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A 1 MW, 1 s UPS-SMES is being developed for a protection from a momentary voltage drop and an instant power failure. As a key technology of the UPS-SMES, we developed a prototype LTS pulse coil with a stored energy of 100 kJ and conducted cooling and excitation tests in 2005. The operation test of the prototype UPS-SMES using this 100 kJ coil with power converters have been performed in 2006. A 1 MJ coil was designed before the fabrication of the 100 kJ prototype coil. The superconductor, the electric insulation technique, the winding method, and the cooling structure used for the 100 kJ coil were based upon the 1 MJ coil design. The successful performance test results of the prototype 100 kJ coil validated the design concept and fabrication technique of the 1 MJ coil. According to the achievement of the prototype 100 kJ UPS-SMES, the 1 MJ conduction-cooled LTS pulse coil has been fabricated successfully. The successful experimental results of the 100 kJ prototype coil with power converters and the fabrication procedure of the 1 MJ full size coil are described.

The stability margin of a conduction-cooled LTS pulse coil with stored energy of 1 MJ, which is used for UPS-SMES as protection from momentary voltage drops, is estimated from thermal analyses using the two-dimensional finite elementary method. How much the 1 MJ coil is able to be downsized is discussed. The winding conductor of this coil is a NbTi/Cu Rutherford cable, which is extruded with aluminum. Dyneema FRP and Litz wires are used as the spacers of this coil. In the design of this 1 MJ coil, the effects of spacers were neglected. In order to estimate the stability margin, thermal analyses was carried out by taking into account the thermal effects of spacers, for the coil during overload operation. And it was checked whether the maximum temperature The results showed that the 1 MJ coil has a large stability margin of 8 times the rated ac loss. In addition, new coils with a stability margin of about two times were designed. Possibilities for reducing the size and the cost of a 1 MJ LTS pulse coil are also shown.

We have been developing a 1 MW, 1 sec UPS-SMES for the protection of production lines of an industrial plant or large-scale experimental devices such as a fusion device from a momentary voltage drop and an instant power failure. A conduction cooled prototype LTS pulse coil of 100 kJ class was developed as a key component of the UPS-SMES. The prototype coil has demonstrated excellent thermal characteristics during cooling and exciting tests. In this paper, measurements of the temperature in the coil during experiments and thermal analysis by using two-dimensional finite element methods were compared to clarify the high heat transfer properties of this prototype coil. This coil was wound with a NbTi/Cu Rutherford cable, which is extruded with aluminum. In order to realize the conduction cooled LTS pulse coil, FRP with polyethylene fibers (Dyneema FRP) and Litz wires were used as spacers. Dyneema FRP improves the heat transfer from layer to layer in the windings. Litz wires increase the heat transfer from turn to turn in the windings and enable conduction cooling of the coil by attaching the end of the Litz wires directly to the cold heads of the cryocoolers. It was clarified that these spacers were very effective and the coil has a large stability margin in terms of the design values

A conduction-cooled low temperature superconducting (LTS) pulse coil has been developed as a key technology for UPS-SMES. We have been developing a 1 MW, 1 s UPS-SMES for a protection from a momentary voltage drop and an instant power failure. A conduction-cooled LTS pulse coil has excellent characteristics, which are adequate for a short-time uninterruptible power supply (UPS). The LTS coil has better cost performance over the HTS coil at present and the conduction cooling has higher reliability and easier operation than the conventional cooling schemes such as pool boiling with liquid helium or forced flow of supercritical helium. To demonstrate the high performances of the LTS pulse coil, we have fabricated a prototype coil with stored energy of 100 kJ and have conducted cooling and excitation tests. The successful performance test results including current shut-off test with a time constant of 1.3 s and repeated excitation of a triangular waveform with high ramp rate are reported

Steady state divertor operation with high performance plasmas (ne ~ 0.7 × 1019 cm−3, Ti ~ 2 keV) was demonstrated for half an hour in the Large Helical Device (LHD), the superconducting helical device (R = 3.6–3.9 m, a = 0.6 m, B = 3 T, l/m = 2/10). The high performance plasmas have been sustained with an averaged heating power of 680 kW and achieved an injected energy of 1.3 GJ. This required both advanced technological integration of heating systems and divertor heat flux control. In particular, optimization of divertor heat flux distribution along the divertor leg trace on divertor plates and real-time magnetic axis sweeping (R = 3.67–3.7 m) have allowed LHD to access a steady state regime with a margin of safety for the actively cooled divertor plates. The distribution of divertor heat load along the traces was investigated with calorimetric measurements and it was found that there was a localized heat load connected with the loss of high-energy ions produced by ion cyclotron radio frequency near-fields. Orbit analysis shows that the behaviour of high-energy ions is qualitatively in good agreement with the experimental result. Long-pulse discharges were terminated by radiation collapse due to penetration of metallic flakes into the plasma.

A long-pulse plasma discharge of more than 30 min duration was achieved on the Large Helical Device (LHD). A plasma of ne = 0.8 × 1019 m−3 and Ti0 = 2.0 keV was sustained with PICH = 0.52 MW, PECH = 0.1 MW and averaged PNBI = 0.067 MW. The total injected heating energy was 1.3 GJ. One of the keys to the success of the experiment was a dispersion of the local plasma heat load to divertors, accomplished by sweeping the magnetic axis inward and outward. Causes limiting the long pulse plasma discharge are discussed. An ion impurity penetration limited further long-pulse discharge in the 8th experimental campaign (2004).

A long-pulse plasma discharge for more than 30 min. was achieved on the Large Helical Device (LHD). A plasma of ne = 0.8× 1019 m−3 and Ti0 = 2.0 keV was sustained with PICH = 0.52 MW, PECH = 0.1 MW and averaged PNBI = 0.067 MW. Total injected heating energy was 1.3 GJ, which was a quarter of the prepared RF heating energy. One of the keys to the success of the experiment was a dispersion of the local plasma heat load to divertors, accomplished by shifting the magnetic axis inward and outward.

We are planning to develop a 1 MW, 1 sec UPS-SMES for a protection from a momentary voltage drop and an instant power failure. As the first step, we have been developing a 100 kJ class prototype UPS-SMES, using a low temperature superconducting coil because of its better cost and performance over the high temperature superconducting coil. However, the difficulty to utilize a pool-boiling LTS pulse coil is the reliability of operation. To solve this problem, a conduction-cooled LTS pulse coil has been designed and fabricated as a key component of the UPS-SMES. The reduction of AC loss and high stability are required for the SC conductor for the conduction-cooled coil because of a limited cooling capacity. The SC conductor of a NbTi/Cu compacted strand cable extruded with an aluminum is designed to have the anisotropic AC loss properties to minimize the coupling loss under the specified orientation of the time varying magnetic field. The coil was wound with a new twist-winding method in which the variation of twist angle of the conductor was controlled with the winding machine designed specifically for this purpose. The fabrication technique and performance of a conduction-cooled prototype LTS pulse coil are described.

During the first two years of the LHD experiment the following results have been achieved: (i) higher Te (Te(0) = 4.4 keV at ne = 5.3 × 1018 m-3 and Pabs = 1.8 MW); (ii) higher confinement (τE = 0.3 s, Te(0) = 1.1 keV at ne = 6.5 × 1019 m-3 and Pabs = 2.0 MW); (iii) higher stored energy, Wpdia = 880 kJ at B = 2.75 T. High performance plasmas have been realized in the inward shifted magnetic axis configuration (R = 3.6 m) where helical symmetry is recovered and the particle orbit properties are improved by a trade-off of MHD stability properties due to the appearance of a magnetic hill. Energy confinement was systematically higher than that predicted by the International Stellarator Scaling 95 by up to a factor of 1.6 and was comparable with the ELMy H mode confinement capability in tokamaks. This confinement improvement is attributed to configuration control (inward shift of the magnetic axis) and to the formation of a high edge temperature. The average beta value achieved reached 2.4% at B = 1.3 T, the highest beta value ever obtained in a helical device, and so far no degradation of confinement by MHD phenomena has been observed. The inward shifted configuration has also led to successful ICRF minority ion heating. ICRF powers up to 1.3 MW were reliably injected into the plasma without significant impurity contamination, and a plasma with a stored energy of 200 kJ was sustained for 5 s by ICRF alone. As another important result, long pulse discharges of more than 1 min were successfully achieved separately with an NBI heating of 0.5 MW and with an ICRF heating of 0.85 MW.

The Large Helical Device (LHD) is a plasma physics experimental device with a magnetic stored energy of 960 MJ, consisting of two superconducting (SC) helical coils and six SC poloidal coils. The trial operation and the first plasma discharge of the Phase I project for LHD were completed on 31 March 1998 as initially planned. The second experimental campaign was conducted with additional heating using two NBI devices. The third campaign started in June 1999 and was completed in January 2000. Many plasma heating tests with a plasma field of up to 2.90 T were carried out. Major test results on the SC magnet system for LHD are as follows: (1) The LHD cryogenic system was successfully operated for 13 400 h and proved its high reliability. (2) A central field of 2.91 T at a radius of 3.60 m was achieved at an inner helical coil current of 11.08 kA, a middle helical coil current of 11.83 kA and an outer helical coil current of 12.02 kA. (3) All six poloidal coils were excited stably. (4) Nine flexible SC bus lines with a total length of 497 m were operated stably and safely.

Precise control is required for the current control of the superconducting coils in the Large Helical Device (LHD) in NIFS, Japan. There are strong mutual couplings between coils, and the effect of structure materials surrounding materials of the coils and plasma, which act like shorted coils seen from the power source, must be considered for the design of the current controller. The authors designed the current controller for the LHD coil, which is composed of an H-infinity controller and a feedforward controller. Features of the controller are as follows. At first, it is possible to design the controller by considering frequency domain characteristics of the closed loop system and stability of the designed controller is guaranteed according to the theory of the H-infinity control. Secondly, the response characteristics to current reference values and DC-coupled characteristics of current control can be determined by the design of the feedforward controller. The designed controller showed excellent characteristics in both simulation and experimental results.

Recently, the SiC-based power electronics device has been under development, which has more advanced electrical characteristics than conventional Si-based device. Main electrical characteristics of SiC-based power electronics device are high operational temperature, high withstanding voltage, low on-state resistance and fast switching frequency. Especially, forward voltage, including on-state resistance characteristics, is the important conductive feature on operational loss reduction of SiC-based power electronics device. To study SiC-based device experimentally, SiC-based Schottky barrier diode (SBD), as an example of SiC-based power electronics device, is examined on its temperature dependence of forward voltage in comparison with Si-based SBD. The application of SiC-based power electronics device to large capacity AC/DC converter is also discussed.

The Large Helical Device (LHD) is a helical-type fusion plasma experimental device using six superconducting coils to form a magnetic field. The fundamental operation of the LHD is performed under a dc magnetic field, and a power system which was suitable for steady state excitation was designed and constructed. With the progress in the fusion plasma research, dynamic experiments such as sweep of magnetic axis are planned, and require dynamic control of magnetic field. For this purpose, the output voltage of the original power supplies were not enough and the voltage enhancement of the dc power supplies were necessary. Therefore, the output voltages of power supplies were enhanced using additional pulse power supplies and the control system was modified for the pulse operation.

The load leveling of an induction motor with pump load using a power angle regulator is described. The power flow is controlled by changing the power angle of the voltage vector. The power angle is controlled using a phase shifter so as to reduce the power pulsations which are produced by pump load. The novel power angle regulator which is called the solid state phase shifter (SSPS) employs an AC switch with PWM schemes for continuous and rapid control. The power pulsations have been reduced from 63% to 10% of their average level. >

An AC-excited variable speed generator with flywheel is proposed as a power supply for a nuclear fusion experimental facility. In this generator, the flywheel, whose speed varies depending on energy charging/discharging, is connected to a power line operated at a certain fixed frequency by means of a synchronous motor generator that is excited by a variable frequency converter, realizing energy charging and discharging. The advantages offered compared to a conventional generator are discussed in detail. It was shown through simulation that it is possible to connect a power load featuring a large time fluctuation without causing oscillation in the system if this flywheel is connected to the receiving point and power is charged or discharged by matching the fluctuation of load power in accordance with the frequency or voltage control of the utility power system. >

The Large Helical Device (LHD) is a fusion plasma test facility that has three sets of superconducting helical coils and three sets of superconducting poloidal coils. This paper introduces a power system to excite these coils. The system consists of large current thyristor rectifiers, quench protection circuits, control system and superconducting DC power feeders. Several components are developed for this power system. A DC circuit breaker using a small power fuse and a vacuum circuit breaker realize a compact and low cost quench protection circuit. A superconducting DC power feeder, that joins the power supply and a coil, moves a large cryogenic joint box to far away from the LHD main structure.

Superconducting cables which are composed of many superconducting strands have the problem of current imbalance. This causes the cables to quench at a current level below its design value. This paper proposes two different methods to solve the imbalance problem, one of which uses current lead resistance and the other one iron core. The current imbalance mechanism is investigated by using Ag sheathed Bi-2223 high temperature superconducting wires. The approach compares two different connection methods for superconducting wires and it shows that the contact resistance is the one important factor that causes current imbalance. The current imbalance correction method using iron core in liquid nitrogen was also experimented.

The vapor phase of saturated superfluid helium suffers from electrical breakdown easily near the Paschen minimum condition of gaseous helium. It is very important from the magnet protection viewpoint to know the surface breakdown voltage in that condition. We measured breakdown voltage of surface or gap breakdown in gaseous helium somewhat higher than the saturation temperature for various pressures (1.33 to 101 kPa) using parallel plane electrodes with a GFRP spacer. In the tested regime the breakdown voltage obeys Paschen's law and changes only with the product of density and thickness. In the lower density-thickness product range surface breakdown can hardly occur before the gap breakdown develops. Whereas in the higher density-thickness product range surface breakdown voltage is lower than the gap breakdown voltage. Within the scatter of the data the breakdown voltage is independent of spacer diameter and thickness.

A new protection circuit for the superconducting coils in the Large Helical Device has been developed. The circuit transfers a high dc current from a power supply to a dump resistor at a coil quench. It is composed of a conventional ac vacuum circuit breaker, disconnectors, power fuse(s), diodes and a dump resistor. Its features include excellent cost performance and reliability and handling of currents up to 32kA in DC.

Achieving steady-state plasma operation at high plasma temperatures is one of the important goals of worldwide magnetic fusion research. A high temperature of approximately 2 keV, and steady-state plasma-sustainment operation of the Large Helical Device (LHD) [O. Motojima, K. Akaishi, H. Chikaraishi et al., Nucl. Fusion 40, 599 (2000)] is reported. High-temperature plasmas were created and maintained for more than 30 min with a world record injected heating power of 1.3 GJ. The three-dimensional heat-deposition profile of the LHD helical divertor was modified and during long-pulse discharges it effectively dispersed the heat load using a magnetic-axis swing technique developed at the LHD. A sweep of only 3 cm of the major radius of the magnetic axis position (less than 1% of the major radius of the LHD) was enough to disperse the divertor heat load. The modification of the heat-load profile was explained well by field-line tracing. The steady-state plasma was heated and sustained mainly by hydrogen minority ion heating using ion cyclotron range of frequencies. The operation lasted until a sudden increase of radiation loss occurred, presumably because of wall metal flakes dropping into the plasma. The sustained line-averaged electron density was approximately 0.7?0.8×10^19 m^?3. The average input power was 680 kW, and the plasma duration was 31 min 45 s. This successful long operation shows that the heliotron configuration has a high potential as a steady-state fusion reactor.

Performance of the LHD cryogenic system in the first year's operation was described making importance on the recovery process after the normal transition of the helical coils. During the excitation tests of the LHD superconducting coils up to 2.75 T, the normal zone propagation was observed in the helical coil and the emergency shut-off of the coil power supplies was carried out. 2,700 l of liquid helium evaporated from the helical coils. The coils and the helium refrigerator were separated automatically and the helium refrigerator could keep its steady state operation. After the pressure and flow rate of the recovery gas from the helical coils were settled down to the normal state, the coils were connected to the helium refrigerator and the cooling was restarted. The system could return to the steady state in which coil excitation is enabling, by only three and a half hours.