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Effective surface passive heat dissipation and durable superhydrophobicity are highly desired for promoting more flexible applications of power-intensive energy systems of aluminum and aluminum-bas...

The present work describes the monitoring system of the real-time strain response on the curing process of epoxy resin from the initial point of curing to the end, and the change in strain during temperature changes. A simple mould was designed to embed the strain gauge, thermometer, and quartz standard sample into the epoxy resin, so that the strain and the temperature were simultaneously measured and recorded. A cryogenic-grade epoxy resin was tested and the Differential Scanning Calorimetry (DSC) was used to analyse the curing process. Based on the DSC results, three curing processes were adopted to investigate their influence on strain response as well as residual strain of the epoxy resin. Moreover, impact strength of the epoxy resin with various curing temperatures were tested and the results indicate that the curing plays a crucial role on the mechanical properties. The method will find cryogenic application of epoxy adhesives and epoxy resin based composites to monitor the strain during the curing process as well as the cryogenic service.

This paper describes a superfluid 4He (He II) production system with a G-M cryocooler developed to supply continuous cooling capacity for cryogenic devices working below 2 K, such as the sealed-cell lambda-point devices and the photon detectors. Unlike the existing cooling method that injecting and pumping liquid 4He (He I) to cool the devices, high purity 4He gas stored in gas cylinder is firstly liquefied by a G-M cryocooler in the condenser, and then cooled down under 2.17 K at the He II pot through a Joule-Thomson valve along with a dry vacuum pump. This design allows a continuous supplement of He II below 2.17 K by using 4He at ambient temperature, which avoids the temperature oscillation to the devices being cooled when refilling He I. Due to the removing of cryogen injection, no additional cryogenic apparatus and professional operation are required. In this system, the He II pot can reach a temperature below 1.8 K for continuous operation.

The ITER project is ongoing in France, and it is expected that a lot of high energy neutrons will be generated, and some neutrons will reach to superconducting magnets. To keep the safe and stable operation of the superconducting magnets, the neutron irradiation data must be piled up and the mechanisms of the irradiation effect must be investigated. A 15.5 T superconducting magnet and a variable temperature insert were installed in a radiation control area at Oarai centre in Tohoku University, and the superconducting properties of the irradiated Nb3Sn wires have been evaluated. The critical current was improved once and drastically dropped after irradiation of 1023 n/m2, and the critical temperature degraded monotonically, and the critical magnetic field decreased around 17 T after the irradiation of 1023 n/m2. The mechanism of the irradiation effect was discussed based on a flux pinning concept.

A cryogenic pulsating heat pipe (PHP), as a novel two-phase heat transfer device, has been studied for cooling superconducting magnets because of its high transfer coefficient, simple configuration, and flexible arrangement. From the results of the room temperature PHP experiments, it was noticed that the flow characteristics and the flow pattern of the PHP are complex, which affect the heat transfer performance of the PHP. In this work, a cryogenic two-phase flow visualization system for cryogenic PHP by using a GM cryocooler is developed to get a better understanding of the oscillating behavior, the distribution of vapor slug and liquid plug, and the flow pattern of the cryogenic working fluid. The details of the design of this system are presented. The performance of the GM cryocooler is verified and the temperature distribution of the cooling system is discussed. The liquid slugs and vapor bubbles were observed during the cool-down period. The visualization experiment will be carried out with the cryogenic PHP prototype installed and operated in a vertical position using nitrogen as a working fluid.

The requirements for Japanese DEMO are to demonstrate (1) steady and stable electrical power generation, (2) plant availability on a level relevant for commercialization, and (3) self-sufficiency of fuels. The DEMO design activity has been mainly focused on challenging DEMO design issues such as divertor heat removal, superconducting magnets, breeding blankets, remote maintenance and the water cooling system, in order to reach the systemic consistency toward a promising DEMO concept. The current status of the DEMO conceptual design activity is presented.

An action plan presented here is the plan toward construction of a Demo reactor in Japan. A Task-force for development strategy of Tokamak Demo Reactors was established and has considered an action plan to construct the Demo in the 2040s. The action plan will lead works in a Joint Special Design Team for Fusion Demo established for design and R&D of the Demo reactor in Japan. Although the first version of action plan was reported in March 2016, a revised version, presented here, is being drafted in order to correspond to the new schedule of the ITER project agreed in 2016, because the ITER progress is one of critical issues in the action plan.

A cryogenic system has been established to measure electrical properties of insulation materials. The cryostat in the system contains two operating modes: 4.2 K and 1.65 K. A G-M cryocooler is used as cold source in these two modes. In 4.2 K mode, samples were directly cooled by the copper plates connected to the second stage of the cryocooler. In the 1.65 K mode, the helium gas is liquefied. The superfluid 4He (He-Ⅱ) of 1.65 K was obtained by throttling and depressurizing the liquid 4He (He-Ⅰ). The thermal switch in the 1.65 K mode can reduce the precooling time by 11.6 h and saved 22.5 L of helium. The transforming of the two modes is controlled by four threaded rods which are connected to the first and the second stage flange. The electrical test unit is designed, including the breakdown strength test and the flashover characteristics test with the voltage from 0 to 100 kV, which realizes the coupling of low temperature and high voltage.

Sealing performance under cryogenic temperatures should be a consideration in the development of a cryogenic valve for use in cryogenic engineering. In the present work, a cryogen-free leak detection system at 20 K to conduct leakage testing of cryogenic valves was developed. The experimental system was composed of a cooling system, a leak detection system, and a PLC (Programmable Logic Controller) system. Compared with the traditional cryogenic leak detection systems, this system using the GM cryocooler instead of cryogen is beneficial for the application for its low operating cost, simple structure, and safe handling. Five valves can be installed simultaneously and tested in sequence. The helium mass spectrometer leak detector is used for its sensitive response and high precision. The reliability of this system was verified by testing the leak rate of the system itself. Three high-pressure cryogenic solenoid valves were tested using helium gas under the pressure varied from approximately 2.5 bar to 80 bar and at both room temperature (300 K) and liquid hydrogen temperature (20 K). The tested valves can be cooled from room temperature to 18 K within 6 h. The test results of the internal leakage rate and the external leakage rate of the samples were discussed. This system will aid in the inspection of many critical cryogenic components such as liquid oxygen and liquid hydrogen valve assemblies for which leakage may lead to serious consequences.

The Joint Special Design Team for Fusion DEMO was organized in 2015 to enhance Japan’s DEMO design activity and coordinate relevant research and development (R&D) toward DEMO. This paper presents t...

ITER is in the integration phase and a fusion DEMO is being designed conceptually. The size of the DEMO will be around 1.5 time larger than the ITER and the weight will be about three times larger than the ITER. The TF coil and the vacuum vessel of the DEMO will become very heavy and extremely large. Although the central support structure will be used in the ITER for the assembly of TF coils and vacuum vessels, the central cylinder concept is proposed in this study to avoid the huge concentrated load at the center of the device floor during assembly. In addition, the welding joint between the vacuum vessels is discussed under the limited conditions supposed on the DEMO. Since the vacuum vessel will become the nuclear boundary, all weld lines will be expected to be examined by ultrasonic testing and/or radiographic testing to ensure the soundness of the welds.

A Nb3Sn wire which was manufactured for the ITER toroidal field coil conductor by a bronze route process was prepared for this study to investigate the effect of neutron irradiation on the critical current in a high magnetic field. The critical current of the virgin wire was measured in liquid helium with a 28 T hybrid superconducting magnet at the High Field Laboratory for Superconducting Materials in Tohoku University. It was also measured in vacuum with a heat conduction type variable temperature insert (VTI) at the International Research Center for Nuclear Materials Science at Tohoku University. The wire was irradiated at below 100 °C by fission neutrons at up to 4.9 × 1022 neutrons m−2 (>0.1 MeV) at BR2 in Belgium, and the critical current after the irradiation was evaluated with a VTI in the range of 8–15.5 T. The difference of the critical current measured with two facilities was discussed, focussing on Joule heating of the sample holder which was made of pure copper, and the neutron irradiation effect on the critical current was investigated in the range of up to 15.5 T. The results show that the critical current measured in vacuum becomes lower than that in liquid helium because of the temperature rise of the sample holder where the sample was soldered, the critical current was increased by the neutron irradiation, and the current ratio (IC/IC0) was almost constant of 1.75 in the range of 8–15.5 T at around 4 K.

To perform post irradiation tests of superconducting strands, a 15.5 T superconducting magnet and a variable temperature insert (VTI) were installed at a radiation control area in Oarai center, Institute for Materials Research, Tohoku University. Both of the 15.5 T magnet and the VTI are conductively cooled with GM refrigeration. The commissioning test of the system is still ongoing because unexpected troubles occurred during the commissioning. Also, a SQUID system with the maximum field of 7 T has been installed at another radiation control area to investigate the magnetic property of uranium and its isotopes. These devices are very useful to study the electro-magnetic properties of the neutron irradiated superconducting strands. This paper will introduce the new superconducting test facility, and some data recently obtained will be presented together with the data of magnetization evaluated with the SQUID, and the discussion on the irradiation effect on superconducting properties will be performed.

For developing the fatigue life evaluation method using small specimen, the effect of specimen size and shape on the fatigue life of the reduced activation ferritic/martensitic steels (F82H-IEA, F82H-BA07 and JLF-1) was investigated by the fatigue test at room temperature in air using round-bar and hourglass specimens with various specimen sizes (test section diameter: 0.85–10 mm). The round-bar specimen showed no specimen size and no specimen shape effects on the fatigue life, whereas the hourglass specimen showed no specimen size effect and obvious specimen shape effect on it. The shorter fatigue life of the hourglass specimen observed under low strain ranges could be attributed to the shorter micro-crack initiation life induced by the stress concentration dependent on the specimen shape. On the basis of this study, the small round-bar specimen was an acceptable candidate for evaluating the fatigue life using small specimen.

Creep resistance is the major concern of vanadium and its alloys for fusion reactor structural applications. In order to elucidate the effects of grain size on the creep behavior of solution and dispersion strengthened vanadium alloys, V–1.6Y–8W–0.8TiC specimens with fine grain sizes from 0.58 to 1.45 μm were prepared by mechanical alloying and HIP without any plastic working and tested at 1073 K and 250 MPa in vacuum. It is shown that the creep resistance of V–1.6Y–8W–0.8TiC depends strongly on grain size and increases with increasing grain size: The creep life for the grain size of 1.45 μm is almost one order longer than that of 0.58 μm, and about two orders longer than that of V–4Cr–4Ti (NIFS-Heat 2) although the grain size of V–4Cr–4Ti is as large as 17.8 μm. The observed creep behavior is discussed in terms of grain size effects on dislocation glide and grain boundary sliding.

A large scale fusion device like ITER requires high performance electric insulation materials against gamma ray and neutron irradiation, since fusion neutrons will reach superconducting magnets and activate them. Cyanate ester resin has been studied as a strong candidate for a new insulation material for fusion devices, and it has been clarified that the blended resin with epoxy has a potential to survive a design period in the radiation environment. This paper describes the results of molecular structure analysis of the blended resin, heat flux measurements by differential scanning calorimetry and the gamma ray and the fission neutron irradiation effects on interlaminar shear strength (ILSS) at 77 K. The gamma ray irradiation of 10 MGy did not change the ILSS significantly, but the neutron fluence of 1.0 × 1022 n/m2 (>0.1 MeV) with over 400 MGy degraded the ILSS. It suggests that cyanurate (triagine ring) will have a resistance against irradiation but oxazolidinon will lose the resistance after heavy irradiation.

A fusion reactor will yield a lot of high energy neutrons, and some of them will stream out of a plasma vacuum vessel and penetrate a blanket system and reach superconducting magnets which provide high magnetic field to confine high energy ionized particles. Under the neutron irradiation, the magnet materials will be activated and the properties will change. In this study, a research network on study of the irradiation effects is introduced and some data recently obtained are presented. By neutron irradiation, the critical current of the Nb3Sn wire increased and the critical field did not change. The organic insulation materials were degraded at neutron fluence of 1 × 1022 n/m2. In addition, the outline of an installation plan of 15.5 T superconducting magnet into radiation control area is described and study issues are explained. The project is undergoing as a program of Nuclear Basic Infrastructure Strategic Study Initiative.

Nanostructured vanadium (V) alloys are expected to exhibit high performance under neutron irradiation environments. However, their ultra-fine or refined grains cause significant decrease in flow stress at high temperatures due to grain boundary sliding (GBS), which is the major concern for their high-temperature structural applications such as future fusion reactors. The contribution of GBS to plastic deformation is known to depend strongly on grain size (GS) and may give more significant influence on long-time creep test results than on short-time tensile test results. In order to improve the creep resistance through elucidation of the effect of GS on the uniaxial creep behavior of nanostructured V alloys, a solution and dispersion hardened V alloy, V–1.4Y–7W–9Mo–0.7TiC (in wt%), with GSs from 0.58 to 2.16 μm was developed by mechanical alloying and HIP processes, followed by annealing at 1473–1773 K, and creep tested at 1073 K and 250 MPa in vacuum. It is shown that the creep resistance of V–1.4Y–7W–9Mo–0.7TiC increases monotonically with GS: The creep life for the alloy with 2.16 μm in GS is as long as 114 h, which is longer by factors of 2–30 than those for the other finer grained alloys and by two orders than that for coarse-grained V–4Cr–4Ti (Nifs heat2, GS: 17.8 μm) that is a primary candidate material for fusion reactor structural applications. The minimum (steady state) creep rate decreases with increasing GS as έ s ∝ (1/ l ) 3 , where έ s is the steady state creep rate and l is the grain diameter. The observed superior creep resistance of V–1.4Y–7W–9Mo–0.7TiC is discussed in terms of GS effects on dislocation glide/climb, GBS, and strain hardening capability enhanced by solution and dispersion hardening.

V3Ga compound has a shorter radioactive decay time compared with Nb-based compounds and it will be one of the candidate superconducting magnet materials for advanced low activation fusion reactor systems including shutdown scenario and material recycle. Recently, we succeeded in developing new V3Ga multifilamentary wires fabricated via the Powder In-Tube (PIT) process using high Ga content Cu-Ga binary phase compounds. We also found that high Ga content in the precursor promoted the formation of the stoichiometric V3Ga phase and it was effective in improving superconducting properties.

The Japan Materials Testing Reactor (JMTR) of the Japan Atomic Energy Agency is a testing reactor with first criticality in March 1968. The reactor has been utilized for various neutron irradiation tests on nuclear fuels and materials, as well as for radioisotope production. The operation of JMTR stopped temporarily in August 2006 for refurbishment and improvement. The renewed JMTR will resume operation in Japanese fiscal year 2011. For the resumption of the new JMTR, the new irradiation facilities, the usability improvements, the target, and the expected roles of the new JMTR have been discussed. As one of the new irradiation facilities, the cryogenic irradiation facility, which is used for the investigation on the low-temperature irradiation behavior of materials such as superconducting magnet materials for fusion reactors, has been desired. In this study, the feasibility of low-temperature irradiation tests with the cryogenic irradiation facility was investigated, and the conceptual design of the facility was carried out. As a result, it was found that irradiation tests at temperatures below 20 K for the development of the superconducting magnet materials can be realized by the installation of an irradiation capsule into an irradiation hole with low γ heating and by the adoption of vacuum jacketed tubes to connect between the capsule and a cooling system.

A fusion reactor generates a lot of 14 MeV neutrons, some of which penetrate shielding blankets, stream out of ports and reach superconducting magnets. Some important studies were performed in the 1970s and a basic understanding of the mechanisms of neutron irradiation effect was established. Advances in the design concept of nuclear fusion reactors led to the need for consistent studies on the neutron irradiation effect of A-15 strands such as Nb3Sn and Nb3Al, which are strong candidates for fusion reactors. In the early 2000s, a progressive attempt to organize the collaborative research of universities and national institutes was started using a 14 MeV neutron source at Japan Atomic Energy Agency. This paper outlines the neutron irradiation issues related to superconducting magnets for fusion, and a brief history of research on the neutron irradiation effect is provided. In addition, experimental results regarding changes in the superconducting properties of Nb3Sn and Nb3Al strands by neutron irradiation obtained in the newly established collaborative framework are presented, and general mechanisms for the property changes are introduced.

The effect of specimen size and shape on the fatigue life of reduced activation ferritic/martensitic steel “F82H IEA-heat” was investigated to develop a fatigue life evaluation method using a small specimen for testing neutron-irradiated materials. The fatigue test was carried out at room temperature in air using three kinds of round-bar specimen (minimum diameter cross sections of 1, 4, and 7 mm) and one hourglass specimen (minimum diameter cross section of 1.25 mm). The effect of specimen size on fatigue life from a level of 103 to 104 cycles was almost negligible for the round-bar specimens. The shorter fatigue life at the total strain range below 0.6% and the slightly longer fatigue life at the total strain range above 0.8% of the hourglass specimen relative to that of the standard specimen were observed from a level of 102 to 104 cycles.

We measured mechanical loss in an AC magnetic field for a short Bi-2223 superconducting tape. Experimental results indicated that mechanical loss decreased as tension applied to the sample tape increased. Furthermore, when a sample holder with a negative coefficient of thermal expansion was used, mechanical loss reduced. The origin of mechanical loss was confirmed to be the vibration of the sample tape. The expansion holder was the most effective means to reduce the vibration.

A fusion reactor is a human dream aiming to realize a terrestrial sun. The ITER project has been running for over ten years to create D-T burning plasma. Since a fusion DEMO design started within a framework of all-Japan system newly established in Japan, the construction site of the fusion DEMO must be investigated based on the discussions which have been performed for a long time in wide engineering fields. Some of the requirements to the site selection are discussed and summarized. The discussions are mainly focused on infrastructure which will be required to the construction site and very long timespan has been imaged from the construction to the site close after the D-T burning plasma operations. The plasma parameters and the in-vessel systems like the blanket and the diverter will not be discussed and the reactor type, such as Tokamak or Helical, is not concerned severely.

The effect of specimen shape on the low cycle fatigue life of reduced activationferritic/martensitic steel, F82H IEA-heat, was investigated to develop a reliable smallspecimen test technique (SSTT) for fatigue. The low cycle fatigue test was carried out at room temperature in air at a total strain range of 0.8–3.0%. One round-bar-type and three kinds of hourglass-type miniature fatigue specimens were used. The relationship between total strain range (δet) and number of cycles to failure (Nf) in one of the hourglasstype specimens, which is the current standard miniature fatigue specimen in Japan, was represented by the Manson-Coffin-type regression formula (δϵt = 376N 0.84 f + 2.16N −0.16 f ). The effect of specimen shape on the lowcycle fatigue life and fracture mode was almost negligible among the four specimen types, and their fatigue lives were almost the same. Similar stress distributions at the minimum cross sections of these specimens were shown above the total strain range of 0.8% by elasto-plastic...

Some technical issues still remain in the manufacturing process of the ITER Toroidal Field (TF) coil. Since the impregnation and curing of its insulation system is one of the issues, the authors performed trial impregnation using Cyanate Ester (CE) resin, which is a new candidate among resins because of its excellent resistance to radiation. The authors also studied the feasibility of curing the Winding Pack (WP). A previous study by the authors showed that a void, which may degrade insulation performance, may be generated as a result of an excessively brief impregnation, which was determined by pot life, namely the time required for viscosity to reach 100 mPas. Since the pot life of the improved CE resin was measured to be 200 h, the insulation system is allowed to be impregnated more slowly. Therefore, in an impregnation test, a 0.6 m sample was impregnated more slowly using CE resin than in previous studies, resulting in no voids in the insulation layers. In addition, calculations by the authors indicate that the required curing temperature tolerance of ±5 °C is difficult to achieve through the conventional method of using external heaters because of the heavy mass of the WP. However, it is shown that this demanding tolerance margin is achievable using the conductor as an additional heater. These results suggest that it is time for full qualification testing of the impregnation and curing process in a dummy double-pancake trial.

On the progress of the Deuterium–Deuterium (D–D) or Deuterium–Tritium (D–T) burning plasma devices, the importance of neutron irradiation on superconducting magnet materials increases and the data base is desired to design the next generation devices. To carry out the investigations on the effect of neutron irradiation, neutron irradiation fields are required together with post-irradiation test facilities. In these several years, a collaboration network of neutron irradiation effect on superconducting magnet materials has been constructed. 14 MeV neutron irradiation was carried out at Fusion Neutronics Sources (FNS) in Japan Atomic Energy Agency (JAEA) and fission neutron irradiation was performed at JRR-3 in JAEA. After the irradiation, the Nb3Sn, NbTi and Nb3Al samples were sent to High Field Laboratory for Superconducting Materials (HFLSM) in Tohoku University and the superconducting properties were evaluated with 28 T hybrid magnet. Also, the organic insulation materials are considered to be weaker than superconducting materials against neutron irradiation and cyanate ester resin composite was fabricated and tested at the fission reactor. One clear result on Nb3Sn was the property change of Nb3Sn by 14 MeV neutron irradiation over 13 T. The critical current was increased by 1.4 times around 13 T but the increment of the critical current became almost zero at higher magnetic fields and the critical magnetic field of the irradiated sample showed almost the same as non-irradiated one.

We have reconsidered V3Ga compound wire for future fusion reactor application. 14 MeV fast neutrons will be formed during the deuterium-tritium fusion reaction, consequently future fusion reactors will be constructed using low activation materials wherever possible. V3Ga compound has a short radioactive decay time and it will be one of the candidate superconductors with both low activation and high field properties so called ldquolow activation superconducting materialsrdquo. The present critical current density, J c, is insufficient for large fusion magnet. We investigated a new PIT process, using high Ga content compound, in order to improve superconducting properties. The upper critical field, H c2, of the samples using high Ga content Cu-Ga compounds, was increased with increasing Ga content, and measured 22.5 T when was used 50 at%Ga compound powder. Moreover, the effect of Mg addition into the powder filament was also investigated. The small amount of Mg addition within 1 at%, was improved H c2 to about 23.0 T. The Ga content dependence and the Mg addition effect on the superconducting properties in the new PIT process are reported in this paper.

Low cycle fatigue (LCF) properties of JLF-1 steel were studied from room temperature (RT) to 873 K in a vacuum condition using engineering size cylinder specimens with 8 mm in diameter. When the fatigue life was plotted against the plastic strain range, the fatigue life curves at RT, 673 K and 873 K were on different lines, which is not in agreement with the Coffin’s model. The TEM images showed that dislocation rearrangements forming cell structure and keeping high density at room temperature. But dislocation cell structure was not observed at 873 K, dislocation density decrease to low level. Loss of dislocation pile up will result in reduction of strain hardening at high temperature. So, the loss of strain hardening will be responsible for the increase of fatigue life at high temperature when plotted against the plastic strain range.

NbTi cable-in-conduit-conductors (CICC's) are employed to fabricate plasma fusion experimental apparatuses. NbTi is a preferable material in terms of low cost and good manufacturability. In contrast, it shows lower stability compared to materials for high magnetic field applications, e. g. Nb3Sn, due to its small critical temperature. A characteristic feature in NbTi multi-strand cable is so called avalanche-like quench, which is specific instability in a parallel electric circuit consisting of superconducting lines. To evaluate stability of a NbTi cable, we employed induced-current methods. Current is increased and decreased by constant voltage source, i. e. magnetic flux change produced by the bias magnet. We evaluated the quenches and equilibrium condition between heat generation due to flux flow voltage and heat transfer to surroundings at critical current. A parameter of experiment was a ramp rate of the bias field that corresponds to electric field in the cable. Of course, the cable was stable with smaller electric field and/or critical current. To survey the influence of heat dissipation during this equilibrium, we changed the surrounding mediums; those are liquid helium, frozen ethanol and ice. Ice is the material with large thermal conductivity, therefore this results in good stability of the cable.

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.

In order to clarify the limitation of bending strain in conductor to fabricate the toroidal field coils of DEMO reactor by react-and-wind (R&W) method, the effect of bending strain on the critical currents (Ic) of Nb3Al cable-in-conduit (CIC) conductor was investigated using the newly developed apparatus which can bend cooled sample from outside of cryostat. The Nb3Al strand manufactured by jelly roll process was 0.74 mm in diameter with the copper/non-copper ratio of 4.0. The CIC conductor sample which consisted of 54 Nb3Al strands and 27 copper wires inserted into a stainless steel conduit was 12.4 mm in diameter and 2.3 m in length. The CIC conductor was wound into a torsion coil spring shape with 4.5 turns and 150 mm in diameter. The bending strain and external magnetic field ranges for Ic measurement were from 0% to 0.6% and from 9.2 T to 11.3 T, respectively. It was clearly shown that the Ic decrease of the CIC conductor by bending is considerably low compared with that of strand, indicating the relaxation of intrinsic strain in Nb3Al filaments in CIC conductor due to the cabling effect. The Ic was not decreased up to 0.4% in bending strain and slightly decreased to 96% of initial Ic at 0.55% in bending strain. This indicates that the R&W method with bending strain of 0.6% is fully applied to the coil fabrication method. The furnace size for heat-treatment for R&W method becomes half of wind-and-react (W&R) method.

A fusion reactor will generate a lot of high energy neutron and much energy will be taken out of the neutrons by a blanket system. Since some neutrons will stream out of a plasma vacuum vessel through neutral beam injection ports and penetrate a blanket system, a superconducting magnet system, which provides high magnetic field to confirm high energy particles, will be irradiated by a certain amount of neutrons. By developing the new NBI system or by reducing the penetration, the neutron fluence to the superconducting magnet will be able to be reduced. However, it is not easy to achieve the lower streaming and penetration at the present. Therefore, investigations on irradiation behavior of superconducting magnet materials are desired and some novel researches have been performed from 1970s. In general, the critical current of the superconducting wire increases under fast neutron environment comparing with that of the non-irradiated wire, and then decreased to almost zero as an increase of neutron fluence. On the other hand, the critical temperature of the wire starts to get down around 1022 n/m2 of neutron fluence and the temperature margin will be decreased during the operation by the neutron irradiation. In this paper, some aspects of irradiated materials will be overviewed and general tendency will be discussed focussing on knock-on effect of fast neutron and long range ordering of A15 compounds.

In this paper, the low cycle fatigue properties of JLF-1 steel were studied from room temperature (RT) to 873 K in a vacuum condition using engineering size round bar specimens. The fatigue life at elevated temperature was almost the same as that at RT when the life was plotted against the total strain range. But when the life was plotted against the plastic strain range, the temperature effect on fatigue life exceeded the 95% confidence band. Based on TEM analysis, this tendency was caused by the interaction of dislocation changing with temperature, which will affect the possibility of crack initiation. Cyclic softening was observed in the fatigue test at elevated temperature. The cyclic stress–strain curve was obtained from the fatigue stress–strain hysteresis curves at around half life. The cyclic yield point was lower than the static one, especially at 873 K. Thus, cyclic softening is a design issue at elevated temperature, which reduces the design margin significantly.

Microstructural changed of JLF-1 steel induced by static tensile loading and tension/compression cyclic loading was investigated following tensile and low cycle fatigue tests in vacuum at elevated temperature. Secondary cracks in radial direction were observed on the fracture surface of the tensile specimen at RT. Initial cyclic hardening followed by cyclic softening was observed during fatigue at RT and 673 K. In contrast, only cyclic softening was observed at 873 K. Transmission electron microscopy images of specimens with a total strain range Delta epsilon(t), = 1.8% at RT, 673 K and 873 K showed that cyclic softening is caused by the reduction of the dislocation density and increment of lath width resulted by partial annihilation of original lath boundaries and cyclic deformation. The peak stress and lath width imply qualitative relationships that correlate with the number of cycles. (C) 2007 Published by Elsevier B.V.

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.

The advantages of MgB2 compound for a future fusion reactor application are not only high Tc (39 K) and low cost but also low activation material. MgB2 compound is non Nb-based superconductor and its decay time of induced radioactivity may be about 10 years and significantly shorter than Nb-based superconductors. In this paper, we fabricated the Mg2Cu doped MgB2 wires using several metal sheath materials to improve Jc property. The selection of metal sheath materials is also important to apply for fusion reactor. Pure Fe, Ta and Cu have faster decay time of induced radioactivity compared the other metals. Tc values of all Mg2Cu doped samples were higher than those of non-doping samples, and Tc values were improved with increasing amount of Cu doping. And the maximum core-Jc value was estimated to be 100 A/mm2 over at 4.2 K under 8 T for 3 at% Cu doping sample. The comparisons of superconducting property between different metal sheath materials are discussed.

V-based superconducting material has a shorter decay time of induced radioactivity and its mechanical properties are better than Nb-based materials. We thought that the V3Ga compound was one of the most promising materials in 'low activation and higher field superconductors' for an advanced fusion reactor. However, the present critical current density (Jc) property of the V3Ga compound wire is insufficient to apply to fusion magnet applications. We investigated a new route powder-in-tube (PIT) process using a high Ga content Cu–Ga compound in order to improve workability and the Jc property of the V3Ga compound wire. It was confirmed that a thicker reaction layer formed along the boundary of the Cu–Ga compound powder filament and V matrix. Critical transition temperature (Tc) values were shown to be almost 15 K, which was higher than conventional bronze processed samples. Comparisons of the transport Jc property and microstructure between different Ga content Cu–Ga compounds are investigated in this paper.

The advantages of the MgB2 compound are the high Tc value (39 K) and lower material cost than other superconducting metallic compounds. In addition, MgB2 wire has a shorter decay time for induced radioactive properties than Nb-based superconductors. However, the Jc value of MgB2 wire is poor compared with Nb-based superconductors, and further improvement of Jc is required if the compound is to be used in a future nuclear fusion reactor. In order to improve Jc, we investigated Cu addition using the Mg2Cu compound in MgB2/Fe-sheathed wires through the powder-in-tube (PIT) process. Tc values of Cu addition samples were higher than those of non-Cu addition samples and they were improved with increasing Cu addition. We obtained maximum core Jc values over 100 A mm−2 at 4.2 K under 8 T for 1 and 3 at.% Cu addition samples. Moreover, we found that Cu addition was more effective at improving the value of Jc in low and middle magnetic fields than the SiC addition process.

In this paper, the tensile and fatigue properties of JLF-1 steel were studied from room temperature to 873 K in vacuum with engineering size round bar specimens. Weak serration occurred in the stress–strain curve at elevated temperature, which is considered as the dynamic strain ageing effect. The serration pattern for the tensile specimens was different from that for the fatigue ones. This was due to the difference in the location of dislocation slip band relative to the position of beams of the gage. The fatigue life at elevated temperature was almost the same as that at RT when the life was plotted against the total strain range. Cyclic softening was observed in fatigue tests at elevated temperature.

Superconductors carrying currents larger than several 10 kA are required for magnetic confinement fusion systems. A cable-in-conduit-conductor (CICC), which has about 1000 of twisted thin superconducting strands in a ridged conduit, is a promising candidate for such large and high magnetic field superconducting system. A specific phenomenon in the CICC is so called avalanche-like quench (AQ). When some strands become resistive due to current saturation at critical current and/or a mechanical disturbance, the current re-distributes from the strands into neighboring ones, which are electro-magnetically coupled more strongly than the others. This causes current saturation in the neighboring strands. Finally, the whole cable becomes normal. We call this phenomenon an avalanche-like quench. Small multi-strand cables were tested by induced-current methods. Quench behaviors of both Nb3Sn and NbTi cables are observed and compared. In the experiment, for the Nb3Sn cable, the critical current measurement was stably carried out at high field without quench. In contrast, for the NbTi cable, we observed a number of AQs during inductive energization. Consequently, we suspected that the reason is based on the difference of critical temperature (TC) between Nb3Sn and NbTi. The difference of TC is several Kelvins, however, their macroscopic behaviors differ significantly.