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Measurement techniques have been developed to characterize form shell targets for the Fast Iginition Realization Experiment (FIREX) project. Interferometry has been adopted to determine the volume of hydrogen fuel in the form shell. A preliminary experiment has been conducted to evaluate the aplicability of interferometry. The paper describes the measurement techniques and discuesses its accuracies.

A way to make a hole in a resorcinol-formaldehyde (RF) foam shell target is described. A target used in fast ignition (FI) experiments consists of an RF foam shell and a guide cone. The fragile RF foam cannot accept conventional mechanical drilling or laser machining. We used the second harmonics of a Nd:YAG laser (532nm) to bore a hole for the cone by utilizing thermal decomposition of the RF foam.To optimize the cutting condition, the thermal characteristic and absorption spectrum of the RF foam were measured with a thermo-gravimetric meter and a spectrometer, respectively. Then, using these results, relationships between the laser power and cutting speed were optimized for RF foams with several densities.

Dy–Ba–Cu–O bulk superconductor has an excellent capability of trapping magnetic flux and lower heat conductivity at cryogenic temperatures as compared with Y–Ba–Cu–O bulk superconductor. The Young's modulus and the bending strength in the range from room temperature to 7 K were measured by the three-point bending tests using specimens cut from a melt-processed Dy–Ba–Cu–O bulk superconductor. They were tested in a helium gas flow type cryostat at Forschungszentrum Karlsruhe and in a liquid nitrogen bath at Iwate University. The Young's modulus was calculated by either the slope of stress–strain curve or that of the load–deflection curve of the specimen. Although the bending strength measured in the two institutes coincided well, there was a significant discrepancy in the Young's modulus. The Young's modulus and bending strength increased with decrease of temperature down to 7 K. The amount of increase in the Young's modulus and the bending strength were about 32% and 36% of those at room temperature, respectively. The scatter of data for each run was significant and did not depend on temperature. The temperature dependence of the Young's modulus coincided with that in Y–Ba–Cu–O obtained by ultrasonic velocity. The temperature dependence of the Young's modulus and the bending strength was discussed from the view point of interatomic distance of the bulk crystal.

Development of foam materials for cryogenically cooled fuel targets is described in this paper. The fabrication development was initiated as a part of the fast ignition realization experiment (FIREX) project at the ILE, Osaka University under a bilateral collaboration between Osaka University and National Institute for Fusion Science (NIFS). For the first stage of FIREX (FIREX-I), a foam cryogenic target was designed in which low-density foam shells with a conical light guide will be fuelled through a narrow pipe and will be cooled down to the cryogenic temperature. Acrylic polymer, resorcinol–formaldehyde (RF) resin, poly(4-methyl-1-pentene) (PMP), and polystyrene-based crosslinking polymer have been investigated as supporting materials for cryogenic fuel. The properties of the material and the present status of the material development are summarized.

In order to investigate the compressive mechanical properties of a single-grain Sm123 bulk superconductor at cryogenic temperature, compressive tests at liquid nitrogen temperature on small specimens with the dimensions of 3 × 3 × 8 mm 3 cut from the bulk with 45 mm in diameter and 15 mm in thickness were carried out. Due to the closure of pre-existing micro-cracks perpendicular to the c -axis, the average Young’s modulus in the c -axis, 93 GPa, was lower than that in the direction perpendicular to the c -axis, 150 GPa. While the former was higher than the value at room temperature (RT), 75 GPa, the latter was slightly lower than that, 165 GPa. The compressive strength in the c -axis, 466 MPa, was higher than that in the direction perpendicular to the c -axis, 368 MPa. This anisotropy is also ascribed to the pre-existing micro-cracks associated with the fracture mode. Both the former and the latter were higher than the values at RT, 350 and 335 MPa, respectively. Most of the specimens loaded in perpendicular to the c -axis fractured along the plane perpendicular to the c -axis into a few pieces. The specimen loaded in the c -axis fractured into many small fragments, although the specimen with extraordinarily low strength fractured in a similar mode as mentioned above.

Tensile and 3-point bending tests of specimens cut from an Y123 and a Gd123 bulk superconductors fabricated by using the modified QMG (Quench and Melt Growth) process were carried out at room temperature. The mechanical properties obtained by two testing method were compared. The average values of the Young's modulus obtained by tensile tests, 122 GPa for Y123 and 109 GPa for Gd123, were lower than those obtained by bending tests, 132 and 117 GPa, respectively. The average values of the tensile strength, 39 MPa for Y123 and 42 MPa for Gd123, were also lower than those of the bending strength, 74 MPa and 71 MPa, respectively. There was no appreciable difference between the Weibull coefficient for the tensile strength and that for the bending strength, 6.9, in Gd123. On the other hand, the coefficient for the bending strength, 17.6, was significantly higher than that for the tensile strength, 6.8, in Y123.

The stress/strain dependence of critical current and the mechanical properties of nickel sheathed PIT MgB2 mono-core tapes with various kinds of metal powders addition to the core were evaluated at 4.2 K in the magnetic field of 2 T. No marked change in the stress-strain curve was found by the powder addition. The critical current was markedly enhanced by In and Sn powder addition. The critical current was almost constant irrespective of strain up to the irreversible strain where a steep degradation of critical current begins. The irreversible strain for the tape increased from 0.25 to 0.50% by the addition of In and Sn powder. The In addition also suppressed a slight increase in the critical current with strain. The effects of Ag and Cu addition are less as compared with those of the low melting point metal powder such as In and Sn. SEM observations on the longitudinal section of In added tape after the tensile test clarified that the cracks were few and rather large as compared with those in the tapes with no addition or high melting point metal powder addition. The strain characteristics were compared with those of MgB2 with other sheath materials and those of a conventional Nb3Sn wire.

In the phase II experiment of the Large Helical Device (LHD) of the National Institute for Fusion Science (NIFS), it is planned to operate the helical coils at 1.8 K by employing pressurized superfluid cooling to raise the magnetic field to 4 T with 17.3 kA. It is important to develop a 20kA-class current feedthrough into the 1.8 K region, but it must have a high current capacity and low heat leakage in the maximum magnetic leakage field of 1 T. Rectangle-shaped YBCO bulk conductors measuring 20 mm wide, 140 mm long and 10 mm thick were manufactured from square-pillar-shaped YBCO bulk materials for a 20 kA current. To check the quality of the bulk conductors, internal defects or cracks were detected by carrying out a precise survey of trapped magnetic flux. An assembled 20 kA current feedthrough was mounted in the λ-plate of a pressurized superfluid cooling cryostat. Experiments of current feeding into the 1.8 K region were carried out by operating the 20 kA current feedthrough. In the experiments, the transport current was kept at 20 kA for longer than 1,200 s. During the 20 kA operation, the current transport section of the YBCO bulk conductors remained in the superconducting state and the voltage drop between the YBCO bulk conductors and the copper electrode was observed to be constant. A contact resistance and the Joule heat generation in the joint region between the YBCO bulk conductors and the copper electrode were obtained as 1.45 nΩ and 0.72 W, respectively in the 20 kA operation. We have demonstrated the feasibility of using a 20 kA current feedthrough for the phase II experiment of the LHD.

The electro-mechanical characteristics of the nickel sheathed powder-in-tube MgB 2 mono-core tapes with 10 vol% indium powder were evaluated. No marked change in the stress–strain curve was found by indium addition. On the other hand, the irreversible strain for the tape where a drastic degradation of critical current I c begins increased from 0.25% to 0.50% by the addition of indium. The indium addition also suppressed the slight increase in I c with strain. These characteristics of I c with strain were compared with those reported by other researchers. Scanning electron microscope observations of the longitudinal section of degraded samples revealed that the number of micro-cracks was appreciably smaller in the indium added tapes.

The heat transfer from a metal to He II is determined by Kapitza conductance at the interface. Surface temperature estimation of the metal is used for the study of Kapitza conductance. The surface temperature is usually extrapolated from the temperature gradient in the metal. In the case of the metal with a coated layer, however, it is difficult to estimate the surface temperature. A similar case is applied for the superconductor of the helical coil of the large helical device (LHD). The copper surface is chemically treated by oxidation in order to improve the heat transfer characteristics in He I. It is planned that the helical coil will be cooled by He II in the phase II upgrade. The Kapitza conductance of the conductor has to be estimated for a stability analysis. Therefore, the heat transfer from the oxidized copper surface to the saturated He II has been measured using an oxygen-free copper cylinder. To investigate the thermal resistance in the oxidized layer, two coating surfaces are prepared: (a) coated with Stycast; (b) coated with Stycast on the oxidation layer. In this paper, the Kapitza conductance of the chemically oxidized copper surface is discussed.

A grant-in-aid project on the database development for He II-cooled superconducting magnet system design has been going on as a 3-year project. Many big projects on He II cooled superconducting magnets and cavities have been planned and are under construction around the world. He II systems consist of many specific technologies such as helium heat transfer coefficients under pressurized and saturated superfluid helium conditions, cooling characteristics in micro channels, Kapitza resistance between the surface of heat exchanger, helium pumping techniques, in situ measurement of temperature, pressure under high magnetic fields, and so on. The four organizations including university and national institutes started their collaborative research on this project to develop a convenient and useful database system. Two activities are going on: The first is to pick up the related papers already published in journals and proceedings in cryogenics and superconductivity fields from 1970. The second is to carry out the original experimental researches on He II technologies to obtain new data and to obtain more accurate and reliable data. As for the first sources, we have already picked up about 1, 000 papers and have carried out several experiments. This paper describes the present situation on this research project on the He II database and future plan.

The first cool-down test of the Large Helical Device (LHD) and the performance of the LHD cryogenic system during the first cycle operation are described. The first cool-down started on Feb. 23, 1998 and finished on Mar. 22. After the cool-down, the excitation tests of the SC coils up to 1.5 T and the first cycle operations for plasma physics experiments were conducted until May 18. The first cycle operation was successfully completed after the warm-up process to room temperature from May 19 to Jun. 15. The cooling characteristics of the LHD, such as temperature distribution during cool-down, heat loads under steady state condition, reliability during long-term operation, are reported.

Stability tests have been carried out on short samples of the aluminum/copper stabilized composite-type superconductors developed and used for the pool-cooled helical coils of the Large Helical Device. The waveform of the longitudinal voltage initiated by resistive heaters shows a short-time rise before reaching a final value, which seems to correspond to the diffusion process of transport current into the pure aluminum stabilizer. The propagation velocity has a finite value even for the transport current being lower than the recovery current, and it differs depending on the direction with respect to the transport current.

Extra AC losses were observed during the Experiments on a Single Inner Vertical coil (EXISV). The Inner Vertical (IV) coils are the smallest poloidal coils for the Large Helical Device (LHD) and their inner and outer diameters are 3.2 m and 4.2 m, respectively. The coil consists of 16 pancake coils wound with cable-in-conduit conductor (CICC) whose strands are NbTi/Cu without any surface coating. Many causes for the extra AC losses were considered, such as the decrease of a contact resistance between strands due to the large electromagnetic force in the conductor or due to the stress during the coil winding process, etc. and possibilities were investigated from the experimental data. Finally, we found that a coupling current with a very long time constant of 124 s caused the AC loss increase. The coupling current with such a long time constant cannot be explained from the symmetric twisting configuration of the CICC but can be explained as a local loop current corresponding to a cyclic change of the non-uniform current distributions in the cable. The non-uniform current distribution could be induced by an asymmetry of the strand transposition in the cable. To verify the above reasoning, we did fundamental experiments on a two-strands-cable, which has an intended asymmetry in the cable twisting. Extra AC losses were also observed for an asymmetric two-strands-cable, and it was demonstrated that the non-uniform current distribution causes an increase of AC losses.

For the purpose of the stability analysis of a pool boiling superconductor, many studies on the heat transfer of LHe have been conducted. There are a number of variables that affect heat transfer. It is well-known that to variables are surface orientation and surface treatment. Surface orientation of a superconductor is varied by magnet winding. The change is associated with the variation of gravitational force on the surface, thus causing heat transfer characteristics to change. Usually, the surface of a superconductor is treated to improve heat-transfer characteristics; for example, oxidation. During the winding process, the winding machine may strip off the treatment at some locations. The resulting damage may change the heat-transfer characteristics and degrade the stability of the superconducting magnet. In this study, the heat transfer of polished Cu, oxidized Cu and partially oxidized surfaces were measured as a function of orientation. The critical and minimum heat fluxes depend on the area fraction of oxidation. The calculation method for the heat flux of a partially oxidized surface was established. Heat-transfer measurements of a prototype superconductor with polished surfaces were also conducted to change the surface orientation. The heat-transfer characteristics with the prototype superconductor were degraded as compared with those of the Cu surface. It became clear that heat transfer for a stability analysis must be measured using a prototype superconductor. Next, the recovery current for a large-sized pool boiling superconductor, which is a helical coil superconductor, was calculated according to the experimental results of heat transfer. The dependence of the recovery current on surface orientation and the area fraction of surface treatment was estimated. The calculated results were compared with the measured recovery currents of the short sample. The calculated recovery current agreed well with the experimental result.

A triplet NbTi/Cu superconductor, high current density superconducting strands for a cable-inconduit conductor (CICC), was used to study the influence of current distribution on stability within the triplet. The initial current distribution among superconducting strands was controlled in terms of a resistive heater wound on the exterior of each strand. The minimum quench energies (MQE) were measured with different initial current distributions within the triplet. We experimentally confirmed that MQE for the triplet was not less than three times MQE for a strand when the transport current of the strand was larger than its minimum propagating current. In addition, MQE changed with the difference of initial current distribution among the strands. This reason is discussed from the viewpoint of the ratio of the triplet's total transport and critical currents, and the difference between the transport current of heated strand and its minimum propagating current.

Preliminary tests on the cryogenic target for the fast ignition realization experiment (FIREX) project has been conducted. A foam shell method is proposed to realize its target design. A foam target consists of three parts: a foam shell, a conical laser guide and a liquid or gas feeder made of glass. The shell is a hollow sphere (500 μm in diameter) with a uniform and thin foam layer (∼ 20 μm in thickness). Epoxy resin is utilized to assemble the parts into the target. Regarding target fabrication, one of the concerns is the influence on various thermal contractions from the different materials when it is cooled down to cryogenic environment. This paper describes the result on the validity check at cryogenic environment and the demonstration of H 2 liquefaction using a dummy target instead of the foam target.

The time constants of coupling losses have been evaluated for the composite-type aluminum stabilized superconducting cables used for the pool-cooled helical coils of the Large Helical Device (LHD). A new method using Hall probes was incorporated to measure the local field change generated by the screening currents in the conductors. During an exponential discharge of the external magnetic field, the current decay was found to be well fitted according to a simple magnetic diffusion equation with a geometrical shape factor. Measurements in the magnetic fields both perpendicular and parallel to the wide surface of the flat cable were performed. The shielding effect of the pure aluminum stabilizer was confirmed by removing it from the cable, which then revealed a longer time constant.

The AC losses of the Inner Vertical (IV) coil have been measured during the Experiments on a Single Inner Vertical coil (EXSIV). The IV coils are the smallest poloidal coils of the Large Helical Device (LHD) and their inner and outer diameters are 3.2 m and 4.2 m, respectively. The coil consists of 16 pancake coils wound with the cable-in-conduit conductors (CICC) whose strands are NbTi/Cu without any surface coating. The AC losses were measured with a calorimetric method during excitation tests and current shut-off tests. In addition to the usual inter-strand coupling losses with short time constants, unexpected coupling losses were observed due to the coupling current of a long time constant.

Composite-type superconductors with NbTi/Cu compacted strands and aluminum/copper stabilizers have been developed for use in the pool-cooled helical coils of the Large Helical Device (LHD). Topics related to the development, fabrication, short sample tests and soldered lap joint of these superconductors are summarized. Evaluation of AC loss has been made based on two measurement techniques. Heat transfer coefficients from the conductor surface to liquid helium have also been measured for the purpose of stability analysis.

One of the most advanced fast ignition programmes is the fast ignition realization experiment (FIREX). The goal of its first phase is to demonstrate ignition temperature of 5 keV, followed by the second phase to demonstrate ignition-and-burn. The second series experiment of FIREX-I, from late 2010 to early 2011, has demonstrated a high (>10%) coupling efficiency from laser to thermal energy of the compressed core, suggesting that the ignition temperature can be achieved at laser energy below 10 kJ. Further improvement of the coupling efficiency is expected by introducing laser-driven magnetic fields. © 2013 IAEA, Vienna.

Heat transfer characteristics of a copper plate (18 mm × 76 mm) to liquid helium have been measured by changing the orientation and treatment of the heat transfer surface. The treated heat transfer surfaces were (a) the polished Cu surface, (b) the oxidized Cu surface and (c), (d) the Cu surface oxidized 50% of area. We observed that the critical heat flux depends upon the heat transfer surface orientation, and the critical heat flux has its maximum between 0 ° and 30 ° of the surface orientation, where the angle is taken from the horizontal axis. On the other hand, the minimum heat flux has less dependence on the heat surface orientation. The critical heat fluxes of the 50% oxidized surfaces almost agree with that of the polished Cu surface. The minimum heat fluxes of the 50% oxidized Cu surfaces approximately agree with that of the 100% oxidized Cu surface.

The degradation of the inter-laminar shear strength (ILSS) of GFRPs was evaluated after electron irradiation at 77K and reactor irradiation at 20K. The GFRPs used in this study were prepared by varying matrix resins. Optical microscopic observation of the fracture surface was carried out to reveal the degradation behavior of the ILSS. The exposed fiber area of the fracture surface was found to depend on the absorbed dose. This suggests that a change in ILSS is induced by the interface failure between fiber and matrix as a result of the change in matrix resin.

Mechanical design of an experimental apparatus for FIREX cryo-target cooling is described. Gaseous helium (GHe) sealing system at a cryogenic environment is an important issue for laser fusion experiments. The dedicated loading system was designed for a metal gasket. We take U-TIGHTSEAL® (Usui Kokusai Sangyo Kaisha. Ltd.) with an indium plated copper jacket as an example. According to its specification, a linear load of 110 N/m along its circumference is the optimum compression; however a lower load would still maintain helium (He) leak below the required level. Its sealing performance was investigated systematically. Our system demanded 27 N/mm of the load to keep He leak tightness in a cryogenic environment. Once leak tightness was obtained, it could be reduced to 9.5 N/mm.

FIREX targets have been developed under two layering strategies: foam shell and cone guide laser heating methods. Basic studies have been conducted by the collaboration research between ILE and NIFS. Then the next stage requires the characterization of a layered solid fuel. The present system is at the disadvantage of optical observations. Therefore, a new apparatus is designed to solve it. Glass windows with a wide aperture are installed for an interferometer and a microscope. To isolate the vibration from a cryocooler, active vibration control units are equipped, and flexible thermal conductive links are utilized. Furthermore, a quick target exchange mechanism is applied to deal with different types of FIREX targets. A target holder is detachable from a main vacuum chamber. A metal gasket with not fixing bolts but a load of ~ thousand newtons on ensures GHe leak tightness for target cooling. Eventually, the design temperature of 10.00 K at a target container has been achieved. The cool-down performance indecates that the new apparatus provides a cryogenic environment for fuel layering demonstrations.

Heat transfer characteristics of a copper plate (18 mm × 76 mm) to liquid heliumhave been measured by changing the heat surface orientation and the channel width of liquid helium. We observed that the critical heat flux depends on the heat surface orientation, and the critical heat flux has the maximum at the angle of 15° – 30°, where the angle is taken from the horizontal axis. On the other hand, the minimum heat flux has less dependence on the heat surface orientation. The channel tends to degrade the heat transfer characteristics compared with the open surface at all surface orientations, especially at 0°.

To investigate the relationship between former strain and stability of a superconducting solenoid, a test coil was constructed and was applied to a training test. In this test, time dependences of the former circumferential strain eθ and quench currents Iq were measured. Iq decreased at the next quenches, when the residual compressive strain eθR of the former occurred at the previous quench. Especially when the coil emerged from boiling helium at a quench, the absolute value of eθR increased, and Iq remarkably decreased at the next quench. These occurrences of eθR could be ascribed to fast change of the Lorents force and thermal condition at the quenches.

The principal Hugoniot for liquid hydrogen was obtained up to 55 GPa under laser-driven shock loading. Pressure and density of compressed hydrogen were determined by impedance-matching to a quartz standard. The shock temperature was independently measured from the brightness of the shock front. Hugoniot data of hydrogen provide a good benchmark to modern theories of condensed matter. The initial number density of liquid hydrogen is lower than that for liquid deuterium, and this results in shock compressed hydrogen having a higher compression and higher temperature than deuterium at the same shock pressure., Comment: 8 pages, 7 figures, 2 tables, accepted for publication in Physical Review B

Bronze processed multifilamentary Nb3Sn superconducting wires, with a CuNb reinforcing stabilizer instead of the conventional Cu stabilizer, were fabricated. The mechanical properties and the strain dependence of the critical current Ic were evaluated at 4.2 K and a magnetic field of 15 T. A remarkable increase in the yield stress (70%) and the plastic flow stress as compared to the values for the wire with Cu stabilizer was observed. The strain for the peak Ic was also increased by 0.2%. Ic on unloading was reversible within the strain range of 1.5%. The strain sensitivity of Ic in the CuNb/Nb3Sn wire was almost the same as that of the Cu/Nb3Sn wire. A decrease in the wire diameter from 0.8 to 0.5 mm resulted in a slight increase in the yield stress of the CuNb/Nb3Sn wire, but no change in the strain dependence of Ic. An increase in the heat treatment temperature from 700 to 750°C resulted in a decrease in the flow stress of 15%, but no change in the strain dependence of Ic. A marked change in the morphology of the Nb filament in the CuNb reinforcing stabilizer was evidenced during heat treatment.

Development of foam capsule fabrication for cryogenically cooled fuel targets is overviewed in the present paper. The fabrication development was initiated as a part of the Fast Ignition Realization Experiment (FIREX) project at the ILE, Osaka University in the way of bilateral collaboration between Osaka University and national institute for fusion science (NIFS). For the first stage of FIREX (FIREX-1), a foam cryogenic target was designed where low-density foam shells with a conical light guide will be cooled down to the cryogenic temperature and will be fueled through a narrow pipe. The required diameter and thickness of the capsule are 500 mum and 20 mum, respectively. The material should be low-density plastics foam. We have prepared such capsules using 1) new material of (phloroglucinolcarboxylic acid)/formalin resin to control kinematic viscosity of the precursor, 2) phase-transfer-catalyzed gelation process to keep density matching of three phases of the emulsion. 3) non-volatile silicone oil as outer oil of emulsion in order to prevent hazard halogenated hydrocarbon and flammable mineral oil. The obtained foam capsule had fine structure of 180 nm (outer surface) to 220 nm (inner surface) and uniform thickness reaching to resolution limit of optical analysis (~0.5 mum). A small hole was made before the solvent exchange and the drying process to prevent distortion due to volume changes. The density of dried foam was 0.29 g/cm . After attaching the petawatt laser guiding cone and fueling glass tube, poly([2,2]paracyclophane) was coated on the foam surface and supplied for a fueling test of cryogenic hydrogen.

In 1991, the superconducting magnet test facility had been constructed in National Institute for Fusion Science. The facility consists of a helium liquefier/refrigerator with the cooling capacity of 600 W at 4.5 K, large cryostats, a mechanical testing machine, DC power supplies with the maximum current of 75 kA and a distributed control system. So far, the development of the superconducting coils for the Large Helical Device had been conducted and many collaborative works have been carried out. However, the test facility with higher bias filed is needed to develop superconducting coils for fusion reactors and the cooling system, which can supply coolant of various temperature to test samples, is required to apply HTS to large-scale conductors. Therefore, the test facility will be upgraded to promote the development of superconducting coils for fusion reactors. The maximum bias field will be upgraded from 9 T to 15 T to investigate the performance of superconductors under the higher bias field and the existing helium liquefier/refrigerator will be replaced with a variable temperature one to test those under various temperature environment. Consequently, it is expected to progress the development of the superconductors for fusion reactors and the development of large-scale HTS conductors.

Fusion power plants need larger scale and higher field superconducting magnets than the ITER magnets. Therefore, higher current superconductors with high strength against strong electromagnetic forces are needed. In order to examine the superconducting properties of such large conductors in real conditions, we are preparing a new test facility that is equipped with a solenoid coil of the highest field of 13 T with the bore of 0.7 m, a pair of temperature-variable current leads, and a vacuum chamber for conductor samples. The highest field can be increased to 15 T by installing an additional coil with the cold bore of 0.6 m. Since the inlet temperature of the samples can be varied from 4.4 K to 50 K, it is possible to examine properties of advanced conductors at actual operating temperatures including high-temperature superconductors. We propose coil-shaped conductor samples instead of straight samples in order to apply electromagnetic hoop forces on the conductors to realize the real condition. A reference design of a sample for a cable-in-conduit conductor is provided.

The measurement technique of thermal impedance at an interface has been developed using the sample with a dummy thermal impedance. 304‐series stainless steel with different thicknesses was utilized as a test material and was attached to copper blocks by Stycast 2850FT which is for adhesive and for dummy thermal impedance. The apparent thermal conductivities of the samples were measured at the range from 15 K to 250 K. Any two thicknesses of them were compared, and the information on the thermal conductivity of stainless steel and on the thermal impedance of the Stycast layer were extracted. According to the results compared with that of other studies, it was confirmed that the comparison between the samples with different thicknesses was one of the useful techniques to obtain the information on the thermal impedance.

Stability tests have been carried out on the aluminum stabilized composite-type superconductors developed and used for the pool-cooled helical coils of the Large Helical Device (LHD). The longitudinal voltage of a normal zone shows a short-time rise before reaching a final value, which seems to be explained by the rather slow diffusion process of transport current into the pure aluminum stabilizer and the copper housing. The propagation velocity has a finite value even below the recovery current, and it differs depending on the direction of transport current. A numerical analysis dealing with transient thermal and electromagnetic relaxation processes in the aluminum stabilizer well explains the experimentally observed results.

Publisher Summary This chapter describes the liquefaction control method of 10 kW class cryogenic system. It also discusses the test results of the refrigeration power of each turbine. It demonstrates the automatic liquefaction operation, and measuresthe refrigeration power of each turbine are in good agreement with that of design values. The cryogenic system has an equivalent design capacity of 9.1 kW refrigeration at 4.4 K, and refrigerates all sets of superconducting coils, their supporting structures and superconducting bus-lines for the fusion experimental device Large Helical Device (LHD). The refrigerator is controlled by two sequential programs; for the start-up control of the eight compressors in sequence, and for the driving control of the seven turbines and the liquefaction control of the cryogenic system. The results of the refrigeration power of each turbine are––the setting-up time of all compressors with full load is about 1.5 hours, all turbines rotate stable and the rate of speed fluctuation is less than ±50 rps at the steady state, and the averaged refrigeration powers show good agreement with that of the designed values.

Publisher Summary The Large Helical Device (LHD) is a heliotron fusion experimental device in which magnetic fields are controlled exclusively by superconducting magnets: a pair of helical coils and three pairs of poloidal coils. A 10 kW class helium refrigerator has been constructed for LHD at National Institute for Fusion Science. During the last twelve months between 1995 and 1996, the refrigerator has been put to the training operations without connecting to any of the superconducting magnets of LHD. As one of its series report, this chapter reviews the paper on significant events and major observations experienced during that period and makes a brief discussion about several interesting topics on preliminary base. The observations revealed that there are two insufficient control system designs that forced the operator to pull the shut-down button. One is the external intervention to the engineering workstation through the integrated services digital network (ISDN) line between the manufacturer and the operator. The other is due to the insufficient computer program proof inspection before loading on the VME.

Voidless fuel solidification within a foam material is attempted for the development of a Fast Ignition Realization EXperiment (FIREX) target. A typical target consists of a foam shell with a thin solid fuel layer, a gold cone guide and a glass fill tube. The foam layer is formed with aggregations of tiny cells. The porous foam material has the advantage of forming a uniform layer in a liquid state by the effect of capillarity. Random liquid–solid transitions of fuel, however, would cause void spaces in each cell because of their different densities. To form a voidless solid fuel layer in the FIREX target, we propose a layering method, which includes the process of controlled solidification and simultaneous liquid fuel supply to the liquid/solid interface. Normal H2 is used as a surrogate fuel for our experiments. By applying a resorcinol/formalin aerogel, which has been developed in the Institute of Laser Engineering, Osaka University, instead of a foam material, formation of a solid H2 layer with reduced void spaces is preliminarily demonstrated. Related solid H2 refractive indices are measured to estimate the void fraction. Eventually, its filling factor reaches ~99%. Furthermore, the application of the proved method is numerically simulated on the FIREX target. The method is confirmed to be applicable in it with a cone guide heating technique.

Evaluations on the mechanical properties such as fracture strength and fracture toughness were carried out at 77 K for an RE-Ba-Cu-O (RE: rare-earth elements) large single-grain superconducting bulk 150 mm in diameter through bending tests for specimens cut from the bulk. The average fracture strength value of the large single-grain bulk at 77 K was slightly higher than that at room temperature. While an inner region of the bulk has pores, a region near the top surface of the bulk has few pores. The maximum fracture strength value was obtained for a specimen cut from the low-porosity region near the top surface. While the porosity of the large single-grain bulk and that of a smaller single-grain bulk are similar to each other, the average fracture strength value at 77 K of the former bulk was lower than that of the latter bulk. The lower fracture strength of the large single-grain bulk corresponded to the lower fracture toughness.

The collaboration research of ILE and NIFS has made progress on the FIREX target developments. We have two fuel layering strategies: a foam shell method and a conical laser guide heating technique. To date, preliminary and principle demonstration stages on both fuel layering methods have been reached. This paper describes the present status of the target developments.

The degradation of interlaminar shear strength (ILSS) of GFRPs has been evaluated after 77K electron and 20K reactor irradiation. The GFRPs used in this work were prepared using several matrix resins. Optical microscopic observations of the fracture surfaces were carried out to determine the degradation behavior of ILSS. The fiber exposed area of fracture (interfacial failure) surface was found to be larger with higher absorbed dose. It suggested that the degradation of ILSS was induced by the interface failure between fiber and matrix. The matrix resin was also found to be degraded along with the interface.

The temperature dependence of the thermo-optic effect in cryogenically cooled Yb:YAG ceramics was evaluated by measuring the thermo-optic coefficient (the derivative of refractive index with respect to temperature, i.e., dn/dT), thermal expansion coefficient (α), and thermal conductivity (κ) between 70 and 300 K. These parameters significantly improved at low temperature. Observed values indicated that a laser gain medium cooled to 70 K can sustain a thermal load up to 20 times higher than that at 300 K, for comparable thermo-optic effects. To our best knowledge, this is the first quantitative evaluation of the improvement in thermo-optic characteristics of cryogenically cooled Yb:YAG ceramics.

The surface orientation dependence of heat transfer characteristics in liquid helium was discussed based on previous studies. Judging from their discussions and experimental data, the critical heat fluxes of our measurements come from the upper limit of the heat flux in the regime of continuous vapor columns and patches. To compensate the surface orientation dependence, we modified the gravitational force term in a theoretical equation for the critical heat flux with a horizontal surface. Then, the evaluations by the modified equation were compared with our experimental results. Film boiling heat transfer coefficient with the variation of surface orientation was also discussed based on two-phase boundary layer treatment of free convection film boiling. It was confirmed that our experiments were consistent with the theory.

Based on the successful result of fast heating of a shell target with a cone for heating beam injection at Osaka University in 2002 using the PW laser (Kodama et al 2002 Nature 418 933), the FIREX-1 project was started in 2004. Its goal is to demonstrate fuel heating up to 5 keV using an upgraded heating laser beam. For this purpose, the LFEX laser, which can deliver an energy up to10 kJ in a 0.5–20 ps pulse at its full spec, has been constructed in addition to the Gekko-XII laser system at the Institute of Laser Engineering, Osaka University. It has been activated and became operational since 2009. Following the previous experiment with the PW laser, upgraded integrated experiments of fast ignition have been started using the LFEX laser with an energy up to 1 kJ in 2009 and 2 kJ in 2010 in a 1–5 ps 1.053 µm pulse. Experimental results including implosion of the shell target by Gekko-XII, heating of the imploded fuel core by LFEX laser injection, and increase of the neutron yield due to fast heating compared with no heating have been achieved. Results in the 2009 experiment indicated that the heating efficiency was 3–5%, much lower than the 20–30% expected from the previous 2002 data. It was attributed to the very hot electrons generated in a long scale length plasma in the cone preformed with a prepulse in the LFEX beam. The prepulse level was significantly reduced in the 2010 experiment to improve the heating efficiency. Also we have improved the plasma diagnostics significantly which enabled us to observe the plasma even in the hard x-ray harsh environment. In the 2010 experiment, we have observed neutron enhancement up to 3.5 × 107 with total heating energy of 300 J on the target, which is higher than the yield obtained in the 2009 experiment and the previous data in 2002. We found the estimated heating efficiency to be at a level of 10–20%. 5 keV heating is expected at the full output of the LFEX laser by controlling the heating efficiency.

A typical FIREX target is assembled with a 500 ?m diameter PS shell, a glass fill tube and a gold cone guide. Each part is glued together by an epoxy resin. To date, several assembled targets have been cooled down to cryogenic environment. However, they could not survive a cool down process and appeared to rupture around the glued boundary between the PS shell and cone guide. To reveal a crucial factor of the target destruction, its thermal stress after cool down was calculated using the ANSYS code. A two dimensional axisymmetric calculation model is composed of the PS shell and cone guide glued by an epoxy resin which is covered on by an epoxy fillet. A cool down process from 293 K to 10 K was simulated. The calculation showed that the rupture of the shell would start from the PS shell and gold cone guide boundary and the target validity depended on how the epoxy fillet could reinforce the PS shell.

This paper reports on way to fabricate a gas-tight targets dedicated for the first stage of Fast Ignition Realization Experiment (FIREX-I) at the Institute of Laser Engineering (ILE), Osaka University. It was found that a Ti;sapphire laser machining can be used to fabricate the target. The performance of the laser machining using a fs Ti;sapphire laser was examined on shell materials. The conditions for accurate machining were determined. Michelson interferometer with two different wavelengths which imitates a white light interferometer is an excellent tool for confirming the gas-tightness of the target after assembly.

Hydrogen at high pressure in the fluid state is of great interest for target design of inertial confinement fusion and understanding the interior structure of gas giant planets. In this work, we successfully obtained the Hugoniot data for liquid hydrogen up to 55 GPa under laser-driven shock loading using impedance matching to a quartz standard. The shocked temperature was determined simultaneously by the brightness temperature. The compression and temperature along the principal Hugoniot are in good agreement with theoretical models. High reflectivity of hydrogen was observed at 40 GPa, which suggests the fluid becomes conducting.

The development of target fabrication for the Fast Ignition Realization EXperiment (FIREX) Project is described in this paper. For the first stage of the FIREX Project (FIREX-I), the previously designed target has been modified by using a bromine-doped ablator and coating the inner gold cone with a low-density material. A high-quality bromine-doped capsule without vacuoles was fabricated from bromine-doped deuterated polystyrene. The gold surface was coated with a low-density material by electrochemical plating. For the cryogenic fuel target, a brand new type of aerogel material, phloroglucinol/formaldehyde (PF), was investigated and encapsulated to meet the specifications of 500 µm diameter and 20 µm thickness, with 30 nm nanopores. Polystyrene-based low-density materials were investigated and the relationship between the crosslinker content and the nanopore structure was observed.