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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.

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...

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.

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.

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.

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.

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.

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...

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

Equipments and techniques have been developed for characterization of the foam shell target for the FIREX-I. An interference measurement technique is planned to use to measure the quantity of fuel in a foam shell. A preliminary experiment of the interference measurement technique was carried out. Experimental results indicate the refractive index of RF foam is near 1.08 and that of vacant RF foam containing liquid H2 is near 1.22. Calculations indicate the refractive index of vacant RF foam is 1.07 and that of RF foam containing H2 is 1.19. Some causes of these differences were discussed.