed tape conductors, which will be effectively used in the temperature range of 20-40 K. The fabrication of the coil has been completed, and excitation tests of the coil were carried out in a helium cryostat. The coil was successfully excited up to the nominal current with a proper PCS operation.

A magnetically levitated superconducting coil system is being developed using high temperature superconductors for examining a new magnetic confinement of high-beta plasmas. A miniature double-pancake coil was fabricated with a Bi-2223 Ag-sheathed tape for the purpose of developing a floating control using laser displacement gauges. The coil was inductively excited with liquid nitrogen cooling and successfully levitated in the air. A persistent current switch is also being developed with a Bi-2223 Ag-0.3wt%Mn-sheathed tape, and a prototype model was successfully tested.

A 60kA HTS current lead has been designed for large fusion magnets such as ITER magnet. The actual refrigeration input power required to cool the current lead is specified to be reduced to one third that of the conventional copper lead. The HTS part of the 60 kA lead consists of 48 units installed with cylindrical array into the enter surface of a stainless steel tube with a diameter of 145 mm, Each unit is composed of six Bi2223/Ag-10at%Au tapes, and its cross-sectional dimension is 6.5 mm x 2.7 mm. The HTS part is cooled by conduction, and the warm and cold end temperature conditions of the HTS part are 50 K and 4.5 K, respectively. The copper part is cooled by helium gas, a flow rate of 3.9 g/s and the inlet temperature of 35 kA. The 60-kA lead has been designed in consideration of safety under the long discharge time condition of ITER-TF coil with a detection time of 2 sec. and a discharge time constant of 15 sec. For the purpose of verifying the reliability of the design for the lung discharge time, one unit sample has been fabricated and tested. The result indicates that the maximum temperature rise of the HTS part is less than 150 k for the ITER like-discharge from 1.25 kA. corresponding to 60 kA of the full lead with 48 units.

A superconducting (SC) current feeder system for LHD has been operated since March of 1998. It consists of 9 SC bus-lines, 9 pairs of current leads and 3 current lead cryostats, SC bus-lines have a total length of 497 m, rated current of 32 kA and withstand voltage of 5 kV under 77 K helium gas condition. After 2 years operation of this system it was demonstrated that the SC current feeder system is reliable and useful. On the basis of these operational experiences, the authors have evaluated the performance, reliability and operation cost of the SC current feeder system in comparison with nonSC type current feeder system. They have concluded that the SC current feeder system with high current capacity is more suitable than other current feeder systems such as using a water-cooled busbar, especially for large-scale SC fusion experimental devices.

A superconducting (SC) current feeder system with total length of 497 m had been kept in cryostable condition during two months. Mass flow rates, pressures and liquid helium levels of the system were controlled automatically in the steady state operation. The stable operation for current-leads were investigated by using a siphon method. The SC current feeder system had been successfully performed with no trouble during the first experimental period.

For pt.I see ibid., vol.7, no.2, p.684-7 (1997). We have been developing HTS current leads for a 1 kWh/1 MW module type SMES system (which we call "ESK" for Experimental SMES of Kyushu Electric Power Co., Inc.). Each module of a module type SMES system has a pair of current leads. For the purpose of reducing the heat load from the current leads, we have employed a bulk HTS. As a step in the preparation of HTS current leads for ESK, we trial produced HTS current leads and tested them to evaluate their characteristics. Our test results indicated that the heat load in a steady state at a rated current of 1 kA and with a rated flow rate of 0.05 g/s was 0.035 W, well below the specified value of 0.1 W. Also, our results indicated that the HTS current leads can be satisfactorily energized for pulsing operation at a rated current for ESK of 500 A-1 kA. The heat load under such conditions was 0.025 W, approximately 80% of that under normal 1 kA operating conditions. It was also verified that operation could be continued for 15 minutes even when the coolant flow was stopped.

We are developing high-temperature-superconductor (HTS) current loads for a 1-MW/1-kWh modular SMES system. In the modular SMES system, a pair of current leads would be installed in each module so that the heat load to the low-temperature end through the leads is minimized. Design study of the lead configuration including the safety lead and its thermal characteristics is presented herein. The safety lead composed of stainless steel is placed in parallel to the bulk HTS. A test unit for verification of safe operation has been developed and the simulated bulk HTS quench test has been demonstrated. The maximum temperature of the safety lead reached 200 K and the terminal voltage of the safety lead was only 1.2 V in the case of a 1000-A quench test. This temperature rise was well below the designed value. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(1): 23–32, 1997

We have been developing high-temperature superconducting (HTS) current leads for a 1 kWh/1 MW module-type SMEs. Each module of a module-type SMES requires a pair of current leads. Therefore, we employed bulk HTS in order to reduce the heat load of the current leads. It is important that HTS current leads for SMES be reliable. The HTS current leads described in this paper have been designed to minimize the heat load and to maintain a high level of reliability. The HTS current leads are designed to hold the heat load at the cold-end terminal to less than 0.1 W. They are also designed with safety leads to bypass current in the event the HTS is quenched and with metal superconductors to assure the continuation of SMES operation even if the HTS should fail or deteriorate in performance. This paper describes an optimal design and the results of a heat load evaluation of HTS current leads for SMES.