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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.

A high average-power solid-state laser system for attaining the current goal of 20 J output energy at 10 Hz with a pulse width of 10 ns has been designed to demonstrate scalability to 100 J output energy. We proposed ring and cascade layouts that were useful for a four-pass zig-zag slab laser amplification system with effective extraction of output energy from a laser medium of an amplifier module. The reduction of thermal effects and the suppression in thermal fracture were evaluated by the measurements of temperature distributions of the laser slab. We analyzed the parasitic oscillation of the slab laser and found the threshold values of the parasitic oscillation. We designed a dual module system in consideration of these results, and examined the extraction energy from the system that incorporated an assembly of serrated apertures to suppress the laser-induced damage of optical components and the laser slab.

The laser damage on a total internal reflection (TIR) surface as well as on entrance and exit faces was evaluated to design a zig-zag slab laser. Incorporating a phase shift by the Goos–Hanchen effect on a TIR surface of a laser slab in contact with water, formulas for a laser-induced damage threshold (LIDT) were modified. We determined a method for the designing the geometry of a zig-zag slab laser based on LIDT. A thin film for anti-reflection (AR) with a high LIDT must be formed on the entrance and exit faces of a laser slab glass. A sol–gel thin film was adopted for the AR coating of a high-power laser due to its high LIDT. We devised an effective dip-coating method to form a sol–gel thin film uniformly on a small area. The LIDT of the sol–gel thin film was measured and evaluated with a single mode Q-switch Nd:YAG laser by a one-on-one measurement technique.

In this paper, we describe the development of a high-average-power solid-state laser system and the derivation of equations for the amplification of a laser beam. This laser system is capable of generating an output energy of 10 J per pulse at a wavelength of 1,053 nm in a 10 Hz operation for scientific and industrial applications. The main amplifier of our system is a laser-diode-pumped solid-state amplifier. A water-cooled Nd:glass slab is pumped with two 803 nm AlGaAs laser-diode modules. The laser beam propagates through zig-zag optical paths four times and is amplified. To estimate laser output energy, we have derived and evaluated equations for the amplification of the laser beam, and designed and constructed a laser system based on the calculated results. Experimental results reveal an output energy of 10.6 J at 1 Hz, which closely fits the results calculated using the derived equations.

A 10-kJ PW laser (LFEX) is under construction for the FIREX-I program. This paper reports a design overview of LFEX, the technological development of a large-aperture arrayed amplifier with modified four-pass architecture, wavefront correction, a large-aperture Faraday rotator with a superconducting magnet, a new pulse compressor arrangement, and focus control.

We experimentally studied laser-hole boring with 100 ps/1 TW laser light and ultra-intense laser-plasma interactions using a 0.5 ps/100 TW laser system. Investigation into laser-hole boring was made by measurements of backscattered light spectra, x-ray image and electron density profiles in under- and overdense regions. For the first time x-ray laser light was used to directly observe the channel in the overdense plasma. We further examined short pulse laser interactions as well as the generation of high-energy particles with 100 TW laser light. Neutrons by D-D fusion reaction in the interaction plasma were measured to diagnose ion acceleration. High-energy electrons (MeV) were measured by a x-ray method.

We report a high average power and high pulse energy diode-pumped Nd:glass master oscillator power amplifier system that is consisted of a thermally-edge-controlled zigzag slab amplifier and stimulated Brillouin scattering (SBS) mirror for use of pumping source of ultra intense laser system. This phase conjugated system produces an average power of 213 W at 10 Hz in 8.9-ns pulse (2.4 GW peak power) with an optical-to-optical conversion efficiency of 11.7 % and a near diffraction limited beam quality. To the best of our knowledge these results represent the highest average power achieved from a Nd:glass based laser amplifier system.

The irradiation uniformity of the GEKKO XII laser has been improved by implementing the state-of-the-art beam smoothing and laser control techniques in the system. Non-uniformity modes higher than 20 are improved to less than 1% RMS by introducing partially coherent light, while the lower modes are improved to 3 - 4% RMS. Implosion experiments were carried out with the new GEKKO XII: a fast-rise square pulse preceded by a pre-pulse was used to suppress the Rayleigh - Taylor instability at the acceptable expense of implosion efficiency. The experimental results are compared with a pusher-fuel mixing model combined with a one-dimensional hydrodynamic simulation. The degradation of experimentally obtained neutrons is mainly attributed to the lower-mode asymmetry. Recent results of planar target experiments on hydrodynamic instabilities are also presented. Particular emphasis is given to the noticeable reduction of fluid instability by the flushing of an external x-ray over the target surface prior to the main drive pulse, leading to suppression of the initial imprint amplitude by the pre-formed plasma

The progress of laser fusion research at the Institute of Laser Engineering, Osaka University is reviewed. They are physics investigations on target implosion, development of laser technologies, and R&D for laser fusion power plants. The long term strategy of Inertial Fusion Energy (IFE) development has been reexamined taking into account the recent progress of implosion physics and reactor related technologies. 15 refs., 12 figs.

An amplified spontaneous emission (ASE) from Nd:glass has been introduced into the high power twelve beam Nd:glass laser system, Gekko XII for obtaining smooth intensity distribution of a focused beam. The angular dispersion of an ASE spectrum with large beam divergence was adopted for efficient beam smoothing without significant reduction of the harmonic conversion efficiency. Temporal evolution of the beam smoothing was evaluated as a function of the beam divergence by using a statistical model of speckle. In Gekko XII, the spectral width and beam divergence of ASE were controlled in a range of 0.4 to 0.6 nm and 6 to 22 times diffraction limited, respectively. Final output energy of 1.3 kJ/beam in a 2.2 ns duration was demonstrated without significant gain reduction and spectral narrowing. The doubling efficiency of 50% was obtained at a low intensity region of around 0.3 GW/cm2 by matching the angular dispersion of spectrum to that of phase matching condition of a frequency conversion crystal. The spec...

A diode-pumped joule class in a 10 Hz output Nd:YLF ring amplifier has been developed. A phase conjugate plate was developed as a wavefront corrector for the residual wavefront distortion of an Nd:YLF rod. We have demonstrated a 0.46 J output of 10 ns pulse duration at 10 Hz repetition rate with 1.5 nJ of input energy. The effective gain of the ring amplifier system was 84.8 dB. To our knowledge, this is the highest magnification with joule-level output energy in a single-stage amplifier system that has ever been built. As a preamplifier system, this system contributed a demonstration of 21.3 J in a 10 Hz output diode-pumped Nd:glass zigzag slab laser system with a stimulated Brillouin scattering- phase conjugation mirror. We describe a robust and effective method of wavefront correction for high-energy laser systems.

Optical parametric chirped pulse amplification (OPCPA) is now recognized as a key technique to generate high peak power ultra short laser pulses [1]. In order to produce high-energy and ultrashort pulses, hybrid OPCPA design is employed, where high gain and broadband OPCPA such as BBO or LiB 3 O 5 (LBO) crystals are used for pre-amplification and large size but lower gain OPCPA such as a Potassium dihydrogen phosphate (KDP) or deuterated KDP crystals are used as a final amplifier. However, amplification bandwidth of KDP or DKDP is narrower and the optimum phase matching wavelength is different to those of BBO.

This paper reports a design overview of a 10-kJ PW laser (LFEX) and technological developments of 2x2 arrayed amplifier, wavefront correction, 1.3-m Faraday ratotor, new scheme of pulse compressor and 91-cm dielectric gratings etc.

The near diffraction limited quality of 21.3 J in 8.9 ns (2.4 GW peak power) at 10Hz rep-rate beam has been obtained by diode-pumped Nd:glass zig-zag slab laser with a stimulated Brillouin scattering (SBS) mirror.

High power laser becomes a key component for the scientific and industry applications. Above all, diode-pumped solid state lasers (DPSSLs) play an important role in inertial fusion energy (IFE) and related high-energy applications as well as material processes. In these applications, repeatable operation (several Hz ~ 100 Hz) is indispensable. In this paper we present the diode-pumped zig-zag slab laser. Average power of 213 W (21.3 J at 10 Hz) was successfully achieved. The diffraction limited focusability was achieved by use of SBS phase conjugator. These results have indicated the 100-J scalability of slab architecture and the 1-kW average power potential of repeatable operation up to 10 Hz for high-energy applications.

A 1053-nm zig-zag slab laser pumped by 290 kW peak power diode array generated 8.4 J pulse energy in 20 ns at 10 Hz, verifying the energy scaling for IFE driver development.

A 1053-nm thermally-edge-controlled slab laser pumped by 800 kW peak power from 8,000 diode bars has been designed for the HALNA project. 20 J pulse energy in 10 ns at 10 Hz is being demonstrated.

We calculated temperature distribution and measured thermally induced birefringence in Faraday glass under laser-light loading. Detailed computation results and experimental results are presented, which will be useful when designing the thermally compensated Faraday rotator.

High peak power laser diode arrays are required for pumping solid-state lasers as the laser fusion driver. We report successful achievement of a 100 kW peak power diode laser module as the pump source of 10 J/spl times/10 Hz Nd:glass slab laser in order to verify a small scale experiment of the diode-pumped solid-state laser (DPSSL) driver for inertial fusion energy development. The AlGaAs laser diode structures were grown by the MOCVD method and employed the quantum well structures.

A simple and flexible method is presented for generating a partially coherent light which obtains the highly smooth focused beam pattern. The beam divergence of 32 times diffraction limited light having a spectral width of 1.6 nm has been easily and reproducibly achieved by injecting a laser pulse from an actively mode‐locked Nd:YLF oscillator to a single mode optical fiber, coupled to a multimode optical fiber. Temporal evolution of the beam smoothing due to the induced incoherency was examined with temporally resolved measurements of the beam pattern. The partially coherent light was focused through a random phase plate after the amplification. Small‐scale intensity perturbation in a focused beam pattern was greatly reduced.

Summary form only given. Diode-pumped solid-state laser (DPSSL) is one of promising candidates as reactor driver for IFE. We have been developing a 10-J output class of diode-pumped water-cooled Nd:glass zig-zag slab laser amplifier module as a smallest-scale test-version of our conceptually designed 4-MJ DPSSL driver. We will show the experimental confirmation of our conceptual design through the investigation of 10-J/spl times/10-Hz module on the key issues such as small signal gain, energy extraction, stored energy, pumping efficiency, thermal effects, beam quality and heat flow in the system.

The long term strategy of Inertial Fusion Energy(IFE) development has reexamined being based on the conceptual design study of laser fusion power plant KOYO, and taking into account the recent progresses of implosion physics research and the reactor related technologies. The design study and the strategy analysis have been conducted as a joint program of university, national laboratory and industries in Japan, and also with international collaborations. It is concluded that the feasibility of inertial fusion power plant is reasonably high and the coordinated R&D throughout the world would be effective.

Laser fusion is one of the most feasible approach in the fusion energy development. The goal of inertial fusion energy (IFE) development is to prove that fusion energy can be available to society as electric power source. Recent progress of laser fusion research and development enable us to examine technical and economical feasibility, and to plan the realistic strategy and program to the commercial power plant. The most important key issue for IFE is driver technologies. The development of the laser fusion driver may establish new industrial technologies based on the photon processes and is attracting attentions in wide industrial fields.Laser fusion is one of the most feasible approach in the fusion energy development. The goal of inertial fusion energy (IFE) development is to prove that fusion energy can be available to society as electric power source. Recent progress of laser fusion research and development enable us to examine technical and economical feasibility, and to plan the realistic strategy and program to the commercial power plant. The most important key issue for IFE is driver technologies. The development of the laser fusion driver may establish new industrial technologies based on the photon processes and is attracting attentions in wide industrial fields.

New GEKKO XII glass laser system has been improved by the precision project of the power balance with the partially coherent light (PCL) source to attain a few percent nonuniformity of the spherical irradiation of the pellet in a directly driven implosion scheme. The technical issues for balancing the twelve beam intensity and the theoretical treatment of the PCL optics are summarized.

We fabricated new Nd:doped SiO2 material for high power laser. The quartz glass possesses very excellent properties for high power laser material. Although the many trials were carried out to make Nd:doped SiO2, it has not succeeded because high Nd concentration causes the clustering in the SiO2 glass host which degrades the laser properties exceedingly, for example, quantum yield, lifetime and so on. To avoid the clustering in the SiO2 matrix, we use zeolite as doping material to make the Nd:doped SiO2. This new material shows the sufficient properties for high power laser medium. The peak fluorescence wavelength is at 1063 nm near YAG laser and stimulated emission cross section is 1.16×10−20 cm2 larger than that of previous works (which is called old Nd:doped SiO2 in this report).

Physics investigation of implosion and relevant technology development at ILE, Osaka University are reviewed. Since the completion of GEKKO XII and its first shot in December 1983, high temperature implosion demonstrated 10 keV and 1013 neutron/shot, and high density compressions more than 600 times over initial solid density were achieve. Fundamental physic relevant to the implosion process such as thermal smoothing, R‐T instability, pre‐heat effect and fuel pusher mixing are investigated. Precision GEKKO XII project and fuel capsule development for cryogenic target with preliminary implosion test are going on.Conceptual design of commercial reactor KOYO has been proceeded. LD pump solid state laser is designed showing the feasibility for reactor driver. Fuel pellet design and gain scaling reveals the key physics issues to be investigated in the next step of program. Applications of ICF technologies performed at ILE with outer collaborators are also shown.

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.

In the recent directly driven laser fusion research, the feasibility of high density compression has been demonstrated experimentally1-3). To obtain such a compression, the improvement of laser irradiation uniformity was one of most important issues, and in these experiments the irradiation uniformity was greatly improved by installing random phase plates (RPPs)4) in front of final focusing lenses. The highest density achieved using a random-phased frequency doubled Ndrglass laser1,2) almost meets that required for the fuel ignition. However, the observed neutron yields were significantly smaller than predictions by one-dimensional hydrodynamics code, suggesting that a hot spot at the center of compressed fuel may be mixed with a surrounding cold part.

We report a high-average-power and high-pulse-energy diode-pumped Nd:glass laser amplifier system consisting of two thermally-edge-controlled zigzag slab amplifiers and a stimulated Brillouin scattering mirror. This phase-conjugated system produces an average power of 213 W at 10 Hz in a 8.9 ns pulse (2.4 GW peak power) with an optical-to-optical conversion efficiency of 11.7% and a near-diffraction-limited beam. To the best of our knowledge, this is the highest performance from a Nd:glass-based laser amplifier system ever built.

The LFEX laser system is under construction to realize the principle of the fast ignition in the FIREX-I program. The 3.1-kJ output power of the chirped pulse has been obtained with the main disk amplifier. The 3.3-nm (FWHM) spectral width, which is enough wide for less than 2-ps rise time in pulse compression, was obtained by controlling the seed pulse both of temporally and spectrally with a spatial light modulation. The beam aberration with the main amplification was reduced to less than 0.2[03bb] rms in far field pattern by using two deformable mirrors in the main amplifier.

We propose cryogenic Yb:YAG ceramic as a novel reactor laser-driver material instead of conventional Nd:glass. Our research on cryogenic Yb:YAG revealed that both of stimulated emission cross section and thermal conductivity can be tuned well by controlling the material temperature. Using the obtained laser parameters in our experiments, a 1.3-MJ, 16-Hz diode-pumped laser system has been conceptually designed. The overall electrical-optical conversion efficiency is numerically calculated to be as high as 17%. The compact main amplifier with about 3000, m3 volume size would be realized by using the active mirror architecture. In addition, a 1-kJ laser system 'GENBU' has been designed as a milestone for the new driver.

Detailed performances of a laser triggered dielectric switch (LTDS) are described. Thin polypropylene films of 25 or 35 μm in thickness were used for insulation of the spark gap. Maintenance‐free operation was achieved by automatic winding of the film after each laser shot. A square optical pulse of 1 ns duration with a rise time of less than 350 ps was generated by using a fast Pockels cell driven by the LTDS with good reproducibility. The switching jitter was measured to be less than the detection limit of 100 ps. Using the LTDS, a single pulse was selected from a mode‐locked pulse train with the success probability of better than 95%. Comparison of the performance of the LTDS with other switching devices are given.