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This study explores the applicability of microwave nondestructive testing to detect a metal pipe’s inner crack. Three-dimensional finite element simulations were conducted to study the inspectability of cracks using microwaves in different modes and the dependency of the reflection characteristics of microwaves on crack size. The simulation results showed that microwaves in the TM01 and TE01 modes can detect circumferential and axial cracks, respectively. The positive relationship between crack size and intensity corresponding to the reflected microwaves was obtained in simulations and then verified by experiments. In both simulations and experiments, the axial crack length showed a small influence on the results, especially for shallow crack detection. In the experiments, circumferential and axial cracks with a width of 0.3 mm were detected using microwaves. The experimental results revealed that signal amplitudes decreased when a slit penetrated a pipe wall, probably due to a microwave leakage.

Porous-stabilized rare-earth barium copper oxide (REBCO) tape is an effective structure to solve a trade-off between the current-limiting and recovery performances of the resistive-type superconducting fault current limiters (RSFCLs). In the present study, a current-transfer-type porous-stabilized REBCO tape was proposed to suppress hot spots on the tape, which caused a recovery delay. In this type of REBCO tape, two porous stabilizers with different roles are utilized. The boiling heat transfer performance is improved with one of the porous stabilizers fixed with insulating epoxy on the tape while the current is transferred to the other porous stabilizer to decrease the Joule heating in the hot spot. To design the structure of the current-transfer-type porous-stabilized REBCO tape, the heat transfer performance with a porous medium and insulating epoxy was evaluated. The results indicated that there were suitable specifications for the porous medium and insulating epoxy for high heat transfer performance. The recovery performance of the current-transfer-type porous-stabilized REBCO tape was evaluated with the prototype using the selected porous stabilizer and insulating epoxy. The experimental results showed that the tolerable Joule heating was dramatically improved via current transfer to the porous stabilizer owing to the suppression of the temperature rise of the hot spot. The recovery time was shortened to 0.8 s. The present study showed one way to suppress the hot spots with a simple structure while maintaining the high electrical resistance of the REBCO tape.

Rare-earth barium copper oxide (REBCO) tape is used for resistive type superconducting fault current limiters (RSFCLs) owing to its quick response and simple structure. To shorten the recovery time, film boiling on the REBCO tape should be suppressed as it exhibits poor heat transfer performance. To address the aforementioned issue, this study utilized low thermal conductivity materials as the thermal/electrical interface material for porous-stabilized REBCO tape. The experimental results showed that the cooling and recovery performances were dramatically improved by utilizing an insulating epoxy, which has a thermal conductivity of 0.6 W/mK. Based on the visualization of the boiling behavior, the liquid-solid contact between liquid nitrogen and the REBCO tape was promoted by the insulating epoxy, in which the film boiling was easily broken.

A quench detection method using a low-temperature superconducting quench detector (QD) has been proposed to protect rare-earth barium copper oxide (REBCO) coils at temperatures below 20 K in magnetic fields. In this study, we first conducted an experiment at 4.2 K in 0–9 T using a locally- degraded REBCO tape and Nb–Ti QDs (Nb–Ti/Cu, Nb–Ti/Cu–Ni). Quick quench detection before thermal runaway of the REBCO tape was demonstrated by the Nb–Ti/Cu–Ni QD in 0–9 T, whereas Nb–Ti/Cu QD exhibited quick quench detection only at 9 T. Furthermore, we numerically evaluated the quench detection time of Nb–Ti QDs attached to a REBCO tape with local degradation at 4.2 K in 0–9 T. The simulation reproduced the experimental results. Moreover, the contact resistance between the REBCO tape and QD should be lower than 1.0 × 10−3 m2K/W for quick quench detection.

This study experimentally investigates the efficacy of TE11 mode microwaves in detecting bend-region cracks. Three types of bends with different dimensions were deployed in a 3 m piping system and ...

This study proposed a simple side-incident TE11 mode microwave probe for the rapid and long-range inspection of cracks in metallic pipes. The probe feeds the microwaves to a metallic pipe, which works as a waveguide; the reflection signal provoked by a defect is measured for detection and localization. The probe enables the detection of both circumferentially and axially oriented cracks, unlike those reported in earlier studies. To achieve better performance, numerical simulations were conducted to evaluate the conversion efficiency and optimize the insertion length of the coaxial cable and the exposed length of the cable core wire. The simulation results suggest that the optimized probe configuration is feasible for pipes with various diameters by proportionally changing the current probe configuration. Although injected microwaves propagated in two directions, the ratio of microwaves to one direction could be controlled by adjusting the inclination angle and the exposed length of the cable core wire. Subsequently, three TE11 probes fabricated according to the simulation results were used to detect circumferential and axial slits in a brass pipe. The results showed that circumferential slits in the vertical positions (parallel to the nontilted coaxial cable) and axial slits in the horizontal positions (perpendicular to the nontilted coaxial cable) caused large reflections, consistent with the electromagnetic field distribution of the TE11 mode microwaves in a circular waveguide. Further experiments verified the feasibility of the designed probe for pipes with different diameters and for directional pipe inspection.

In this study, a TE01 mode converter was developed to locate an axial crack on the inner surface of a metal pipe. Three-dimensional finite element simulation was conducted to evaluate the effect of inserted coaxial cables on the transmission characteristic of the mode converter. The result showed that the energy of TE mode microwaves leaned to transmit to one side when the cables penetrated with inclination. To demonstrate the effectiveness of the mode converter, experimental verification was conducted. The mode converter was fabricated based on the simulation result. Microwaves were emitted through the pipe wall of the converter to propagate on both sides (‘right side’ and ‘left side’) of a pipe with an artificial slit. Compared with the signals from each side, the reflection from the slit on the right side was more significant than that on the left side. This result is consistent with the numerical simulation result.

This study evaluated the effect of pipe diameter on the applicability of a technique using TE11 mode microwaves for in-pipe crack detection. Three TE11 mode converters of different inner diameters were designed based on theoretical calculations and verified via numerical simulations. The working bandwidths of these mode converters were 7.0, 4.0, and 1.9 GHz. Experimental verification was carried out using brass pipes with the corresponding three inner diameters, and with pipe lengths up to 21–25.5 m. An axial and a circumferential slit were introduced to simulate cracks and deployed at multiple positions along the pipes under test. The results showed that both axial and circumferential slits could be detected and located for an inner pipe diameter up to 39 mm and at a distance of 15 m–24 m.

This study proposes to make effective use of plutonium transmuted from minor actinides (MA) by fusion reactors as fertile fuel in a light water reactor (LWR). The plutonium transmuted from MA, particularly Pu-238, has a large neutron capture cross section and becomes fissile Pu-239 by the reaction. If the plutonium transmuted from MA is loaded into LWR appropriately, there is a possibility to maintain a constant effective multiplication factor. This study evaluated the effects of the plutonium transmuted from MA on a full MOX boiled water reactor (BWR) core by Monte Carlo-based neutron transport and burnup calculation. We revealed that some fuel assemblies achieved the effective multiplication factor is greater than 1.0 and its decrease was significantly small (−0.013) during 400 days operation. However, under this condition, the power peaking factor was 1.4, which should be unacceptable from the viewpoint of thermal design. Inventories of heavy nuclides in fuel cycles was evaluated using a simple diagram and its result indicated that the accumulation of MA in the fuel cycle was reduced by introducing the MA transmutation and the Pu recycle system. Furthermore, the amount of MA production and MA transmutation are balanced by introducing two fusion reactors with 3 GW thermal output into the fuel cycle, and the MA inventory was equilibrated.

Joint-winding of high-temperature superconducting (HTS) helical coils has been proposed as a challenging option for the large helical device (LHD)-type fusion reactors, FFHR series. The coils are wound by connecting helical conductor segments using bridge-type mechanical lap joints with indium foils inserted into the joints. A previous study achieved 1.8 nΩ using a 100-kA-class STARS (Stacked Tapes Assembled in Rigid Structure) conductor joint at 100 kA, 4.2 K and 0.45 T. However, it took eighteen hours to join the conductors because each REBCO (Rare-earth Barium Copper Oxide) tape was joined individually and in series. This study introduced an integrated joint piece to join 10-kA-class STARS conductors, which possibly makes the fabrication time to be less than three hours per one conductor joint. We also applied heat treatment at 120 °C during the joining process to obtain uniform joint pressure distribution and a large true area of contact. The joint resistivity and critical current for each REBCO tape of the joint were comparable to those for a single lap joint at 77 K. The joint resistivity was also evaluated at 4.2 K, which was one-fourth to one-third of the previous result obtained during testing of the 100-kA-class STARS conductor joint.

A partly insulated fin (PIF) structure has been proposed to mitigate the temperature stratification of flowing-type liquid metal divertors. This study conducted a three-dimensional magnetohydrodynamics (MHD) flow and heat transfer numerical simulation to investigate the influence of the PIF on the velocity and the temperature field inside the film flow. An analytical model simulated a liquid metal film flow of 25 mm thickness under a transverse magnetic field up to 1.0 T. The simulation was conducted with a PIF, a metal fin, and without a fin. The results showed the generation of a high velocity region near the surface and the velocity was 2.68 times higher than the case without a fin due to the MHD effect. In addition, an upward flow was observed in front of the PIF. The contribution of the flow field generated by the fin to the temperature field was confirmed through numerical analysis, and the highest temperature of the liquid metal surface decreased by 0.96% at its maximum. A deformation of the surface due to the fin was estimated using the pressure distribution obtained from the simulation. The result indicated expected deformation is 21.8 μm at the magnetic field of 1.0 T.

Quench detection in a rare-earth barium copper oxide (REBCO) coil is difficult due to the slow normal zone propagation in it. To address this issue, we have proposed a quench detection method using a low temperature superconducting (LTS) wire as a quench detector. A demonstration experiment in our previous research using a REBCO tape showed that the quench detector was able to detect quench in the tape before it burned out. Therefore, to verify the applicability of this method to a real magnet system, we first numerically evaluated the quench detectability of Nb-Ti and Nb 3 Al detectors attached on stacked REBCO tapes at 4.2 K under magnetic fields up to 20 T assuming the coil excitation process. The results indicated that the detectors were not needed to be placed in each layer of the REBCO coil. Furthermore, we have conducted an experiment using a REBCO single pancake coil with the Nb-Ti quench detector at 4.2 K and self-field. As the results of the experiment, we demonstrated that one Nb-Ti detector was able to detect the quench in the REBCO coil even if there were at least three REBCO tapes and the polyimide tapes between the detector and the hot-spot in the REBCO coil.

Easy and simple fabrication of a lap joint of high-temperature superconducting (HTS) tapes is desirable for various HTS applications such as power cables and current feeders. We have developed mechanical joints with a low-temperature heat treatment at around 370 K where indium foils are inserted between joint surfaces. Though the joint showed a good performance, it took over 10 min to fabricate one joint and ideally the fabrication time should be further shortened. In this study, we proposed to use ultrasonic welding (UW) to assist a heat generation at the joint for lap joints with indium foil, which can be completed within 1 s. We prepared various lap joint samples of HTS tapes: REBCO (REBa 2 Cu 3 O 7 , RE: rare-earth) or BSCCO (Bi 2 Sr 2 Ca 2 Cu 3 O 10 ) tapes were pressed together with an indium foil using a UW machine or pressing machine with cartridge heaters. Then, the current-voltage curve was evaluated at 77 K and the joint resistance characteristics were discussed depending on the joint methods and HTS tapes. The samples with copper-stabilized REBCO tapes all showed joint resistivities of 31.6-35.6 nΩcm 2 without critical current (I C ) degradation for both joint methods. The samples with silver-sheathed BSCCO tapes showed joint resistivities of 18.6-46.5 nΩcm 2 for both joint methods; however, some of those fabricated by the UW showed I C degradation. The degradation was able to be prevented by optimizing UW conditions.

The multi-layered structure of a rare-earth barium copper oxide (REBCO) tape causes the resistance of Cu/Ag and Ag/REBCO interfaces (interface resistance), which is a major factor of the resistance of tape-to-tape joints (joint resistance). Interface resistance has been evaluated by the current transfer length (CTL) method. However, induced damage of a part of the measured REBCO tape in this method may cause variations in estimated values, as well as making the tape itself unable to be reused for applications. Therefore, this study proposes the contact-probing CTL method, which is a nondestructive evaluation method of interface resistance. We evaluated the interface resistance in a REBCO tape by the conventional CTL method and the contact-probing CTL method. The estimated interface resistance values by the contact-probing CTL method were more reliable and less varied than those by the conventional method. We also evaluated the temperature dependence of the interface resistance, which is important to analyze joint resistance for various applications of REBCO tapes. The result showed that the interface resistance remained constant at different temperatures.

The conceptual design of a helical fusion reactor was studied at the National Institute for Fusion Science in collaboration with other universities. Two types of the force free helical reactor (FFHR) are FFHR-d1 and FFHR-c1. FFHR-d1 is a self-ignition demonstration reactor that operates with a major radius of 15.6 m at a magnetic field intensity of 4.7 T. FFHR-c1 is a compact subignition reactor that aims to realize steady electrical self-sufficiency. Compared to FFHR-d1, FFHR-c1 has a magnetic field intensity of 7.3 T and a geometrical scale of 0.7. The location of the superconducting coils in both types of FFHR is based on that of the Large Helical Device (LHD). LHD has a major radius of 3.9 m. According to the design of LHD, the deformation must be within the required value to compensate for the accuracy of the magnetic field. According to this concept, the magnet support structure of LHD was fabricated using thick Type 316 stainless steel to impart sufficient rigidity. Thus, the stress of the magnet system of LHD is sufficiently below the permissible stress. In the case of FFHR, from the viewpoint of the reactor, a large access port is required for the maintenance of the in-vessel components. The mechanical design of the support structure is conceptualized by considering the basic thickness of the material and residual aperture space by referencing the mechanical analysis results. Details of the design concepts of LHD and FFHR-d1/FFHR-c1 as well as the results of mechanical analyses are introduced in this paper.

FFHR-d1A and c1 are the conceptual design of a helical fusion reactor. The positional relationship among superconducting coils, a pair of helical coils with two sets of vertical-field coils, are observed to be similar in both type of FFHR. Such a relation of coil configuration is based on the coil configuration of the Large Helical Device, which has been designed and constructed at the National Institute for Fusion Science. There is increasing demand to achieve an optimized coil configuration to anticipate improvements in plasma-confinement conditions. In this study, the structural design of FFHR based on the fundamental set of parameters of coil configuration is depicted, which satisfies the soundness of the structure. Further, the effects of the coil configuration parameters on the stress distributions are investigated. An effect of radius of curvature on a winding scheme of the helical coil is also discussed.

Lap joints between Rare-Earth Barium Copper Oxide (REBCO) tapes are intended to be employed for segmented fabrication of high-temperature superconducting magnet as innovative design of fusion magnets. This study evaluates the shear strength and failure mechanism of lap joints between commercial REBCO tapes from two different manufacturers while using different mechanical joining procedures or soldering alloys. Additionally, the shear strength along the width direction of the different REBCO tapes was experimentally evaluated. Overall results using REBCO tapes with soldered Cu stabilizer revealed that they are unsuitable for field application due to low shear strength of the tapes. On the other hand, REBCO tapes with electro-plated Cu stabilizer are more versatile; soldered lap joints using Pb37Sn63 showed shear strength higher than required value. However, failure mechanism of the soldered joints always involve delamination of the REBCO tape which instantly increase joint resistance. Conversely, mechanical lap joints with indium fabricated using the improved method with heat treatment or indium pre-coating with heat treatment exhibited cohesive failure followed by plastic deformation of indium, which reduces the shear stress while maintaining acceptable joint resistance. This fact makes the mechanical lap joint a more reliable option so far.

Superconducting magnets may potentially become larger and more complex for use in future demonstration and commercial fusion reactors. For such situations, the innovative design concept of remountable (or demountable) high-temperature superconducting (HTS) magnets has been proposed for both tokamak and helical reactors that uses the HTS material features of high thermal stability and low cryogenic power. In Japan, the derived concept of joint-winding of HTS coils has been proposed, in which the coils are wound by connecting short conductor segments. This paper first briefly summarizes the design proposal history and current research and development (R&D) status of the important technologies for joint design, and then reports recent R&D progress in mechanical lap joints with inserted indium foils. Based on the joint resistance as a function of temperature, the applied magnetic field and the numbers of layers and rows of HTS tapes in the conductor, the joint resistance of a 100-kA-class HTS conductor joint can be reduced to one third of its initial value by low-temperature heat treatment. Mechanical behavior of these joints indicates that stress relaxation of indium does not affect the fabrication time and that mechanical joints containing indium are preferable to soldered joints under high electromagnetic forces.

This study proposes ″multi-layered trenches″ as a new liquid metal flow stirring structure for the flowing-type liquid metal divertor to prevent the temperature stratification and the evaporation of liquid metal.Then it investigates the characteristics of the liquid metal flow induced by this structure experimentally and numerically. The proof-of-principal experiment using a torus channel simulating the trenches successfully demonstrated the liquid metal wavy flow where the ratio of meandering component to the main stream was 27% under the magnetic field up to 5 T. The experiment and three-dimensional magnetohydrodynamics flow simulation also proved that magnitudes of the wavy flow and the vorticity increased with a rise in the magnetic field. Moreover, two-dimensional heat transfer model simulation showed the heat transfer enhancement by the wavy flow. Maximum temperature was 26.6 K lower than that in the case without the wavy flow.

The LHD-type helical fusion reactor FFHR has been studied to realize steady-state fusion power generation without a need for current drive and free from disruption. The conceptual design studies of FFHR are steadfastly progressing based on the presently ongoing experiments in the Large Helical Device (LHD). In order to enhance the attractive features of the base option of FFHR-d1A, which is similar to LHD, configuration optimization is being considered for FFHR-d1C. Slight modification of the helical coil trajectory gives an improved condition both for the plasma confinement and the MHD stability. In order to overcome the difficulty for construction and maintenance associated with the three-dimensional structure, innovative ideas are being explored for the superconducting magnet, divertor, and blanket. For the superconducting helical coils, the joint-winding method confirms a fast manufacturing process. The helical divertor is reexamined and practical feasibility is discussed. The maintenance method of the helical divertor and the helically-segmented breeder blanket is a serious issue and a plausible solution is proposed.

This study demonstrated the applicability of a nondestructive microwave testing method for the long-range inspection of pipe inner surfaces. Experimental verifications were conducted using pipes with lengths of 19–26.5 m, inner diameters of 11, 19, or 39 mm, and made of either brass or austenitic stainless steel. Microwaves were emitted from a probe attached to one of the pipe ends, and the reflections due to full circumferential wall thinning 50 mm in length and 1 mm in depth situated at various distances from the end were measured. The frequency range and the probe characteristics were decided so that microwaves propagating only in TM01 mode affect signals. After signal processing to compensate for microwave dispersion, the measured signals showed clear reflections caused by wall thinning situated 23.5 m from the pipe end at which microwaves were emitted. This study also discussed the attenuation of microwave reflection with the distance between a probe and wall thinning. The experimental results agreed well with a theoretical model describing the attenuation of microwave propagating inside a pipe, although the model did not consider the reflection of microwaves due to a flaw, the dependence of the reflection on the frequency of microwaves, or mode conversion of the microwave due to the reflection. The agreement revealed that not only frequency but also the inner diameter, electromagnetic property, and surface roughness of a pipe are needed to evaluate the length of the pipe that can be inspected by the method.

Quench detection for the high-temperature superconducting tape, especially rare-earth barium copper oxide (REBCO) tape, is difficult due to the small normal zone propagation velocity in it. To address this issue, we have proposed a novel quench detectable REBCO tape with a low-temperature superconducting wire used as a quench detector, “REBCO/LTSd tape.” In this paper, we numerically and experimentally demonstrated the validity of the tape consisting of the yttrium barium copper oxide (YBCO) tape and the Nb–Ti wire used at 4.2 K under no external field. Results of the numerical simulation showed that sufficiently high voltage of 0.1 V can be detected before the maximum temperature of the YBCO tape reaches 50 K with the Nb–Ti wires having high resistance at normal conducting state. Experimental results also demonstrated that the quench in the YBCO was detected with the 1 V scale voltage by using the pure Nb–Ti wire.

This study demonstrates by numerical simulations how transmuting minor actinides in fusion reactors can contribute to nuclear fuel cycles in Japan. Minor actinides are installed into blankets of a helical-type fusion reactor to transmute them rather moderately so that neither large neutron wall loading nor removal of large heat are required. Neutron transport and burn-up simulations are conducted to optimize the position of minor actinides and the blanket's structure for transmutation. Whereas the ratio of transmuted minor actinides is not large, the total amount of transmuted minor actinides is revealed to be sufficient because of the system's large inventory. The simulations are followed by a plan that introduces fusion reactors intended for transmutation. Discussion based on Japan's present situation confirms that introduction of fusion reactors with a thermal fusion output of 1 GW by the year 2050 would mitigate many issues concerning disposal of high-level radioactive waste.

A conceptual design for a helical fusion reactor is currently being undertaken by the National Institute for Fusion Science, Japan. The coil support structure is designed from the perspective of the allowable stress of the material. A continuous helical coil winding with a low temperature superconductor and a water-cooled divertor made of tungsten and copper alloy have been considered for use in this reactor, and it is defined as the basic option. Several flexible design options have also been proposed; these options solve existing issues in the basic option and they are treated as the challenging options. They can be implemented by modifying the structural components of the basic option. The structural design modifications need to be made in order for the challenging options and the mechanical soundness were investigated.

This paper discusses the applicability of simple low-frequency eddy current testing to the detection of deeply embedded flaws. The study specifically considered a double tank in a reprocessing plant for extracting plutonium-uranium from spent nuclear fuels. The tank was modelled by two type 304 austenitic stainless steel plates situated with an air gap of 80 mm, and the change in the thickness of one of the plates was detected through the other plate and the air gap. Axisymmetric two-dimensional finite element simulations were conducted and found that a simple circular coil with a large diameter enabled to detect the thickness based on the magnetic flux density at the centre of the coil although the plates were as thick as 30 mm. The results of the numerical simulations were validated by experiments.

This study evaluates the mechanical strength, failure mechanism and change in electrical resistance under shear stress of a mechanical lap joint of Rare-Earth Barium Copper Oxide high-temperature s...

The present study evaluates the thermal design of a blanket system using Flinabe in order to facilitate further discussions on its applicability as a self-cooled liquid blanket system. Molecular dynamics simulations were performed to evaluate the Prandtl number of mixtures in five compositions (LiF–NaF–BeF2 = 31–31–38, 36–27–37, 42–22–36, 49–16–35, and 67–0–33). Thermofluid analysis was carried out to estimate the temperature margin and pressure drop per unit length in a simple geometry model of the blanket system. The Prandtl number of Flinabe is above 100 at 400°C. The present study reveals that Flinabe remarkably relaxes the design conditions compared to Flibe as a coolant owing to its low melting point. In contrast, the pressure drop per unit length of Flinabe is higher than that of Flibe because the viscosity exponentially increases at low temperature. The temperature margin is quite dependent on the heat load on the first wall. If the pressure drop per unit length is around 1.0 MPa/m, the he...

A mechanical lap joint of REBCO high-temperature superconducting (HTS) tapes with an indium foil inserted between joint surfaces has been proposed for segment-fabrication of HTS magnet of an advanced fusion reactor and various HTS applications. In a previous study, we successfully achieved a joint resistivity of about 3.5 pΩm 2 by heat treatment during fabrication of the joint. In this study, improved heat treatment method was developed by controlling contact pressure and combining bake-out process. We evaluated joint resistance and critical current of the joint depending on the heating condition and bake-out condition. The joint resistivity after heat treatment was 2.5 pΩm 2 without decreasing critical current with conditions of heating temperatures of 90-140 °C, a heating time of 30 min, a contact pressure of 100 MPa, a bake-out time of 30 min, and a bake-out temperature of 150 °C. Owing to applying the contact pressure when the indium becomes softer, the true area of contact increased and the thickness of the indium decreased greater than those in the previous method. In addition, it was shown that voids appearing by heating were removed by bake-out process from thermal desorption spectroscopy test.

This study focuses on bending characteristic of a bridge-type mechanical lap joint of Rare-Earth Barium Copper Oxide (REBCO) tapes, which has been proposed for segment fabrication of high-temperature superconducting magnet especially for a heliotron-type fusion reactor. Previous studies have developed the joint with a straight geometry, showing sufficiently low joint resistance from a view point of electrical power required to run a cryoplant for removing heat from a superconducting magnet, though conductors connected at the joint section are ideally curved to form a helical-shaped coil. In this study, we experimentally evaluated joint resistance as a function of bending strain for a bridge-type mechanical lap joint of REBCO tapes with indium foils inserted between the joint surfaces. The results showed critical bending strain, where joint resistance starts to increase, was almost the same as irreversible strain of the REBCO tape in the case of a 10-mm-long joint. The critical bending strain varied as a function of both joint resistivity and joint length; lower joint resistivity and longer joint can achieve higher critical bending strain. The fabrication process “Joining-then-bending” can be also applied to a curved joint for reactor-scaled helical coils, where 25-mm-long joint is planned to be applied, though “Bending-then-joining” would be needed for the joint of small prototype coils.

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.

In this study, the applicability of a nondestructive method using microwaves to the detection of multiple pipe wall thinning defects was investigated. In numerical simulation, a single full circumferential defect was introduced into a straight pipe of 19 mm inner diameter, and the effect of the defect on transmitted waves was investigated. The transmitted energy ratio of the TM01 mode to the emitted wave was found to be 93% in average even in the presence of a 1-mm depth thinning. After that, experimental measurements were performed using a flanged brass tube with two artificial thinning defects. By applying signal processing to the measured S-parameters, clear reflections from the two defects were observed. By comparing the signals, we confirmed that the first full circumferential defect provides at most 12% amplitude attenuation of the reflected signal from the second defect, which should be related to the distance and size of the defects.

This study proposes the construction of the probability of detection (POD) as a function of both the depth and length of a flaw. In addition, this study discusses how to censor signals in constructing the POD. The general effects of the flaw parameters on signals are evaluated by numerical simulations, and the scattering of signals, which is critical to the POD, is estimated by signals obtained in experiments. A new likelihood function is introduced, and the proposed method is demonstrated using eddy current signals caused by various artificial flaws on a flat type 316L stainless steel plate obtained in a laboratory test. The demonstration confirms that the proposed method can provide a reasonable POD with a small amount of experimental signals, and reveals that proper censoring significantly decreases the detrimental effect of noise on the POD.

A mechanical joint of high-temperature superconducting (HTS) conductors has been investigated for segmented fabrication of HTS magnets proposed for a future fusion reactor. In a previous study, joint resistance of 1.8 $\text{n}\Omega$ at 100 kA was achieved for a large-scale simple-stacked gadolinium barium copper oxide (GdBCO) conductor having a bridge-type mechanical lap joint with indium foils inserted between joint surfaces. The obtained joint resistivity (the product of joint resistance and contact area) was, however, ten times larger than that of a single mechanical lap joint achieved in previous studies, which may be caused by nonuniform distribution of contact pressure. In order to reduce this joint resistance, therefore, we propose a new method by introducing heat treatment in fabricating process of the joint. By applying the method, the joint resistance became the lowest at heating temperatures of 75 °C-90 °C, which was reduced to 60% of the value obtained without heat treatment. The reduction of joint resistance was due to a decrease in thickness of the inserted indium and an increase in true area of the contact surface. On the other hand, relatively large voids were observed in the indium by X-ray computed tomography when the heating temperature exceeds 120 °C. The voids were caused by emission of the gases absorbed in the tape and foil, and the voids decreased by baking the conductor tapes and indium foil before the heat treatment. Applying the heat treatment to a large conductor joint, the voids can be formed due to the temperature exceeding 120 °C since the temperature control in the large joint becomes more difficult. Therefore, the baking process before the joint fabrication with heat treatment is one of the options to avoid the void formation.

Mechanical joints of rare-earth barium copper oxide (REBCO) tapes and stacked tape conductors have been investigated for segmented fabrication of high-temperature superconducting (HTS) magnets for future fusion reactors. To design the optimized joint, we need to predict joint resistance depending on operating conditions and the conductor's structure. The prediction method was proposed in our previous study, taking into account the model of contact resistance, resistance of constituent materials, and interfaces. However, that was just based on data of mechanical lap joints for one kind of REBCO tape and a magnetic field perpendicular to the tape. Therefore, in this paper, we first evaluated joint resistances in mechanical lap joints of Fujikura's and SuperPower's REBCO tapes as a function of temperature and magnetic field with different orientations. The results showed that the joint resistance significantly increased with an increase in magnetic field parallel to the tape at temperatures of 10 K and lower. In addition, the reduction rate of joint resistance for the SuperPower's tape with a decrease in temperature was lower than that for the Fujikura's tape. Based on experimental results for Fujikura's REBCO tapes with different field orientations, we modified the model of contact resistance. Further improvement of the prediction method is still needed for different stabilizer structures and a temperature range around 10 K.