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As a high-efficiency cooling technology for high-temperature superconducting coils, we have begun research and development to examine the feasibility of a cooling assist technology that maintains a cryogenic state by combining the magnetic force generated by the superconducting coil with magnetic refrigeration technology. Magnetic refrigeration requires the magneto caloric material to change the magnetic field. In many cases, the magnetic field change is obtained by moving the magnetic source, but moving the superconducting coil is not a good idea. Although it is possible to generate a change in the magnetic field by turning on and off the power supply of the high-temperature superconducting coil, it is unlikely to be established as a system that assists cooling of the superconducting coil because the coil generates heat due to AC loss. Therefore, it was considered that the magnetic field change can be obtained if the magnetic force generated from the superconducting coil can be controlled by the magnetic shield. As a verification of the principle, it was clarified experimentally that a magnetic field change can be obtained by repeatedly inserting and removing the magnetic shield into and from the gap between the magnetic field generation source and the magneto caloric material, and the temperature change can be extracted by the magneto caloric effect. In the experiment, the temperature change obtained when a magnetic shield was inserted into the air gap was measured by a simple test device using an iron-based magneto caloric material having high magneto caloric effect performance at room temperature and a permanent magnet. The principle verification confirmation test was performed using several types of magnetic shielding materials such as Permalloy bulks and the electrical steel sheets. In addition, numerical analysis is performed on the magnetic shielding effect, and the shielding effect is improved by increasing the thickness of the shielding material and the possibility of using high temperature superconductors as magnetic shielding materials were also analyzed. In this study, we report the possibility of cooling the high-temperature superconducting coil with high efficiency by combining the magnetic field created by the superconducting coil and magnetic refrigeration.

As a high-temperature superconducting (HTS) conductor with a large current capacity applicable to a nuclear fusion experimental device, REBCO (REBa 2 CuO y ) tapes and high-purity aluminum sheets are alternately laminated, placed in a groove of an aluminum alloy jacket having a circular cross section, and the lid is friction-stir welded. To make the current distribution and mechanical characteristics uniform, the conductor is twisted at the end of the manufacturing process. In the early prototype conductor, when the I c was measured in liquid nitrogen under self-magnetic field conditions, I c degradations were observed from the beginning, and the characteristic difference between the two prototype samples under the same manufacturing conditions were large. Furthermore, I c degradation was progressed by repeating the thermal cycle from room temperature to liquid nitrogen temperature. This I c degradation did not occur uniformly in the longitudinal direction of the conductor but was caused by local I c degradation occurring at multiple locations. If the conductor was not manufactured uniformly in the longitudinal direction, the difference in thermal shrinkage between the REBCO tape and the aluminum alloy jacket caused local stress concentration in the REBCO tape and buckling occurred. Element experiments to explain this mechanism were conducted to clarify the conditions under which I c degradation due to buckling occurs. Then prototype conductors were tested with improved manufacturing methods, and as a result, I c degradation could be suppressed to 20% or less. We have achieved the prospect of producing a conductor with uniform characteristics in the longitudinal direction.

Magnetic refrigeration technology is expected to be a highly efficient process at a temperature of around 20 K, which is the hydrogen liquefaction temperature. In magnetic refrigeration, a magnetic field change is applied to a magnetocaloric material (MCM) to obtain a magnetocaloric effect. In addition, since the greater the magnetic field change, the better the cooling characteristics, the magnetic field strength of permanent magnets is insufficient, and the use of superconducting coils that can generate strong magnetic fields with low power consumption is essential. We are researching and developing a static magnetic refrigeration system (SMRS) that does not have moving parts to obtain magnetic field changes, and generates them by altering the energizing current to the coil. The key to devising this system is an AC loss of the superconducting coil. In this study, we measured the AC loss under energization conditions of multiple palm-sized REBCO coils at liquid nitrogen temperature, and calculated the efficiency of the SMRS by improving the accuracy of the analysis method for evaluating the AC loss, based on the results. We report the results and discuss the technical feasibility of SMRS.

In a cold test of the JT-60SA CS1 module, which is composed of six octa-pancake coils and one quad-pancake coil wound with Nb3Sn CIC conductors, the measurement of self-magnetic fields generated by the CS1 module was conducted. As a result, the decay of the self-magnetic field was observed after the CS1 module was degaussed. The time constant of the decay was from 90 s to 269 s. The measurement results indicate that loop currents with long time constants occurred in the CS1 module due to coil energization. The loop currents with long time constants, which are also called ‘supercurrents’, are irregular coupling currents that are an unexpected phenomenon from the conductor design. Loop currents with long time constants in CIC conductors have only been observed in large superconducting coils so far. To verify an occurrence of loop currents with long time constants in a CIC conductor without coil winding configuration, self-magnetic fields were measured using a short straight CIC conductor where a multi-strand cable was connected electrically at the conductor ends. As a result, loop currents with long time constants ranging from 57 s to 105 s occurred in the short straight CIC conductor, the length of which is approximately 1 m. The measurement results indicate that the conductor length is not always related to loop currents with long time constants. The occurrence of loop currents with long time constants in the CIC conductor depends on the presence of current paths at the conductor ends.

The Large Helical Device (LHD), built in the 1990s, is a heliotron-type fusion plasma experimental device with the world's first fully superconducting magnetic confinement system. The LHD cryogenic system operated stably for 18 years from 1998 to 2015 with high availability exceeding 99%. Unfortunately, in August 2015, a fire occurred in the cold box of the He refrigerator during maintenance, and nonmetallic components such as multilayer insulation films, temperature sensors, and measuring instruments were burnt down. Repair work started in November 2015 and completed at the end of July 2016. In August 2016, a test operation of the He refrigerator was conducted, and the refrigeration power was compared with that measured in the initial performance test conducted in 1995. The measured equivalent refrigeration power at 4.4 K was 9.19 kW, representing a decrease ~2% from the value of 9.38 kW measured in 1995. We attributed this slight decrease in refrigeration power to performance deterioration owing to aging over 18 years and not to the fire. The LHD restarted operation in January 2017, and its 19th operational cycle for a deuterium plasma experiment was conducted successfully up to August 2017. This paper reports the operational history and restart of the LHD superconducting magnet and cryogenic system.

The Large Helical Device (LHD) superconducting magnet system consists of two pairs of helical coils and three pairs of poloidal coils. The poloidal coils use cable-in-conduit (CIC) conductors, which have now been adopted in many fusion devices, with forced cooling by supercritical helium. The poloidal coils were first energized with the helical coils on March 27, 1998. Since that time, the coils have experienced 54,600 h of steady cooling, 10,600 h of excitation operation, and nineteen thermal cycles for twenty years. During this period, no superconducting-to-normal transition of the conductors has been observed. The stable operation of the poloidal coils demonstrates that a CIC conductor is suited to large-scale superconducting magnets. The AC loss has remained constant, even though a slight decrease was observed in the early phase of operation. The hydraulic characteristics have been maintained without obstruction over the entire period of steady cooling. The experience gained from twenty years of operation has also provided lessons regarding malfunctions of peripheral equipment.

A 600 W helium refrigerator/liquefier with variable temperature supplies was constructed in the National Institute for Fusion Science (NIFS). The mass flow rate of supply Supercritical Helium (SHe) is important information for its performance tests and experimental uses. A quarter circle orifice, according to Verein Deutscher Ingenieure and Verband Deutscher Elektrotechniker (VDI/VDE) 2041, is equipped as a mass flow meter. In the case of the configuration of our system, it is recommended to calibrate the orifice under identical operation conditions with regard to Reynolds number and installation. Therefore, a new in-situ calibration method of the orifice is developed. A dummy heat load was attached between SHe supply and return ports, and the resulting SHe enthalpy rise gives us the information for a mass flow rate. The obtained coefficients of discharge are consistent with those of previous studies.

Highly efficient liquid hydrogen storage system is studied with magnetic levitation using high-temperature superconducting (HTS) coils. The system also has high safety in case of emergency, such as an earthquake, with a seismic isolation to absorb vibrations provided by HTS levitation coils setup on the ground side. In such an emergency case, it is considered that a large amount of ac losses are generated in HTS coils, and the winding temperature may rise to lead to a coil quench. In this study, the self-oscillation-type heat pipe (OHP), whose thermal transport property is much greater than that of solid thermal conduction, is used to cool the coil windings. As a result, an HTS coil equipped with an OHP cooling system can be realized, supporting both low heat loads in the usual operation and high heat loads in an emergency.

The high-temperature superconducting (HTS) option is employed for the conceptual design of the LHD-type helical fusion reactor FFHR-d1. The 100-kA-class STARS (Stacked Tapes Assembled in Rigid Structure) conductor is used for the magnet system including the continuously wound helical coils. Protection of the magnet system in case of a quench is a crucial issue and the hot-spot temperature during an emergency discharge is estimated based on the zero-dimensional and one-dimensional analyses. The number of division of the coil winding package is examined to limit the voltage generation. For cooling the HTS magnet, helium gas flow is considered and its feasibility is examined by simple analysis as a first step.

High-temperature superconducting (HTS) current lead has been prepared by the trifluoro acetates-metal organic deposition (TFA-MOD) processed YBCO tapes. The YBCO tape has higher critical current density J c and better magnetic property than the Bi2223 tape. Furthermore, the thermal conductivity of the YBCO tape with a thin Ag layer is much lower than that of the Ag-sheathed Bi2223 tape. The critical current of the YBCO tapes ranges from 150 to 180 A in liquid nitrogen and self-field. A current lead is composed of eight bundles stacked by two YBCO tapes (16 tapes), Cu end caps, and a pair of stainless-steel boards. Rogowski coils and Hall sensors used for estimation of current distribution are attached on each YBCO bundle. The transport current of 1.6 kA was carried with no voltage on every YBCO tape at 77 K. Although voltage of several tens of microvolts on some YBCO tapes appeared, transport current of 1.8 kA was successfully carried with no quenching. The voltages between the Cu cap and YBCO tapes almost linearly increased with increasing transport current, and they range from 0.75 to 0.93 mV and 1.02 to 1.21 mV at 1.8 kA, respectively. The low voltages correspond to 0.42-0.52 μΩ and 0.57-0.67 μΩ. The low contact resistance results in low joule heating at Cu joints. The current calculated by Rogowski coils in eight bundles varies from 137 to 355 A at 1.8 kA and sweep rate of 800 A/s. Therefore, the imbalance among eight bundles is evaluated to be 218 A. The current of eight bundles calculated by Rogowski coils sums up to 1808 A, which is the slight difference of 8 A in comparison with transport current of 1800 A. Rogowski coils and Hall sensors are effective at evaluating of current distribution for current lead.

The superconducting magnet test facility in the National Institute for Fusion Science has been upgraded for excitation tests at a wide temperature range and a higher magnetic field of 13 T. As part of the upgrade, the helium refrigerator/liquefier that operated for 24 years was replaced with a variable-temperature helium refrigerator/liquefier. The required liquefaction rate is 250 L/h, and the required refrigeration capacity is 600 W at 4.5 K, same as the previous one. In addition, it has a new feature that can supply helium gas of a wide temperature range. The typical design cooling capacity is 1 kW under the condition of 20-K supply/30-K return and 1.5 kW under the condition of 40-K supply/50-K return. After the replacement, a series of commissioning tests were performed under the various operational conditions. From the results, the satisfactory thermodynamic performance was confirmed. In the future, it is expected that the substantial progress will be made in the development of large-scale superconducting magnets with advanced superconductors such as high-temperature superconductors and MgB2. The design of the variable-temperature helium refrigerator/liquefier and the results of the commissioning tests are reported in detail.

In 2007, JAEA and NIFS launched the test project to evaluate the performance of cable-in-conduit (CIC) conductors and conductor joints for the JT-60SA CS and EF coils. In this project, conductor tests for four types of coil conductor and joint tests for seven types of conductor joint have been conducted for the past eight years using the NIFS test facility. As a result, the test project indicated that the CIC conductors and conductor joints fulfill the design requirement for the CS and EF coils. In addition, the NIFS test facility is expected to be utilized as the test facility for the development of a conductor and conductor joint for the purpose of the DEMO nuclear fusion power plant, provided that the required magnetic field strength is within 9 T.

In this paper, it is shown that the results of experiments on which four oscillating heat pipes (OHP) were simultaneously operated. The OHP is formed into board shape and the four OHPs were put radially. Two FRP disks as dummy winding held the OHPs. Stainless steel sheet heaters were inserted between OHPs and FRP disks, to simulate heat generation in the winding. This equipment was cooled by circulation helium gas. The helium gas flowed in the copper pipe. The copper pipe was soldered to copper plates mounted to cooling edge of the OHPs. Four buffer tanks were connected to each OHP to control the pressure of each OHP individually. The working fluid of the OHPs was Neon. It was demonstrated that all OHPs were simultaneously operated.

This study is aimed at the development of high-temperature superconducting (HTS) magnets for application in a fusion experimental device next to the Large Helical Device (LHD). By applying the features of an HTS, high current density and high stability can be balanced. As a candidate conductor, REBCO tapes and pure aluminum sheets are laminated and placed in the groove of an aluminum alloy jacket with a circular cross-section, after joining a lid to the jacket using friction stir welding, and twisting the conductor to homogenize its electrical and mechanical properties. The FAIR conductor derives its name from the processes and materials used in its development: Friction stir welding, an Aluminum alloy jacket, Indirect cooling, and REBCO tapes. Initially, the degradation of the critical current of the FAIR conductor is observed, which was eventually resolved. The development status of the FAIR conductor has been reported.

As a final check of the coil manufacturing method of the JT-60 Super Advanced (JT-60SA) central solenoid (CS), we verified the performance of a CS model coil. The model coil comprised a quad-pancake wound with a Nb 3 Sn cable-in-conduit conductor. Measurements of the critical current, joint resistance, pressure drop, and magnetic field were conducted in the verification tests. In the critical-current measurement, the critical current of the model coil coincided with the estimation derived from a strain of −0.62% for the Nb 3 Sn strands. As a result, critical-current degradation caused by the coil manufacturing process was not observed. The results of the performance verification tests indicate that the model coil met the design requirements. Consequently, the manufacturing process of the JT-60SA CS was established.

Based on the successful plasma experiments in the Large Helical Device (LHD), design activities of the LHD-type helical fusion reactor FFHR-d1 are progressing at National Institute for Fusion Science (NIFS). A 100 kA current capacity is required for the winding conductor under the maximum magnetic field of ~12 T. The high-temperature superconductor (HTS) is a promising option for the helical coil conductor. For the development of such a HTS conductor suitable for the helical fusion reactor, we fabricated 30 kA-class HTS conductor samples, and the excitation tests were successfully carried out. We then fabricated and tested a 100-kA class HTS conductor. The conductor sample is a one-turn short-circuit coil with a race-track shape having a bridge-type mechanical lap joint. The transport current of the sample was induced by changing the external magnetic field, then the critical current of the sample was measured. A numerical analysis of the critical current is being performed by self-consistently solving the spatial distributions of the current density and magnetic field among the simply-stacked HTS tapes to verify the measured critical current of the samples. The critical current characteristics of a single HTS tape is evaluated by the percolation model in the precise analysis.

We developed several new Cu-Sn-Zn-(Ti) alloys, which are so-called “CSZ alloy,” for practical bronze-processed Nb 3 Sn wires. Several CSZ alloys were fabricated via the Mizuta method. The Zn was uniformly dissolved in the α phase of the Cu-Sn alloy, and the Zn-Ti compounds were not found in the CSZ alloy. Various 19 Nb-filament CSZ Nb 3 Sn precursor wires were fabricated using CSZ matrices with different Sn and Zn nominal compositions. We also found that the CSZ/Nb composites showed excellent workability at room temperature. In this paper, changes in microstructure and superconducting properties of Nb 3 Sn wires with different CSZ matrices as a function of the heat treatment condition were investigated. We clearly observed that the amount of Zn in the matrix remained constant after the Nb 3 Sn formation. Thicker Nb 3 Sn layers were formed by the Zn addition. T c transitions of about 18 K have been obtained. Finally, we have succeeded in fabricating multifilamentary wires using CSZ matrices through the restacking method. The number of Nb filaments is 7771 with a diameter of 3.4 μm. The effect of the Zn addition on microstructures and superconducting property of the CSZ wires are also reported.

Manufacturing of central solenoid (CS) for JT-60SA is in progress under the JA-EU broader approach activities. The terminal joints and the lead extensions are used for connecting the power supply and the CS. The terminal joint connects a Nb 3 Sn conductor of CS and a NbTi conductor of current feeder. The terminal joint sample was manufactured and tested to verify the design and manufacturing process. The test results indicate that the lap type Nb 3 Sn-NbTi joint is available for CS terminal joint. The lead extension is located between the terminal joint and the CS. The extension joint designed by butt joint is used for connecting the CS and lead extension. A repairable extension joint is useful to exchange the terminal joint when a serious problem occurs at the terminal joint. The resistance of the extension joint after the repair process of disconnecting and reconnecting was measured to validate the repairability. The resistance of repaired joint was almost the same as the resistance of original joint. The lead extension is subjected to the electromagnetic force derived from the plasma operation. The lead extension is supported by the CS structure. The structural analysis of the lead extension and its support structure was conducted to confirm the support design. The analysis results showed that the stresses of all components were less than the allowable stress limit. The components for terminals were successfully developed.

In a cold test of the JT-60SA CS model coil, which has a quad-pancake configuration consisting of a Nb 3 Sn cable-in-conduit (CIC) conductor, magnetic fields were measured using Hall sensors. For a holding coil current of 20 kA, measured magnetic fields varied slightly with long time constants in the range 17–571 s, which was much longer than the time constant derived from a measurement using a short straight sample. To validate the measurements, the magnetic fields of the model coil were calculated using a computational model representing the positions of Nb 3 Sn strands inside the CIC conductor. The calculated results were in good agreement with the measurements. Consequently, the validity of the magnetic field measurements was confirmed. Next, we investigated other coils consisting of CIC conductors and having long time constants. The only commonality among the coils was the use of CIC conductors. At present, there is no obvious way to prevent generation of such magnetic-field variations with long time constants.

Thermal stability for a butt joint of a central solenoid (CS) is experimentally and numerically estimated. The butt joint is fabricated using a strand bundle for the CS conductor, and the quench current of the butt joint is measured by changing the temperature of supercritical helium (SHe). The results show that even if the SHe flow slows to 50% of the rated flow, the temperature margin of the joint is 4 K, the butt joint is sufficiently stable. We also calculate the thermal stability of the butt joint by changing certain operating conditions. According to the simulation data, when connection resistance becomes high, from 2 to 5 nΩ, there is little change in the temperature margin. The experimental and numerical results suggest that the butt joint does not quench and can be operated with stability.

In this report, we propose segment-fabricated high-temperature superconducting (HTS) magnets as candidates for the FFHR-d1 heliotron-type fusion reactor. The FFHR-d1 requires 100-kA-class superconducting conductors used at 12 T for a pair of helical coils. We fabricated and tested two 30-kA-class GdBCO conductors with bridge-type mechanical lap joints (mechanical bridge joints). This report details the design of the joint section and the experimental results of those samples, especially, those of their joints. We improved the geometry of the joint region in a second sample, based on our results from the first. The second sample has sufficiently low joint resistance (less than 5 nΩ), and we could apply 70 kA to it without causing quenching at the joint. Its joint resistance was also acceptable for providing the electric power required to run the cryoplant for the segmented HTS helical coils.

Design activities on the helical-type fusion DEMO reactor, FFHR-d1, are progressing at NIFS. A 100 kA current-capacity is required for the helical coil conductors under the maximum magnetic field of ~ 13 T. High-temperature superconducting conductor has been proposed as one of the conductor options for the FFHR-d1 magnet. In this study, a 30 kA class HTS conductor sample has been fabricated and tested. The sample had no current feeders and the current was induced by changing the background magnetic field generated by the 9 T split coils in the cryostat. Rogowski coils and Hall probes were used for the measurement of the transport current of the sample. The critical current of the sample was measured at various temperatures and bias magnetic fields. To verify the self-field effect of the sample, a numerical analysis was performed by considering the current and magnetic field distribution among the tapes self-consistently. The analysis result was compared with the experimental observation.

The high-temperature superconducting magnet option is being explored in the conceptual design studies of the LHD-type helical fusion reactor FFHR-d1. A 100 kA-class conductor is being developed by simply stacking REBCO tapes in a copper and stainless-steel jacket. One of the design options of the HTS conductor includes internal insulation so that the windings do not require vacuum pressure impregnation process. Innovative winding method of the huge helical coils is being investigated based on the segment fabrication of half-helical-pitch conductors by developing a bridge-type mechanical lap joint. A “30 kA-class” prototype conductor sample was fabricated using GdBCO tapes and successfully tested. The critical current was measured at various temperatures at 4.2-40 K and magnetic field

The upgrading plan of the Mini-RT which is an experimental device for plasma physics was initiated to replace the magnetically-levitated high temperature superconducting (HTS) coil by the new one wound with the latest REBCO tapes. The performance of the new HTS coil has been examined at the National Institute for Fusion Science. The coil was indirectly cooled to 35 K by forced flow of cold helium gas. The coil was successfully excited up to 100 A with a proper persistent current switch (PCS) operation. The central field given by the Hall probe was 0.21 T, which was the same with that expected by a numerical calculation. The time constant of the current decay during the persistent current mode was evaluated at the coil temperature of 36 and 41 K to be 306 and 228 h, respectively. These values are consistent with the estimation based on the measured joint resistances and they are much longer than that of the previous floating coil of Mini-RT wound 10 years ago (41 h before aged deterioration). By the test results, it has been confirmed that the promising performance of the manufactured REBCO floating coil has been achieved to further promote the Mini-RT project.

A central solenoid (CS) model coil (CSMC) was manufactured by using real manufacturing jigs and procedure to validate the CS manufacturing processes for JT-60SA. The winding accuracy and the temperature control precision during the heat treatment met the requirements. The vacuum pressure impregnation process was also successfully finished. The cold test of the CSMC was performed as a final check of the manufacturing process. The joint resistance, the Ic, and the pressure drop measurements were conducted as the verification test. The results of verification test satisfied the design requirements. These results indicate that the manufacturing processes of the JT-60SA CS has been established. The manufacturing of real CS pancakes just started after finishing the CSMC test.

To evaluate the joint fabrication technology for the JT-60SA EF coils, joint resistance measurements were conducted using a sample consisting of pancake and terminal joints. Both joints are shake-hands lap joints composed of cable-in-conduit conductors and a pure copper saddle-shaped spacer. The measurements demonstrated that both joints fulfilled the design requirement. Considering these measurements, the characteristics of both joints were investigated using analytical models that represent the joints. The analyses indicated that the characteristics of the conductors used in the joints affect the characteristics of the joints.

A new method of including cryogenic oscillating heat pipes (OHPs) in the HTS coil windings as a thermal transport device has been studied. In this work, two type of OHPs are tested in low temperature. Employed working fluids are H2, Ne, N2. We have attained high performance thermal property using a bent-pipe cryogenic OHP as a prototype. Obtained effective conductivities have reached to 46000 W/m K. Then a flat-plate cryogenic OHP has been developed, that is suitable for imbedding in magnet windings. Preliminary experiments have been conducted and the result has been promising.

In order to understand the current behavior inside a cable-in-conduit conductor (CICC), self magnetic field measurements on the CICC were conducted. A prototype NbTi CICC, the configuration of which is a racket shape, for JT-60SA EF coil was used as a short conductor sample. By using Hall sensors arranged around the short conductor sample, the variation in the self magnetic field was measured. Taking into account the measurements, the current behavior inside the CICC was analyzed by using analytical models consisting of line currents. The analytical results indicate that the current distribution in the cross-section of the CICC would be non-uniform before a normal propagation, and the current distribution would be improved after the normal propagation.

The conductors for plasma equilibrium field (EF) coils of JT-60SA are NbTi cable-in-conduit (CIC) conductor with stainless steel 316L jacket. The production of superconductors for actual EF coils started from February 2010. Nine superconductors with 444 m in length were produced up to July 2010. More than 300 welding of jackets were performed. Six nonconformities were found by inspections as go gauge, visual inspection and X-ray test. In order to shorten the manufacturing time schedule, helium leak test was conducted at once after connecting the long length jacket not just after the welding. The maximum force to pull the cable into jacket was about 7.6 kN on average. The mass flow rates of 9 conductors showed almost same values indicating that there are no blockages in the conductors. The measured current sharing temperature agreed with the expectation values from strand performance indicating that no degradation was caused by production process. The coupling time constants of conductors ranged from 80 to 90 ms which are much smaller than the design value of 200 ms.

The fast ignition realization experiment (FIREX) project is progressing. The new short pulse laser system, LFEX laser, has been completely assembled and one of the four beamlets is now in operation. A fast-ignition experiment was performed using this single short pulse combined with the Gekko XII implosion laser. The energy of the GXII implosion laser was about 2 kJ and the pulse width was 1.5 ns. The energy of the LFEX laser was increased upto 800 J and two pulse durations 5 and 1.6 ps were compared. Targets were deuterated plastic shells with gold cones. It was found that the neutron yield was increased by a factor of 30 as a result of the fast electron-induced heating in LFEX 1.6 ps shot. The estimated coupling efficiency between the LFEX laser pulse and the compressed fuel was low (less than 5%). This may be due to pre-plasma formed by light arriving at the target before the main laser pulse. Further investigations and attempts to overcome these problems are now in progress. © 2011 Elsevier B.V.

Feasibility studies on applying high-temperature superconductors (HTS) to the heliotron-type fusion energy reactor FFHR are being carried out. Using HTS, we consider that the three-dimensional helical coils with a ~40 m diameter can be constructed without preparing a huge winding machine. A practical method for realizing this concept is proposed. The electromagnetic stress inside the helical coil packs is examined using an FEM analysis for double-pancake windings. The effect of error magnetic field generated by the shielding currents in HTS tapes is also examined.

The cryogenic oscillating heat pipe (OHP) for conduction cooling of superconducting magnets was developed and the function was demonstrated successfully. OHP is a highly-efficient heat transfer device using oscillating flow of two-phase mixture. The working fluids that are employed in the present research are Nitrogen, Neon and Hydrogen, and the operating temperatures are 67–91 K, 26–34 K and 17–27 K, respectively. The estimated effective thermal conductivities from the measurement data of the OHP were higher than one of the solids such as copper at low temperature. These results revealed that the cryogenic OHP can enhance the performance of cooling system for magnets.

Oscillating heat pipes (OHP) for cryogenic use are being developed to improve the heat removal characteristics of high-temperature superconducting (HTS) magnets. It is generally difficult to remove the heat generated in HTS windings, because the thermal diffusivities of component materials decrease with an increase of the operating temperature. Therefore, a local hot-spot can be rather easily generated in HTS magnets, and there are possibilities of observing degradation of superconducting properties and/or mechanical damages by thermal stresses. As a new cooling technology to enhance the heat removal characteristics in HTS magnets, the cryogenic OHP is proposed to be imbedded in magnet windings. The feasibility of cryogenic OHP has been confirmed by fabricating proto-types and by observing stable operations using hydrogen, neon and nitrogen as the working fluid. A high thermal conductivity was achieved that surpasses those of high-purity metals. We also propose a modified-type OHP to mitigate the orientation dependence.

Propagation and recovery of normal zones were observed several times in a pair of helical coils, which are H1 and H2 coils, of the Large Helical Device. In order to detect the position of a propagating normal zone, pick-up coils were installed along the helical coils by the pitch of 30 degree of the poloidal angle. The diameter and turn number of the pick-up coil are 99 mm and 10,000, respectively. They detect the change of magnetic field by current transfer between the superconducting strands and an aluminum stabilizer. The position and the velocity of propagating normal-zones were detected successfully 15 times since the pickup coils had been installed. They were induced 12 times in H1 coil and three times in H2 coil. Most of the normal zones were induced at the bottom of the coils, and all of them propagated to one side, which is the downstream of the transport current, with recovery from the opposite side. The propagation velocity can be estimated from the delay time of the peak voltage of the pickup coils. These obtained data are very useful to investigate the cause of the normal-zone propagation and cryogenic stability of the helical coils. Furthermore, this method is applicable for other superconducting magnets made of asymmetrical conductors.

The MgB2 coil wound with a parallel conductor composed of two MgB2 tapes with insulation has been developed. The tape is obtained by the rolling process from the round wire which is fabricated by in-situ method. The performances of tapes, such as critical current density and ac losses, are higher than that of round wires in case that the transverse magnetic fields are applied to the tape in direction of the parallel to the flat face of the tape. We proposed as a candidate of applying the tape to a conductor with large current capacity that the parallel conductor is composed of insulated tapes. We fabricated a tape with aspect ratio of two from a round wire with diameter of 0.8 mm as a strand for the conductor. The coil is composed of two coils that are jointed each other in series. At the joint, two strands are transposed to be identical the inductances of two strands. The inner and outer diameters of the coil are 65 mm and 107 mm, respectively. The length of the coil is 53 mm. The total turn number is 540 turn. The magnetic flux density at the center in the coil is 1.31 T when the transport current is 200 A. In this paper, the design and the test results of the MgB2 coil wound with the conductor are shown and the critical current properties and ac loss properties of the coil are discussed.

HTS current leads have been prepared by the TFA-MOD processed YBCO tapes 5 mm in width and around 120 μm in overall thickness. The YBCO superconducting thin layer with 1.5 μm is formed on the oxide buffer layers deposited on Hastelloy substrate tape. The critical current of the YBCO tapes is around 100 A at 77 K in self-field. A current lead unit is composed of a GFRP board and five YBCO tapes soldered to Cu cap at both ends. The transport current of the ten current lead units ranges from 536 A to 408 A at 77 K in liquid nitrogen. The YBCO current lead is assembled from the ten current lead units in parallel. The transport current of 4,000 A at 77 K was stably carried with no voltage generation on the YBCO tapes of the assembled current lead for ten minutes. Furthermore, the trans port current of 5,000 A was successfully applied with voltage of around 200 μV on YBCO tapes. The heat leakage of the assembled YBCO current lead with 150 mm in length between 77 K and 4.2 K is estimated to be 470 mW. Therefore, the heat load of it at transport current of 4000 A corresponds to 0.12 W/kA, which is one order of magnitude smaller than that of conventional Cu current lead. The small heat load results from high current performance and low thermal conductivity in the present YBCO current lead.

The equilibrium field (EF) coil conductors are designed with the NbTi cable in conduit (CIC) conductor because the maximum magnetic field of each conductor is up to 4.8 T for the EF-L conductor and 6.2 T for the EF-H conductor. The first EF conductor was manufactured and processed into the performance verification test sample. Then the performance verification tests had been operated before beginning the mass production. The current sharing temperature (Tcs) measurement tests and the quench tests were operated as the performance verification test to evaluate the Tcs and the minimum quench energy (MQE). The results of the Tcs measurement test indicate that both the Tcs of the EF-H conductor and the EF-L conductor are almost same as the expected value estimated by the strand characteristics. Additionally, the Tcs results satisfied the requirements of the EF coils, therefore the conductor design and the fabrication process are going well. The results of the quench test indicate that the EF conductors have enough stability marginuch as more than several hundred mJ/cc at the temperature margin of more than 0.5 K and the current of 20 kA. In addition, the supercritical helium (SHe) mass flow seldom influenced on the MQE. Thus the EF coil can be operated in stable condition even if the SHe mass flow will be decreased.

In the performance verification tests of the JT-60SA central solenoid model coil, the hydraulic characteristics of the model coil improved during the coil energization. As a cause of this phenomenon, it was suggested that a new cooling channel is made by the cable deformation due to the electro-magnetic (EM) force. To investigate the new cooling channel, the EM structural coupling analysis was conducted. The analytical results indicated that new cooling channels in the coil winding occur, and the cooling channels become larger as the position of the cable moves away from the center of the coil winding.

The helical coils in the Large Helical Device (LHD) are large-scale superconducting magnets for heliotron plasma experiments. The cooling system of the coils was upgraded in 2006 to improve the cry...

The Large Helical Device (LHD) is a heliotron-type device employing large-scale superconducting magnets to enable advanced studies of net-current-free plasmas. The major goal of the LHD experiment ...

Performance of the superconducting helical coils of the Large Helical Device (LHD) during the past 12 cooling cycles is reviewed. The pair of helical coils are pool cooled by liquid helium and woun...

The Large Helical Device (LHD) is one of the world’s largest superconducting systems. It consists of a pair of pool-cooled helical coils, three pairs of forced-flow-cooled poloidal coils, nine supe...

AC losses in poloidal coils of the Large Helical Device (LHD) have been measured during single-pulse operations. The superconductors of the coils are Nb-Ti cable-in-conduit conductors (CICC) cooled by supercritical helium. The enthalpy increase due to the losses was observed at the inlet and outlet of the helium coolant. Time constants of the coupling losses can be estimated using analytical expressions with a circuit model and by considering the magnetic field distribution in the coil. The estimated time constants are 80 and 200 ms for two of the poloidal coils. We found a clear difference in the time constants, even though the two coils were fabricated with exactly the same design.

An imbalanced current distribution is often observed in cable-in-conduit (CIC) superconductors which are composed of triplet type multi-staged sub-cables, and hence deteriorates the performance of the coils. Since it is very difficult to control homogeneous current distribution in the triplet type CIC, we propose a coaxial multi-layer type CIC to obtain the homogeneous current distribution. We use a circuit model to analyse the current distribution in the coaxial multi-layer CIC. After calculating inductances between adjacent layers in the coaxial multilayer cable, we can derive a generalized formula governing the current distribution as explicit functions of the superconducting cable construction parameters, such as twist pitch, twist direction, layer radius and SC and Cu strands number. We apply the formula to design the coaxial multi-layer CIC for SC magnet of Force Free Helical-type Fusion Reactor (FFHR). We can design the coaxial multi-layer CIC with the homogeneous current distribution, and investigate several SC strand arrangements in the CIC, and optimize the superconducting strand volume.

Magnetic field measurements on a shake-hands lap joint were conducted using the joint sample of cable-in-conduit conductors for JT-60SA EF coil in order to understand electrical phenomena in the joint region. In the measurements, the magnetic field which is perpendicular to the mid-plane of the cross-section in the joint region was measured using hall probes. In the case that the joint region is subjected to an external magnetic field, the magnetic field has an influence on the current distribution in the joint region.