中村 一也

The JT-60SA Poloidal Field (PF) coils system comprises four Central Solenoid (CS) modules and six Equilibrium Field (EF) coils, and the cryostat, vacuum vessel (VV), and stabilizing plate (SP) are installed around the PF coil. Evaluating the voltage between conductors in the PF coils is one of the most critical factors in the energized coil operation and the insulation design. The power supply voltage of the PF coils has some frequency components caused by the voltage control system. The resonance phenomena caused by the voltage fluctuations of the power supply induce a non-uniform voltage distribution in the coil. The locally concentrated voltage may exceed the withstand voltage of the insulation, affecting the operation of the JT-60SA. In this study, a circuital model was assembled including four CS modules and six EF coils, to estimate the resonance frequency effect into the coil voltage distribution. In addition, the effects of the cryostat, VV, and SP on the voltage between conductors in the PF coils were evaluated.

The JT-60SA central solenoid (CS) module is cooled to the operating temperature by the gas and supercritical helium. The cool-down tests of center solenoid model coil (CSMC), manufactured to validate the CS module manufacturing process for JT-60SA, showed that the maximum temperature in the CSMC occurs at the innermost turn of the coil. In the JT-60SA cool-down operation, the temperature can only be measured at the flow pipes connected to the outlet and inlet. Therefore, the maximum temperature in the CS module, which has a flow path similar to that of the CSMC, cannot be measured directly. In this study, we developed an analytical model to estimate the temperature distribution for the helium, strands, and jacket in the conductor from the measurement of the JT-60SA CSMC cool-down test. When the inlet temperature of the CS module in the JT-60SA cool-down test in 2023 was used for the inlet temperature in the analytical model, the coil was safely cooled to the operating temperature. Furthermore, the maximum temperature difference in the CS module and the cool-down speed were evaluated when the inlet was adjusted to keep the temperature difference between the inlet and outlet constant.

We have studied a magnetic levitation system including an HTS bulk as magnetic shield. This system consists of a ferromagnetic rail, HTS bulks, and HTS coil. In this paper, the vertical attractive force was discussed in two types of the analytical models. First, the model with the racetrack coil was proposed. We analyzed the vertical attractive force depending on the length of the coil's straight section. As a results, the maximum vertical attractive force became large with increasing a straight section, and the force reached approximately 20 kg when the straight section was 1 m. Next, we proposed a scaled-up model which was a 10-fold scaled-up of the HTS racetrack coil, HTS bulk, and ferromagnetic rail assuming an actual train size. Here we considered that one vehicle body, whose weight was 25 t, was levitated by four racetrack coils. Hence the vertical attractive force of each racetrack coil needed to be about 6.25 t. According to the analysis, the maximum vertical attractive force was about 16 t. The force was generated by one racetrack coil. Therefore, the vertical attractive force was enough larger than the weight of the actual vehicle body.

The authors previously proposed a quench protection method for a high-temperature superconducting (HTS) coil that uses Cu tape co-wound with HTS tape (Cu-CW method). In this method, when a quench occurs in the HTS coil, part of the current in the HTS coil is quickly transferred to the co-wound Cu tape coil by shorting the Cu coil with a resistor due to the tight magnetic coupling of both coils, and the hot-spot temperature of the HTS coil is reduced due to the quick reduction of the HTS coil current. The previous work showed by a numerical simulation that the method is effective to improve the quench protection performance compared with a commonly used detect and dump method (D&D method). In this work, the effectiveness of the method was studied by an experiment using small-scale test coils wound with yttrium barium copper oxide (YBCO) coated tapes to experimentally simulate quench behaviours of HTS coils of significant scale.

The JT-60SA central solenoid (CS) module is cooled to 4.5 K using liquid helium. This cool-down operation of the coil is planned to take about one month. During cool-down, the maximum temperature difference in the coil (dT(C-M)) must be kept below 40 K to avoid excessive thermal stress. Thermometers are attached only at the inlet and outlet on the outer perimeter of the coil. However, because the maximum temperature inside the coil occurs near its inner edge, dT(C-M) cannot be measured directly. Hence, estimating dT(C-M) is important to allow the cool-down process to be completed within one month without excessive thermal stress on the CS. In the present study, based on the results of the JT-60SA cool-down test performed in 2020, a temperature simulation model of the CS module, including the strands, jacket, and He in the bundle and hole regions was created using MATLAB, and dT(C-M) was estimated during the cool-down test. This model was then used to evaluate the effect of the mass flow rate and inlet pressure on the cool-down speed.

For the large research Tokamak JT-60SA, evaluating the withstand voltage between conductors is one of the most critical factors for the coil design. The power supply feeding the coil injects a wide spectrum of frequency components. The transient response and resonance phenomena induce a non-uniform voltage distribution in the coil. The transiently concentrated voltage can cause the insulation breakdown between conductors, and JT-60SA may not be able to operate safely. This study conducted an impulse test to evaluate the withstand voltage between adjacent conductors for an equilibrium field (EF) coil. The actual power supply voltage for the EF coil was measured to investigate the ramp rate (dv/dt) and frequency spectrum. In addition, based on the results of a resonance test of the EF dummy coil, a circuit simulation model for the EF coil was developed to evaluate the effect of the transient response and resonance phenomenon on the voltage in the coil. The results of this study provide fundamental data that can be used to ensure that the EF coil does not cause insulation breakdown.

The JT-60SA power supply of the central solenoid (CS) module contains multiple frequency components, and the transient response phenomenon and the resonance phenomenon cause the voltage non-uniformity in the CS. There is thus a possibility that a local voltage between the conductors that is larger than the withstand voltage of the insulation between conductors will be occurred, and the insulation is damaged. Therefore, an estimation of the transient response phenomenon and the resonance phenomenon is important for verifying that the CS is electrically stable. In the present study, the power supply voltage of the JT-60SA CS module was measured, and the effect of the transient response and the resonance phenomenon on the voltage between CS conductors was evaluated using an analysis model that included CS (four modules), the current lead between the power supply and the CS, and the structures surrounding the CS. The simulation results show that when the actual power supply is applied to the CS, the maximum voltage between CS conductors affected by the transient response and the resonance phenomenon is lower than the withstand voltage of the insulation between CS conductors. The results of our analyses provide fundamental data that can be used to estimate the voltage distribution in the CS and thus ensure the electrical stability.

The JT-60SA central solenoid (CS), which consists of four stacked modules, is cooled to an operating temperature of 4.5 K by supercritical helium (SHe). During cool-down from room temperature to 4.5 K, in order to avoid a damage on the coil, it is required to reduce thermal stress due to temperature difference in the coil. In the cool-down operation, the CS needs to control the maximum temperature difference in the longitudinal direction of the cable-in-conduit (CIC) conductor below 40 K. However, since that could damage the coil, correctly estimating the temperature distribution in the conductor is important for ensuring the cool-down operation is performed safely. In the present study, the JT-60SA CS module was prepared and the inlet and outlet temperatures, as well as the SHe mass flow rate, were measured until the CS module was cooled to operating temperature, after which aCSmodule simulationwas performed in order to determine the effect of the inlet temperature on the cool-down speed. From the simulation results, it was concluded that the average cool-down speed was 0.73 K/h and the CS module reached 80 K in approximately 8.3 days while maintaining the maximum temperature difference within 40 K. Furthermore, the temperature difference between the inlet and outlet was kept within 25 K in order to limit the thermal stress. Taken together, the results of our analyses provide fundamental data that can be used to evaluate the safe cool-down operation of the coil and thus protect the coil systems.

In this article, we studied a magnetic levitation system which was composed of a high-temperature superconducting (HTS) coil, HTS bulks, and a ferromagnetic material (rail). We set two magnetic levitation systems in one ferromagnetic rail. We analyzed the levitation force depending on changing the distance between HTS coils of each direction of current flowing. As a result of comparing two poles that has current flowing through its coils in one direction and two poles that has current flowing through its coils in different directions, the model which had the coils in one direction was better. A reason for this result is discussed comparing with the magnetic flux density and the stress tensor in a ferromagnetic rail. In addition, the distance between coils was also important for the magnetic interaction. We show the tendency of levitation force obtained by changing the distance between coils in each direction of current flowing and explain the reason. In this article, it is found that a greater force can be obtained by using magnetic field interaction effectively. We optimized arrangement and magnetic pole combination and we succeeded to improve the levitation property.

In the authors' previous work, a new quench protection method to reduce hot-spot temperature and increase quench detection voltage was proposed to protect an HTS magnet composed of multiple pancake sub-coils from quench damages (Toriyama et al., 2019). In the method, a current of a quenching sub-pancake coil is transferred to the other sub-coils of the magnet forming auxiliary resistive shunt loop (ARSL) by resistively shorting the other sub-coils. In this work quench behaviors of the pancake sub-coils of a model magnet was investigated by a simulation experiment using small scale test pancake coils wound of YBCO wire. Current patterns of the sub-pancake coils of a model magnet at a quench event were calculated for the case that ARSL method was applied, and in the experiment, the same patterns of the calculated currents were applied to the quenching test coils by a controllable current supply. Experimental results show effectiveness of the new method.

The maximum voltage between the CS module terminals is designed to be 10 kV, the voltage between the layers under ideal conditions is then about 0.38 kV because the CS module has 52 layers. But, in operating condition, there is a possibility that the voltage between the layers is higher than 0.38 kV due to the voltage fluctuations of the power supply and the inhomogeneous voltage distribution in the CS module induced by the resonance phenomenon. The JT-60SA CS consists of four electrically independent stacked CS modules. It might even produce voltages high enough to damage the insulation between the CS conductors. In the present study, which is based on the results of previous studies, we created a circuit simulation model for the JT-60SA CS (four CS modules), including the structures (ground resistances, CS structure, TFC cases, and EFC covers), and used it to estimate the voltage distribution produced by the resonance phenomenon. From these results, we conclude that the resonance phenomenon caused by power supply frequency components does not impact the insulation between the CS conductors, and hence the JT-60SA CS is electrically stable under normal operating conditions.

The most probable cause of quench damage of HTS magnets is over-heating at the highest temperature spot (hot-spot) in the magnet wire during the quench protection sequence. Therefore, to avoid damage, it is necessary to reduce heat generation in the hot-spot. The authors propose a method to reduce the hot-spot temperature of a magnet composed of multiple sub-pancake-coils by reducing current in a quenching sub-coil. A quench occurs most often in a sub-coil that has critical current I-c lower than that of the other sub-coils. When a quench is detected in the lower I-c sub-coil, the current of the quenching sub-coil is transferred to the other coils with higher I-c's resistively shorting the other sub-coils by forming auxiliary dump resistor loop. In this paper, the effectiveness of the method is shown by a case study of a magnet composed of eight pancake sub-coils.

Relations between quench protection conditions and hot-spot temperature of epoxy-impregnated Bi2223/Ag sheathed wire coils are experimentally investigated and limits of hot-spot temperature for the coils to be safe from damage caused by quenches are estimated using small-scale test coils. In the previous work using test coils wound with yttrium barium copper oxide (YBCO) wires, it was found that the hot-spot temperature limits are around 300 K, which is much lower than the temperature (500 K) where critical current of YBCO wire is degraded thermally. Structure and mechanical property together with superconducting property of Bi2223/Ag sheathed wire are different from those of YBCO wire. Therefore, relations between quench protection conditions and hot-spot temperature of epoxy-impregnated Bi/Ag sheathed wire coils together with hot-spot temperature limit are different from those of YBCO wire coil. The test coils used in the experiment are wound with Bi/Ag sheathed wire and epoxy impregnation, and their configurations are basically the same as those in the authors' previous work on YBCO coils, because both wires need to test in the same conditions. In this paper, it is found that the hot-spot temperature limits are also lower than temperature for Bi/Ag sheathed wires to degrade thermally. A reason for those results is discussed comparing with the case of YBCO coils.

An inhomogeneous voltage distribution in the coil is induced by the resonance phenomenon and transient response caused by the power supply. Therefore, damage to the insulation between conductors in the coils is possible. We created the circuit simulation models of the central solenoid (CS) module (the 52-layer pancake coil) with the room-temperature busbar and superconducting current feeder between the power supply and CS module. Then, we evaluated the behavior of the voltage between the conductors for the resonance phenomenon and transient response caused by the supply voltage in the CS module. Based on the results, we can conclude that the influences of resonance phenomenon and transient response on the voltage between the conductors in the CS module is negligibly small under the expected operating conditions. These results therefore represent important information for the safe operation of the JT-60SA.

In the large-scale superconducting magnet, a nonuniform voltage distribution is present due to the resonance phenomenon resulting from inductance and capacitance. To investigate this phenomenon, a central solenoid (CS) model coil (four-layer) was prepared and the frequency dependence of the voltage in different coil turns was measured. Simulation was also performed for a CS model coil in order to determine the effect of factors such as the relative permittivity and the electrical conductivity of the insulation between the conductors. The results indicated that both the maximum voltage in the turns and the frequency at which resonance occurs strongly depend on the relative permittivity, whereas only the maximum voltage depends on the conductivity. Based on this simulation, we investigated the resonance phenomenon of JT-60SA CS. Consequently, the maximum voltage between layers at the resonance frequency is about 2 times higher than that at 5 kHz, and the maximum voltage between turns is about 20 times higher, so there is a possibility that the insulation between the turns is damaged at the resonance frequency. These results, therefore, represent important information for the safe operation of the JT-60SA.

Broader Approach activities accompany the ITER project, and these activities have been carried out between the EU and Japan. The type of the joint between the conductors of the JT-60 Super Advanced (JT-60SA) central solenoid (CS) is a butt joint. The butt joint is a compact joint, but it has limited stability because of poor helium cooling. In this paper, the butt joint of the JT-60SA CS is analyzed, for the temperature rise at the butt joint, by using the FEM simulation software COMSOL Multiphysics. As a result, when the plasma disruption occurs, the butt joint of the CS in the JT-60SA has enough high T-CS margin even if the mass flow decreases from 6.0 to 4.0 g/s. These analyzed results become the fundamental data to evaluate the thermal stability of the coils and to protect coil systems from quench.

Two kinds of joints (the lap and butt joints) for JT-60SA poloidal field coil are evaluated. We measured and analyzed AC losses of the joints for JT-60SA under a time-varying magnetic field, and based on the analytical AC loss, the temperature rise of the joints is estimated at the plasma disruption by using the FEM simulation software COMSOL Multiphysics. As a result, the maximum temperature of the lap joint is constant, on the other hand, the maximum temperature of the butt joint strongly depends on the mass flow of the supercritical helium. When the mass flow of the supercritical helium is 4 g/s and more in all the initial temperatures (4.2-7.0 K), the butt joint has a sufficient temperature margin at the plasma disruption. The obtained results become fundamental data to estimate the temperature margin of the coil.

We prepared two kinds of ionic liquids (DEME BF4, DEME TFSI), and measured the thermal conductivity of the ionic liquids and the ac loss of the Bi-2223 tape using the ionic liquid impregnation under dc magnetic field in liquid nitrogen. As a result, the thermal conductivity of DEME BF4 is approximately twice that of the epoxy. When the mechanical loss is maximum value, the loss can be reduced about 98.5% by using the ionic liquid compared with no impregnation. We consider that the tape impregnated into the ionic liquid is effective for reducing the ac loss. These results produce fundamental data to reduce the ac loss of conduction-cooled high-temperature superconducting coils.

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 n Omega, 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.

The upgrading project of the JT-60 to the JT-60 Super Advanced (JT-60SA) at Japan Atomic Energy Agency has started as a joint effort between Japan and the EU. The coils of the JT60-SA are separately fabricated in Japan and EU. The CS and EF coils are fabricated in Japan. In this paper, we describe the butt joint of the JT-60SA CS and measured the ac loss under a time-varying external magnetic field, and analyzed the ac loss at the butt joint by using the FEM simulation software COMSOL Multiphysics. As a result, the measured ac loss of the butt joint were found to be the smallest of all joints (the CS butt joint, the EF pancake joint, the terminal EF joint and the EF prototype joint), and to strongly depend on the thickness of the copper sleeve in full-size coil joint.

We experimentally and numerically evaluated the AC loss occurred in joints of the EF coil in JT60-SA. There are two kinds of the joints (pancake joint and terminal joint) in the EF coil. Firstly, the AC losses of the joints were experimentally obtained. The experimental results showed that the AC losses in the pancake and the terminal joints were smaller than the loss in the prototype joint which was the previously designed joint and we measured the loss data in two years ago. And next, we numerically analysed the AC loss of the joints. The simulated results well agreed with the experimentally measured data. And we numerically tried to decrease the loss. The AC loss could be decreased to be half of the initial loss.

Mechanical loss in superconducting coils is one of the important parameters for the stability of high temperature superconducting coils (HTS coil). We prepared three kinds of impregnations (no impregnation, ionic liquid grease), measured the AC loss of the Bi-2223 tape under DC magnetic field in liquid nitrogen and quantitatively estimated the relation between the impregnation and losses. As a result, the loss can be reduced about 40% by using ionic liquid compared with no impregnation. We consider that the tape impregnated with ionic liquid is effective to reduce the AC loss. The results become fundamental data to reduce the AC loss of conduction-cooled HTS coils.

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 444m 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. (C) 2011 Elsevier B.V. All rights reserved.

Three kinds of conductors for JT-60SA poloidal field coil are evaluated: (1) a cable consists of Nb(3)Sn superconducting strands and pure copper strands (CS conductor); (2) a cable consists of only NbTi superconducting strands (EF-H); and (3) a cable consists of NbTi superconducting strands and pure copper strands (EF-L). We measured the AC loss under a time-varying external magnetic field. The AC loss experiment on these conductors was carried out with a calorimetric method. As a result, the coupling time constant of CS, EF-H, and EF-L conductors was 50, 45, and 40 ms (n tau = 100, 90, and 80 ms), respectively. The obtained results become fundamental data to estimate the current sharing temperature margins (T(cs)) of the conductor.

Japan and EU are promoting the project that the coils in JT-60 installed in Japan Atomic Energy Agency are changed to superconducting coils. We numerically and experimentally evaluate thermal stability of a conductor joint in the planning superconducting coil (EF coil). We measured an AC loss of the prototype joint, and estimated the coupling time constant. The AC loss at the EF coils joint segment was calculated based on the coupling time constant and the magnetic field distribution. Moreover, the temperature rise due to the AC loss was numerically derived. The calculation result showed that the temperature rise was small and the coil was enough stable.

Recently, the recombination event of norovirus (NoV) has been reported with high frequency, suggesting that RNA recombination is a major driving force in Nov evolution. To assess the incidence of Nov recombination in a residential area, we conducted a molecular biological survey of NoVs existing in sewage water in Toyama Prefecture, Japan. Although GII/4 was predominantly detected in sewage water that was associated with a high frequency of outbreaks caused by this genotype, other genotypes, including two types of recombinant strain, were identified during the survey period. One of the recombinants is the WUG I type, which was first detected in Saitama Prefecture in 2000. The other recombinant is a novel type derived from two parent strains of genogroup II, GII/7 for the RNA-dependent RNA polymerase and GII/13 for the capsid. This suggests that certain NoVs circulating in the area are occasionally changing their genetic properties by recombination events.

It was reported that Lorentz force caused degradation of critical current in the ITER-TFMC conductor. We have used our novel experimental setup, which utilizes the closed electric circuit concept for critical current and stability measurements of multi-stand superconducting cables. The feature of this setup is mechanical loading applied to the multi-strand cable in the transverse direction. Significant degradation in the critical current of the cable was observed when the average compressive stress was about 20 MPa. This degradation was found irreversible after unloading. We tested the cable with epoxy or ice molds as well. No degradation was observed in the molded cables. We also tested the cable with smaller void fraction. In this case, significant degradation in critical current was observed.

We investigated the effect of thermal cycles between room and liquid-nitrogen temperatures on the critical current (Ic) and AC loss of two superconducting coils: one with a bobbin that expands during cooling from room temperature to cryogenic temperature and one with a bobbin that contracts during the cooling. After 100 cycles, neither bobbin suffered degradation. The Ic of the contraction-bobbin coil did not decrease, and that of the expansion-bobbin coil decreased due to the repeated thermal strain. The expansion coil's AC loss was significantly smaller than the contraction coil's loss at the first cycle; however, it increased with the thermal cycles and eventually surpassed the contraction coil's loss because the Ic of the expansion coil decreased due to the thermal fatigue. These results indicate that a moderate expansion in the size of the bobbin effectively decreases AC loss and that excessive expansion reduces the Ic and increases the AC loss.

We have developed a novel critical current and stability measurement experimental setup, which utilizes a closed electric circuit with a multi-strand superconducting cable. The feature of this setup is mechanical loading applied to the multi-strand cable in the transverse direction. It was reported that Lorentz forces caused degradation in the critical current of the ITER-TFMC conductor. Furthermore, these phenomena were mainly observed in the ITER full-size conductors with large Lorentz forces under high magnetic fields. The advantage of our setup is critical current measurement under mechanical stresses comparable to those in the full-size conductor under high magnetic fields. By employing an inductive critical current measurement technique, we conducted an experiment with a transport current of about 10 kA without any power supply or current leads. In our experiments, we observed significant degradation in critical currents due to a compressive stress of about 30 MPa. We applied an innovative technique to mitigate the critical current degradation in mechanically loaded Nb₃Sn superconducting multi-strand cables. We molded one such cable with ice and tested it. No degradation occurred in the icemolded cable. In addition, stability was also ensured due to the large thermal conductivity of ice. Thus, we have successfully mitigated the degradation in the critical current of the Nb₃Sn conductor by ice molding.

In conduction-cooled HTS magnets, it is important to design effective heat drains from the HTS tapes to a cold head of a refrigerator. Aluminum nitride (AIN) is often used as a heat-drain material because thermal conductivity and electric insulation of the material are good. However, the AIN is hard and brittle, and hence it is difficult for magnet makers and users to process the AIN. To address this issue, we have studied to use the plastic having both high-thermal conduction and high-electric insulation properties as the heat drain material. The plastic is the Dyneema fiber reinforced plastic (DFRP). We fabricated an experimental arrangement in which the Bi-2223 tape and the DFRP block were contacted, and a steady current was applied to the tape. Based on the measured voltage between terminals, thermal drain properties from the tape to the DFRP block were estimated. In the same conditions, experiments using the glass fiber reinforced plastic (GFRP) were also performed. Furthermore, the coil shape sample made of the AIN and DFRP were used. The paper presents the difference of the thermal drain performance in the AIN, the DFRP and the GFRP.

Recent studies of RHQ-processed Nb3Al wires for future accelerator magnets are presented and discussed. Test wires were prepared with different fabrication parameters, such as the Nb matrix ratio, RHQ current, area reduction ratio of the wire after an RHQ treatment and the 2nd heat treatment condition. Measurements of the critical current density (J(c)) and n-value have been performed during the past several years. Recently, we performed the critical temperature (T-c) measurement, and the relationship between J(c) and T-c was investigated by and RHQ treatment, a 2nd heat treatment, the Nb matrix ratio, and the area reduction effect.

We developed a novel critical current and stability experimental setup utilizing a closed electric circuit with a multi-stand superconducting cable. The feature of this setup is transverse mechanical loading implied to the multi-strand cable in the transverse direction. It was reported that Lorentz force caused degradation of critical current in the ITER-TFMC conductor. Furthermore, these phenomena were only observed in the ITER full size conductors with large Lorentz forces under high magnetic field. The advantage of our setup is a critical current measurement with comparable mechanical stress under high magnetic field. Employing an inductive critical current measurement technique, we conducted the experiment with transport current of around 10 kA without any power supply nor current lead. As experimental results, we observed significant degradation due to compressive stress of around 30 MPa. This degradation was found irreversible, when it was unloaded.

We have measured the contact resistance between superconducting strands, which is a parameter of coupling losses in cable-in-conduit conductors. Varying the twist pitches of sub-cables in the multi-stranded cables, we prepared triplets made of three strands with four different intersection angles between the strands (0, 5, 7 and 13 degrees). The experimental results showed that the range of the surface resistances estimated by both the contact areas and the contact resistances depended on the intersection angles between the strands of the cable. When we estimate inter-strand coupling losses in the cable, we have to take the contact condition of the cable into consideration. The obtained results enlarge the database used to simulate an electric circuit model of multi-stranded conductors and to estimate inter-strand coupling losses in the conductors.

Recent developments in RHQ Nb3Al wire for accelerator magnets are presented and discussed. The main items of the development are to increase the critical current density (J(c)) and to find a good stabilization method. For the former item, test wires with different Nb-matrix/filament ratios were made, and the effect of the ratio on J(c) was investigated in correlation with the heat treatment and the area reduction after the RHQ process. For the latter item, a special copper electroplating technique was developed to deposit a thick Cu layer on the surface of the wire. The mechanical bonding strength and the electrical characteristics of the Cu layer were studied by bending and drawing the wire, and by measuring the resistance. Although the present piece length of the Cu stabilized Nb3Al wire is about 40 cm, we can draw and reduce the wire down to 60% of the original diameter, without breaking the wire or damaging the bonding of the Cu stabilizer. The highest 2 noncopper J(c) achieved during this study was 2156 A/mm(2) at 10 T and 4.2 K.

In order to obtain a high-intensity neutron beam for neutron scattering experiments, strong sextupole magnets with large apertures are useful as a focusing element. We have started the development of a Nb3Sn sextupole magnet for this purpose. The design field strength of the sextupole is 12780 T/m(2) and the aperture of the. coil is 46 mm in diameter. As a first step of the development, three model coils have be en made in order to learn about Nb3Sn technology and to evaluate the fabrication techniques., Single-pole tests of these coils have been performed to assess the conductor stability and training behavior of the Nb3Sn coils. This paper describes the design of the magnet, the fabrication of the model coils and the results of single-pole tests.

Contact resistance on strands is an important parameter for characterizing stability of cable-in-conduit conductors. We prepared triplets made of three strands with four different twist pitches (25, 50, 65 mm and untwisted). The electrical and mechanical properties of the strands were measured, after cycling 10 times with a peak force of 667 N/m. The contact area between the strands could be precisely estimated, and then the surface resistance was evaluated. As a result, the contact resistance and the surface resistance between the strands depended on the twist pitches of the wires. The results become fundamental data to simulate an electric circuit model consisting of more than one sub-cable, and also to estimate the inter-coupling losses in the cables.

We prepared sample doublets made of two strands with three different twist pitches (25, 50 and 65 mm). One of the doublets is equally twisted to each other, and the other is a strand twisted in spiral around a straight strand. We measured the contact resistance between the strands in the doublets in liquid helium. Since not only the resistance but also mechanical properties of the strands (applied force to the strands and deformation of the doublet due to the force) were measured, the contact area could be precisely estimated, and then the surface resistance was also evaluated. As a result, the difference between the virgin-contact resistance and the contact resistance with the compressive force applied on the doublets increased when the twist pitch of the doublets became short, and the difference in the shapes of the doublets have an influence on the contact resistance. The obtained results enlarge the database used to simulate an electric circuit model of multi-stranded conductors and to estimate inter-strand coupling losses in the conductors.

We have fabricated an experimental equipment which mechanically applies a force to stacked strands with two kinds (0 and 60 degrees) of crossing angles. We measured the contact resistance between the strands, the contact force applied to the strands and the deformation of the strands at three temperatures (293 K, 77 K and 4.2 K). We investigated two kinds of strands (bare surface and with Cr plate). From the experimental results, in the case of a line contact (when the intersection angle's 0 degree) at liquid helium temperature, the ranges of the surface resistance of the Cr plated strands were between 4.3 x 10(-8) and 1.7 x 10(-7) Omegam(2), While the resistance of the bare strands were between 9.2 x 10(-9) and 2.7 x 10(-8) Omegam(2). Hence, effects of the Cr coating on the surface of the strands were clearly observed. The difference of the surface resistance appears to slightly depend on the intersection angles of the contact strands. The obtained results become fundamental data to simulate an electric circuit model of multi-stranded conductors and to estimate inter-strand coupling losses in the conductors.

We have measured contact resistance between superconducting strands that was a parameter of coupling losses in cable-in-conduit conductors. Assuming some kinds of twist pitches of sub-cables in the multi-stranded cables, we measured the contact resistance under the condition of the intersecting strands. Since not only the resistance but also mechanical properties of the strands (applied force to the strands and deformation of the strands due to the force) were measured, the contact area could be precisely estimated, and then the surface resistance was also evaluated. The experimental results showed that the surface resistance hardly depended on the twist pitches of the sub-cables at both liquid nitrogen and liquid helium temperatures. The results become fundamental data to estimate the coupling losses caused by current loops including more than one sub-cable with long-time constants of current decay.

Local temperature rise due to insufficient cooling is one of the important problems in conduction-cooled high-temperature superconducting (HTS) coils. Assuming two types of plastics having high thermal conductivity as spacers in HTS coils, we measured temperature irises of a conduction-cooled HTS tape under various contact conditions between the plastics and the HTS tape. The cooling performance from the Zylon fiber reinforced plastic to the HTS tape was not so good, since the fibers in the plastic were oriented in one direction. The Dyneema fiber reinforced plastic effectively cooled the HTS tape, because the fibers in the plastic were two dimensions. The possibility to use the plastics with high thermal conductivity as the spacers in the conduction-cooled HTS coils was demonstrated.

Contact resistance on surfaces of strands is one of important parameters for characterizing stability of cable-in-conduit conductors. Redistribution of strand currents after occurring a local-normal segment in a strand and AC losses due to inter-strand coupling currents strongly depend on the contact resistance between strands in the multi-stranded conductors. Applying compressive force to a cross segment of the stacked strands, we have measured the contact resistance between the strands at three temperatures. We have prepared two kinds of strands for this experiment. The experimental results showed that the contact resistance strongly decreased with increasing of the contact force to the strands. Furthermore, we quantitatively estimated surface resistance based on the contact resistance and the deformation of the strands. When the strands were made of copper, the range of the estimated surface resistance was between 2x10(-10) and 8x10(-10) OmegaM(2) at the liquid helium temperature. In case that we used the superconducting strands, the surface resistance was 1-2x10(-10) Omegam(2) at the same temperature.

Dyneema (R) and glass fiber reinforced plastics (DGFRPs) expand when they are cooled down to cryogenic temperature. Therefore, they may have applications as structural materials in superconducting magnets to increase stability against abrupt motion of superconductors in the magnets. To obtain the mechanical properties of DGFRPs, we measured coefficients of friction on surfaces of DGFRPs under various experimental conditions at three temperatures. four different forces, and two types of DGFRPs. In all of the experimental conditions, the measured coefficients of friction were quite low. The range of measured coefficients was 0.07-0.14. (C) 2001 Elsevier Science Ltd. All rights reserved.