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

In this study, we experimentally evaluate the ion transportation through a cone guide target, which accelerates ions up to MeV energies via target normal sheath acceleration, and transports them onto the position of imploding fuel in the fast ignition scenario of nuclear fusion. We measured the electric and magnetic fields (EM-fields) induced by return current streaming along the cone wall by proton radiography, and we report that the EM-fields are predominantly induced within a temporal window up to 30 ps after the laser injection. The magnitude of the electric field is maximized around 13 ps, reaching $4.0\times 10^{10} \mathrm {V}\ \mathrm {m}^{-1}$ , when the magnetic field is below 200 T. The present scheme provides insights on the EM-fields evaluation in the time region that is difficult to treat with simulations due to the computing resources.

The scientific and technical advances continue to support novel discoveries by allowing scientists to acquire new insights into the structure and properties of matter using new tools and sources. Notably, neutrons are among the most valuable sources in providing such a capability. At the Institute of Laser Engineering, Osaka, the first steps are taken towards the development of a table-top laser-driven neutron source, capable of producing a wide range of energies with high brightness and temporal resolution. By employing a pure hydrogen moderator, maintained at cryogenic temperature, a cold neutron ($$\le 25\hbox { meV}$$ ≤ 25 meV ) flux of $$\sim 2\times 10^3\hbox { n/cm}^2$$ ∼ 2 × 10 3 n/cm 2 /pulse was measured at the proximity of the moderator exit surface. The beam duration of hundreds of ns to tens of $$\upmu \hbox {s}$$ μ s is evaluated for neutron energies ranging from 100s keV down to meV via Monte-Carlo techniques. Presently, with the upcoming J-EPoCH high repetition rate laser at Osaka University, a cold neutron flux in orders of $$\sim 1\times 10^{9}\hbox { n/cm}^2/\hbox {s}$$ ∼ 1 × 10 9 n/cm 2 / s is expected to be delivered at the moderator in a compact beamline.

Mechanical properties of a DyBaCuO single-grain superconducting bulk material obtained through the melt-processing in 100% O2 atmosphere were evaluated through tensile tests for the specimens cut from the bulk material. DyBaCuO bulk materials melt-processed in 50 and 75% O2 atmosphere were also evaluated for comparison. The porosity was decreased with increasing the O2 pressure in the melt-processing, and few pores were observed for the bulk material melt-processed in 100% O2 atmosphere. The tensile test specimens were glued to metal rods, and the metal rods were connected to the universal joints for loading. The tensile strength was improved with decreasing the porosity. The tensile strength values obtained in this study were lower than bending strength values reported elsewhere. Flow-like patterns formed by the crack propagations were observed on the fracture surfaces of the tensile test specimens. Through the observations on the fracture surfaces, differences in the fracture mechanisms between the porous and the low porosity bulk materials are discussed.

In inertial confinement fusion (ICF), a fuel target containing deuterium and tritium is used. In recent ICF experiments on the Gekko XII LFEX facility at the Institute of Laser Engineering at Osaka...

A counter-propagating laser-beam platform using a spherical plasma mirror was developed for the kilojoule-class petawatt LFEX laser. The temporal and spatial overlaps of the incoming and redirected beams were measured with an optical interferometer and an x-ray pinhole camera. The plasma mirror performance was evaluated by measuring fast electrons, ions, and neutrons generated in the counter-propagating laser interaction with a Cu-doped deuterated film on both sides. The reflectivity and peak intensity were estimated as ∼50% and ∼5 × 1018 W/cm2, respectively. The platform could enable studies of counter-streaming charged particles in high-energy-density plasmas for fundamental and inertial confinement fusion research.

The scientific and technical advances continue to support novel discoveries by allowing scientists to acquire new insights into the structure and properties of matter using new tools and sources. Notably, neutrons are among the most valuable sources in providing such a capability. At the Institute of Laser Engineering, Osaka, the first steps are taken towards the development of a table-top laser-driven neutron source, capable of producing a wide range of energies with high brightness and temporal resolution. By employing a pure hydrogen moderator, maintained at cryogenic temperature, a cold neutron ([Formula: see text]) flux of [Formula: see text]/pulse was measured at the proximity of the moderator exit surface. The beam duration of hundreds of ns to tens of [Formula: see text] is evaluated for neutron energies ranging from 100s keV down to meV via Monte-Carlo techniques. Presently, with the upcoming J-EPoCH high repetition rate laser at Osaka University, a cold neutron flux in orders of [Formula: see text] is expected to be delivered at the moderator in a compact beamline.

In order to investigate the mechanical properties of a (Gd,Y)BaCuO large single-grain bulk material, bending tests were carried out at room temperature for the specimens cut from the bulk material. A precursor, which consisted of three regions with different Gd and Y contents, was used for the fabrication of the large single-grain bulk material in order to overcome the undesirable nucleation apart from the seed crystal. The precursor was prepared such that Y content increased with increase in the distance from the center, where the seed crystal was placed, to decrease the peritectic decomposition temperature. Two types of bending test specimens were cut from the bulk material—the specimens that included the interface between two regions with different Gd and Y contents and the specimens that did not include the interface. Although the bending strength data of the specimens that did not include the interface scattered widely in comparison with the specimens that included the interface, the average bending strength values of these specimens were similar to each other. This result demonstrates that the interface does not cause the deterioration of the mechanical properties of the whole bulk material.

Fast heating efficiency when a pre-imploded core is directly heated with an ultraintense laser (heating laser) was investigated. ‘Direct heating’ means that a heating laser hits a pre-imploded core without applying either a laser guiding cone or an external field. The efficiency, η, is defined as the increase in the internal core energy divided by the energy of the heating laser. Six beams (output of 1.6 kJ) from the GEKKO XII (GXII) green laser system at the Institute of Laser Engineering (ILE), Osaka University were applied to implode a spherical deuterated polystyrene (CD) shell target to form a dense core. The DD-reacted protons and the core x-ray emissions showed a core density of 2.8 ± 0.7 g cm−3, or 2.6 times the solid density. Furthermore, DD-reacted thermal neutrons were utilized to estimate the core temperature between 600 and 750 eV. Thereafter, the core was directly heated by a laser for fast-ignition experiments (LFEX, an extremely energetic ultrashort pulse laser) at ILE with its axis lying along or perpendicular to the GXII bundle axis, respectively. The former and latter laser configurations were termed ‘axial’ and ‘transverse modes’, respectively. The η was estimated from three independent methods: (1) the core x-ray emission, (2) the thermal neutron yield, and (3) the runaway hot electron spectra. For the axial mode, 0.8% < η < 2.1% at low power (low LFEX energy) and 0.4% < η < 2.5% at high power (high LFEX energy). For the transverse mode, 2.6% < η < 7% at low power and 1.5% < η < 7.7% at high power. Their efficiencies were compared with that in the uniform implosion mode using 12 GXII beams, 6% < η < 12%, which appeared near to the η for the transverse mode, except that the error bar is very large.

The critical heat flux in liquid hydrogen is ten times higher than that in liquid helium and is approximately half of that in liquid nitrogen. Since the resistivity of pure metal such as copper or silver at 20 K is less than one-hundredth of that at 300 K, HTS magnets immersed in liquid hydrogen are expected to satisfy the fully cyostable condition or to be stable against high resistive heat generation enough for quench detection at a practical current density. In order to examine cryostability of HTS magnets in liquid hydrogen, a pool-cooled Bi2223 magnet with a 5 T magnetic field at 20 K has been designed, fabricated and tested in liquid nitrogen prior to excitation tests in liquid hydrogen. The magnet consists of six outer double pancake coils with the inner diameter of 0.20 m and four inner double pancake coils with the outer diameter of 0.16 m. The resistive voltage to initiate thermal runaway in the coil assembly in liquid nitrogen was higher than 1 V that is sufficient high for quench detection.

A laser inertial fusion energy (IFE) reactor requires repetitive injection of fuel pellets and laser engagement to fuse fusion fuel beyond a few Hz. We demonstrate 10 Hz free-fall bead pellet injection and laser engagement with γ-ray generation. Deuterated polystyrene beads with a diameter of 1 mm were engaged by counter illuminating ultra-intense laser pulses with an intensity of 5 × 1017 W cm−2 at 10 Hz. The spatial distribution of free-fall beads was 0.86 mm in the horizontal direction and 0.18 mm in the vertical direction. The system operated for more than 5 min and 3500 beads were supplied with achieved frequencies of 2.1 Hz for illumination on the beads and 0.7 Hz for γ-ray generation; these frequencies were three times greater than with the previous 1 Hz injection system. The duration of operation was limited by the pellet supply. This injection and engagement system could be used for laser IFE research platforms.

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

The injection and engagement of pellets using laser beam irradiation is one of the key technologies to realize a laser-driven inertial fusion energy (IFE) reactor. We irradiated ultra-intense laser...

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

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

The infrared (IR) heating method for a central ignition target with spherical symmetry is modified for the axisymmetric Fast Ignition Realization EXperiment (FIREX) target. The challenge is that the FIREX target pretends to be a thermally spherical shell. Our previous simulation studies (A. Iwamoto et al., Fusion Sci. Technol. 56, 427 (2009), A. Iwamoto et al., J. Phys.: Conf. Ser. 244, 032039 (2010)) have shown that the combination of volumetric heating in a fuel and cone temperature control has the potential to finish a uniform fuel layer. We have developed the IR heating system, dedicated to the FIREX target, with exclusive cone temperature control. The ability of solid fuel layering was examined by using an 826 µm polystyrene (PS) shell with a gold cone of 1.2 mm in length instead of the 500 µm FIREX target for easy observation. The system could control the profile of a solid fuel layer in the PS shell target. Eventually, the solid layer with the best sphericity of 92% was formed, and the RMS roughness of the inner surface was 44 - 49 µm in modes 1 to 100 and 14 - 26 µm in modes 5 to 100.

The applicability of tokamak, helical and laser fusion reactors as a volumetric fusion neutron source has been examined using the systems codes that have been utilised for the conceptual design of DEMO and commercial reactors in Japan. This study has clarified the characteristics of reactor-based volumetric neutron sources that can be designed based on the current physics and engineering basis with a reasonable running cost (∼5B Yen/year). Although the achievable neutron flux is 2–3 orders lower than that of accelerator driven neutron sources, tokamak and helical neutron sources can provide a much larger irradiation area for the test of large components. Laser neutron sources have both high operability and tritium breeding capacity. These reactor-based neutron sources also serve as an integrated test bed of the entire reactor system.

A multi-purpose high repetition laser facility, the so-called Japan establishment for power-laser community harvest (J-EPoCH) is proposed as a next generation laser facility. J-EPoCH will operate at the maximum rate of 100 Hz. The omnidirectional 12 laser beams with 8 kJ would yield ∼1013 neutrons with a large high aspect ratio target. As one of the applications of J-EPoCH, a laser fusion subcritical research reactor has been conceptually designed based on existing technologies. Moreover, a variety of fusion engineering studies: energy conversion, tritium breeding, neutron irradiation effects, etc, can be conducted. The feasibility of the subcritical research reactor is considered in terms of neutron-thermal (n-t) conversion and tritium breeding. Lead–lithium alloy (Li17Pb83) and boron carbide (B4C) have the potential to be studied for preliminary fusion power generation. The subcritical reactor will generate 21.4 W and 20.0 W of the thermal fusion power with the Li17Pb83 and the B4C layers of the thickness of 80 cm, respectively at 1 Hz operation. The Li17Pb83 layer of a 5 mm thickness will achieve the temperature rise of 0.203 mK per shot. The thermal fusion energy is detectable with conventional measurement techniques. The core with the Li17Pb83 layer thickness of 100 cm will yield more than one tritium from a deuterium–tritium fusion neutron. However, laser windows reduce the efficiency of n-t conversion and tritium yield., ** After this version, the author still has made a few minor corrections.

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

ITER Central Solenoid (CS) pulse operation induces significant flow disturbance in the forced-flow Supercritical Helium (SHe) cooling circuit, which could impact primarily on the operation of cold circulator (SHe centrifugal pump) in Auxiliary Cold Box (ACB). Numerical studies using Venecia®, SUPERMAGNET and 4C have identified reverse flow at the CS module inlet due to the substantial thermal energy deposition at the inner-most winding. To assess the reliable operation of ACB–CS (dedicated ACB for CS), the process analyses have been conducted with a dynamic process simulation model developed by Cryogenic Process REal-time SimulaTor (C-PREST). As implementing process control of hydrodynamic instability, several strategies have been applied to evaluate their feasibility. The paper discusses control strategy to protect the centrifugal type cold circulator/compressor operations and its impact on the CS cooling.

International audience; Mechanical properties of a high packing ratio MgB 2 bulk sample processed by spark plasma sintering (SPS) were evaluated at 77 K through bending tests for specimens cut from the bulk sample. The effects of the packing ratio and temperature on the fracture strength of MgB 2 bulk are discussed. The fracture strength at 77 K of the MgB 2 bulk processed by SPS was higher than that at room temperature. The packing ratio of the MgB 2 bulk processed by SPS was higher than those of other MgB 2 bulks processed by sintering under the ambient pressure and hot isostatic pressing. The fracture strength was improved by increasing the packing ratio. The fracture strength at a very low temperature was estimated from the bending test results at 77 K and room temperature. The fracture strength at 20 K of the MgB 2 bulk processed by SPS was estimated to be around 425 MPa. © 2015 IEEE.

We conducted fast heating of a core plasma produced by counter-illumination of two beam bundles of implosion lasers for two incident directions of the heating laser. For the axial heating configuration, wherein the heating laser was incident along the coaxial direction of the implosion lasers, the size and density of the counter-imploded core plasma were estimated. The incident timing of the heating laser was determined. Although the heating laser was incident during the stagnation of the core plasma, there were no considerable increases in the intensity of X-rays with energies beyond 1 keV and the yield of the thermonuclear fusion neutrons. Effective core heating did not occur due to the interaction of the heating laser with the low-density ablated plasma far from the core. On the other hand, for the transverse heating configuration, wherein the heating laser was incident from the transverse direction of the implosion laser axis, optimization of the incident timing of the heating laser was not achieved. However, two-dimensional radiation-hydrodynamic simulation revealed that a heating laser can directly interact with a core plasma and effective heating is expected. In our scheme, transverse heating configuration appears to be significant.

The Japan Establishment for a Power-laser Community Harvest (J-EPoCH) is proposed as a next generation laser facility having multi-purpose high repetition laser beams at the maximum rate of 100 Hz. One of the experimental areas is assigned for laser fusion research. The omnidirectional 12 laser beams with 8 kJ and a 5 PW heating laser are arranged. A Large High Aspect Ratio Target (LHART) with the laser beams would yield ~1013 neutrons. The fusion energy corresponds to ~22 J/shot. As one of the J-EPoCH applications, a laser fusion subcritical research reactor with replaceable cores has been conceptually designed based on existing technologies. Moreover,we can conduct a variety of fusion engineering studies: energy conversion, tritium breeding, neutron irradiation effects and other subjects. The feasibility of tritium breeding and preliminary energy conversion studies are proven according to calculations by the Particle and Heavy Ion Transport code System (PHITS).

A Monte Carlo (MC) code simulating the kinetics of two-particle collisions has shown to have certain advantages in the spectral analysis of neutrons from fast-ignition inertial confinement fusion (ICF). The MC code quickly displays neutron spectra produced in the expected fusion plasma kinetics (incl. ion energy spectrum, anisotropy, and collision geometry). This allows to explore various potential mechanisms of neutron production without using time-consuming hydrodynamic or particle simulations and makes it possible to give a feedback to ongoing experiments. The validity of this model was demonstrated in the ”cone-free-shell” fast ignition experiment at the GEKKO-XII - LFEX laser facility, where the model provided a clear explanation for the significant spectral modulation of D(d, n)3He neutrons generated through the interaction between a high-power laser and a long-scale plasma.

We study fuel layering for the Fast Ignition Realization EXperiment (FIREX) cryogenic target with a foam shell. A void free solid fuel layer within a porous foam material must be formed ideally. We have demonstrated the residual void fraction of ∼1% in a foam wedge with temperature controlled solidification. ANSYS simulations have shown that the residual void reduction technique will be applicable to the FIREX target. We examined each step in the simulated solidification process using a dummy foam shell target. In several attempts, solid fuel formation with a reduced void fraction in the foam shell succeeded.

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

International audience; In order to improve the mechanical properties of bulk MgB2 superconductor, high packing ratio bulk samples were processed by spark plasma sintering (SPS) at various temperatures. Effects of the SPS temperature on the mechanical properties of the bulk MgB2 superconductor are investigated through bending tests of specimens cut from the bulk samples. Both Young's modulus and bending strength were improved by the increase in the SPS temperature. The improvements of the mechanical properties are mainly due to the improvement in the packing ratio. The relationships between the bending strength and packing ratio of the spark plasma sintered bulk samples are similar to those of other bulk MgB2 pieces processed by capsule method and hot isostatic pressing.

A clear understanding of mechanical properties of superconducting bulk materials is required for their reliable integration together with other materials constituting the superconducting devices. In this study, evaluations of mechanical properties of a high packing ratio MgB 2 bulk processed by Spark Plasma Sintering-SPS were carried out. Mechanical properties of the high packing ratio MgB 2 bulk processed by SPS were improved than those of MgB 2 bulks processed by conventional route. The improvement of the mechanical properties with increasing the packing ratio could be approximated by using exponential equations.

An ultra-intense short pulse laser induces a shock wave in material. The pressure of shock compression is stronger than a few tens GPa. To characterize shock waves, time-resolved velocity measurement in nano- or pico-second time scale is needed. Frequency domain interferometer and chirped pulse laser provide single-shot time-resolved measurement. We have developed a laser-driven shock compression system and frequency domain interferometer with CPA laser. In this paper, we show the principle of velocity measurement using a frequency domain interferometer and a chirped pulse laser. Next, we numerically calculated spectral interferograms and show the time-resolved velocity measurement can be done from the phase analysis of spectral interferograms. Moreover we conduct the laser driven shock generation and shock velocity measurement. From the spectral fringes, we analyze the velocities of the sample and shockwaves.

Process studies for Toroidal Field Structure (TF ST) system with a dedicated Auxiliary Cold Box (ACB-ST) have been conducted under 15 MA baseline, including plasma disruptions. ACB-ST consists of two heat exchangers immersed in the Liquid Helium (LHe) subcooler, which are placed at the inlet/outlet of a Supercritical Helium (SHe) cold circulator (centrifugal pump). Robustness of ACB-ST is a key to achieve the stability of TF coil operation since it provides the thermal barrier at the interface of the TF Winding Pack (WP) with ST. The paper discusses the control logic for the nominal plasma operating scenario and for Mitigation to regulate the dynamic heat loads on ST. In addition, the operation field of a cold circulator is described in the case of plasma disruptions. The required performance of heat exchangers in the ACB-ST is assessed based on the expected operating conditions.

In laser-driven inertial fusion energy reactors, injected fuel pellets are continuously delivered into the reaction chamber and irradiated by laser beams injected at a frequency of tens of hertz. Thus far, a spherical shell pellet has been confirmed as the most conventional target design to achieve high-energy-density state thorough an implosion process that is required for fusion burn. To demonstrate repetitive fuel implosion with a 1 Hz joule-class laser system, a testbed of a shell injection system delivering a spherical shell (500 m diameter and 7 m thickness) was developed. The developed testbed demonstrated that (i) repetitive implosion (maximum frequency: 0.5 Hz) of shell injection was possible for more than ten shells at a shell speed of 191 mm s−1, and (ii) the distribution of the injected shell after 18 cm free-fall was within a circular region, 6.4 mm in diameter. The estimated laser-hit-ratio to the pellet was on the order of 10%.

The FIREX-1 project, the goal of which is to demonstrate fuel heating up to 5 keV by fast ignition scheme, has been carried out since 2003 including construction and tuning of LFEX laser and integrated experiments. Implosion and heating experiment of Fast Ignition targets have been performed since 2009 with Gekko-XII and LFEX lasers. A deuterated polystyrene shell target was imploded with the 0.53- μm Gekko-XII, and the 1.053- μm beam of the LFEX laser was injected through a gold cone attached to the shell to generate hot electrons to heat the imploded fuel plasma. Pulse contrast ratio of the LFEX beam was significantly improved. Also a variety of plasma diagnostic instruments were developed to be compatible with harsh environment of intense hard x-rays (γ rays) and electromagnetic pulses due to the intense LFEX beam on the target. Large background signals around the DD neutron signal in time-of-flight record of neutron detector were found to consist of neutrons via (γ,n) reactions and scattered gamma rays. Enhanced neutron yield was confirmed by carefully eliminating such backgrounds. Neutron enhancement up to 3.5 × 107 was observed. Heating efficiency was estimated to be 10-20% assuming a uniform temperature rise model. © Owned by the authors, published by EDP Sciences, 2013.

Implosion and heating experiments at the Institute of Laser Engineering, Osaka University on Fast Ignition (FI) targets for the FIREX-1 project have been performed with Gekko-XII laser for implosions and LFEX laser for heating. We tried to reduce the prepulse level in the LFEX laser system and have improved the plasma diagnostics to observe the plasma in the harsh hard X-ray environment. A plastic (CD) shell target, 7-μm thick and 500 μm in diameter with a hollow gold cone was used in this experiment to guide the short-pulse laser at the time of the maximum compression. The shell target was imploded with 9 or 12 beams of Gekko-XII laser (527 nm) with energy of 300 J/beam in a 1.5 ns pulse. Two of the four LFEX laser (1053 nm) beams were injected into the inside bottom of the cone with an energy up to 0.7 kJ/beam in a 1.5 ps pulse at the time around the maximum implosion. We have observed neutron enhancement up to 3.5 × 107 with total heating energy of 300 J, which is higher than the yield obtained in the previous experiment in 2002 [R. Kodama et al. Nature 418, 933 (2002)]. We found the estimated heating efficiency is at a level of 10–20%. Fuel heating to 5 keV is expected when the full output of LFEX is used.

Mechanical properties of Dy123 (DyBa2Cu3Ox) dense bulks which had few pores and those of conventional Dy123 bulks which had micro-pores were evaluated. While the dense bulks were fabricated by heating precursors in O2 atmosphere, the porous bulks were fabricated in air. Mechanical properties of the dense bulks were superior to those of the porous bulks. The average sizes of Dy2BaCuO5 particles dispersed in the bulks with 0.5 wt.% Pt were smaller than those dispersed in the bulks with 0.25 wt.% Pt. Bending strength values of the former bulks were higher than those of the latter bulks.

Fracture strength and Young's modulus in a GdBa2Cu3Oy based large single-grain bulk were evaluated through three- and fourpoint bending tests of specimens cut from the bulk. The diameter of the bulk was about 150 mm. The bulk was fabricated in air. Scatter of the bending strength data in the bulk was mainly attributable to dispersion of pores. There was no significant difference in the bending strength value among the bending test specimens cut from inner region of the bulk. The bending strength of the large single-grain bulk was comparable to that of a smaller GdBa2Cu3Oy bulk.

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

Mechanical properties of low porosity Dy123 (DyBa 2 Cu 3 O x ) bulks with different Dy211 (Dy 2 BaCuO 5 ) content were evaluated through bending tests and Vickers indentation tests of specimens cut from the bulks. The bending strength and the Young’s modulus of the low porosity bulks were higher than those of porous Dy123 bulks evaluated in the previous study. The bending strength of the low porosity bulks, which was decreased by the oxygen annealing, increased with increase of the Dy211 content. The fracture toughness of the area with uniform dispersion of the Dy211 particles was superior to that with low density of the Dy211 particles.

Fuel layering of a cryogenic target with a conical laser guide such as the FIREX target is complicated because of its non-spherical symmetry appearance. To simplify the layering, a foam layer is pl...

In order to improve the mechanical properties of the Dy123 bulk material, a precursor was melted in 100% O2 atmosphere to obtain bulk with low porosity. Evaluations of the mechanical properties in the Dy123 bulk were carried out through bending tests and Vickers indentation tests. The mechanical properties such as Young's modulus and bending strength were improved by eliminating pores. The mechanical properties of Dy123 bulks melt-processed in 50 and 75% O2 atmosphere were also evaluated and the relationship between the mechanical properties and the porosity associated with the oxygen annealing was discussed.

Bending tests of the as-grown and the annealed specimens cut from Dy123 bulk samples were carried out. The Young’s modulus of the annealed specimen was lower than that of the as-grown specimen. On the other hand, the fracture strength of the annealed specimen was slightly higher than that of the as-grown specimen. The fracture toughness of the annealed specimen was also higher. The smaller bulk showed excellent mechanical properties in comparison with the larger bulk. The mechanical properties of these bulks are discussed in terms of the porosity and the mechanical properties of the matrix evaluated by Vickers indentation tests.