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The GEKKO⁄HIPER-laser driven shock experiments were characterized in detail for studies on equation-of-state (EOS) in ultra-high pressure regime. High-quality shock waves were produced with optically smoothed laser beams. Key issues on EOS measurement with shock waves, the spatial uniformity and the temporal steadiness of shock, and the preheating problem were investigated by measurements of the self-emission and reflectivity from target rear surface. Our experiments and analysis based on impedance matching method were validated by use of double-step targets consisting of two Hugoniot standard metals. Extreme shock waves previously only achieved in nuclear explosion experiments were generated using the laser direct-drive experimental scheme.

The temporal evolution of the density profile of a directly laser-driven polystyrene target was observed for the first time using an x-ray penumbral imaging technique coupled with side-on x-ray backlighting at the GEKKO XII [C. Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)]–High Intensity Plasma Experimental Research laser facility (IL=0.7×1014 W/cm2, λL=0.35 μm). This density measurement makes it possible to experimentally confirm all physical parameters [γ(k),k,g,ṁ,ρa,Lm] appearing in the modified Takabe formula for the growth rate of the ablative Rayleigh–Taylor instability. The measured density profiles were well reproduced by a one-dimensional hydrodynamic simulation code. The density measurement contributes toward fully understanding the ablative Rayleigh–Taylor instability.

We developed and improved high space- and time-resolution x-ray imaging techniques (3–6 μm, 100–200 ps). These techniques were coupled with side-on x-ray backlighting for observing hydrodynamics of directly laser-driven polystyrene plasmas. As a high sensitive x-ray image detector, Imaging Plate was introduced to penumbral imaging technique [S. Fujioka et al., Rev. Sci. Instrum. 73, 2588 (2002)]. Noise in penumbral images was reduced by this improvement, and high quality density profiles of plasmas were obtained. A large-magnification slit camera, whose slit width and magnification were 4 μm and 82.8, was coupled with an x-ray streak camera (XSC). Due to the advantage of the large-magnification, high spatial resolution of 5.8 μm was found even with an XSC, the photocathode of which usually has low spatial resolution. These techniques are useful in understanding of hydrodynamic instability in inertial confinement fusion.

Rayleigh–Taylor (RT) instability is one of the most important physical issues in the inertial confinement fusion research. The reduction of its growth rate due to laser-driven ablation, which is described by the modified Takabe formula, is of great importance for a precise understanding of the target physics. We have developed various diagnostic techniques for the measurement of the parameters appearing in the Takabe formula, such as the growth rate, the acceleration, the mass ablation rate, and the density structure including ablation density and density scalelength. Face-on and side-on x-ray backlighting were coupled with moire interferometry imaging, penumbral imaging, and Fresnel zoneplate imaging. All the physical parameters related to RT instability were measured experimentally, and compared with simulation results.

A uniform laser ablation was observed in a polystyrene film coated with a photovoltaic perylene/phthalocyanine bilayer when an incident took place at an intensity range of 10 9 –10 10 W/cm 2 ( λ =1064 nm, 1.1 ns FWHM). Without the bilayer coating, the laser pulse formed spiky structures in the polystyrene film as self-focusing traces of the laser pulse, while for the coated film, the uniform surface ablation trace without the spiky interior structures was observed. In the case of incident of 532 nm where the coating material exhibits no reflection, such difference depending the coating was not observed. These phenomena were coupled with the reflection and conduction properties via its photovoltaic effect, and agreed with the required ablation to achieve high-density compression of the fuel capsule for inertial fusion energy (IFE).

One-dimensional (1D) penumbral imaging technique with high spatial resolution has been developed, and applied to density profile measurements in laser-driven Rayleigh-Taylor instability experiments at a GEKKO XII-HIPER laser system. A laser-irradiated planar target was observed with side-on x-ray backlighting. A penumbral image of an x-ray radiograph was made by using a knife-edge imager. The x-ray radiograph was, then, reconstructed by differentiating the penumbral image with a proper Wiener filtering. A density profile was deduced from the reconstructed x-ray radiograph. In a proof-of-principle experiment, the density profile of a polystyrene (PS) target before laser irradiation was measured by using this method, and high spatial resolution of 3–4 μm was demonstrated. A laser-irradiated PS target in the acceleration phase was observed. The experimentally observed density profile was found to be consistent with the prediction by a 1D hydrodynamic simulation code. The x-ray penumbral imaging is a very sim...

The initial imprint caused by the spatial non-uniformity of laser intensity which generates a mass perturbation is one of the critical issues for directly driven laser fusion research. To mitigate such laser imprint a foam-hybrid target, which has low-density foam layer and is coated with high-Z material, was proposed. In such targets, the X-ray radiation plays a significant role in the formation of the initial imprint. For short wavelength perturbations, a clear suppression of the perturbation growth is observed in the foam-hybrid target. The perturbation growth is reduced by radiation preheating since the ablation surface is smoothed. Moreover, the emitted X-rays from the coated high-Z material suppresses the hydrodynamic instability at the interface of plastic and foam. However, the ablation structure is sensitive to the opacity model. Thus, it is important to analyze the role different radiation models play in the hydrodynamics. In this work we will compare both CRE and LTE models.

We are preparing a laser triggered lightning method for the winter thunderstorm. Using a - 150-J electron beam excited CO2 laser, we obtained a laser plasma channel consisting of many plasma beads made of ionized aerosols in the atmosphere. The laser plasma channel initiates an electrical streamer and propagates a leader. We discuss experimental results on the critical conditions for initiating streamer from a laser plasma channel and for leader propagation through a laser plasma channel.

The control of the refractive index of laminated coatings consisting of alternating stacks of nanoscale Al2O3 and TiO2 sublayers grown by atomic layer deposition has been achieved. The refractive index of the coating linearly changed from 1.870 to 2.318 as the thickness of the single TiO2 sublayer was varied from 2.0 to 39 A while that of the single Al2O3 sublayer was kept constant at 5.5 A. The refractive index could be varied by adjusting only the number of growth cycles of each material. This approach will have potential applications to optical multilayer coatings consisting of well-controlled extremely thin layers.

An x‐ray emission computed tomographic technique has been demonstrated to diagnose the laser implosion uniformity of spherical targets. Three‐dimensional images of compressed core were reconstructed from five pinhole camera images obtained at different viewing directions by using an iterative algorithm. Good agreement between the experimental results and two‐dimensional hydrodynamic simulations showed that the uniformity of initial target shell thickness has a significant influence on implosion symmetry.

Laser oscillations with nearly diffraction limited output beam of a copper vapor laser (CVL) were investigated. The beam divergence less than 0.3 mrad in the pulse duration of 75ns was attained by the long pulse excitation. Also, good beam quality in the whole pulse duration was obtained by the injection locking, with the threshold injection power of only 0.1 W. The CVL beam quality has been successfully established for the master oscillator power amplifier.

We measured the two-dimensional birefringence compensation in laser-diode (LD) pumped Nd:glass (HAP4) laser rods with a high-sensitivity polarimeter. The compensation was performed by inserting a 90° quartz rotator between two identical LD pumped Nd:glass rods with conductive cooling. A considerable reduction of birefringence loss from 9.8% to less than 0.9% after compensation was observed.

Hydrodynamic instability in laser-irradiated targets have been investigated in detail by using ultra high-speed x-ray radiographic technique. Recently developed high-resolution x-ray imaging for laser-driven Rayleigh-Taylor (RT) instability experiments as well as data including RT growth rate, ablation density and plasma density profile are desribed. Results are of great importance for comprehensive understanding of the dispersion relation of the laser-driven RT instability. Especially, direct observation of the ablation density was first achieved with temporal and spatial resolutions of 100 ps and 3 μm, respectively. Imaging techniques includes x-ray Moire imaging, x-ray penumbral imaging and Fresnel phase zone plate imaging coupled with x-ray streak cameras or x-ray CCD cameras. Experiments were performed by using Gekko-XII/HIPER laser system at the Institute of Laser Engineering, Osaka University.

We present recent results on the hydrodynamic instability experiments on the HIPER (High Intensity Plasma Experimental Research) laser facility at Institute of Laser Engineering, Osaka University. We measured the Rayleigh‐Taylor growth rate on the HIPER laser. Also measured were all parameters that determine the RT growth rate. We focused on the measurements of the ablation density of laser‐irradiated targets, which had not been experimentally measured. The experimental results were compared with calculations with one dimensional simulation coupled with and without Fokker‐Planck equation for electron heat transport.

The bulk laser damage threshold of lithium triborate (LiB3O5) single crystals were determined to be 45.0 GW/cm2 at 1.064 μm, 1.1 ns pulse width. The damage threshold was dependent on pulse duration and wavelength: at 1.064 μm, 25 ns the damage threshold was 11.2 GW/cm2, at 0.532 μm, 1.0 ns the damage threshold was 26 GW/cm2; and at 0.355 μm, 0.9 ns the damage threshold was 21.9 GW/cm2. To the best of our knowledge this is the highest value reported among inorganic frequency conversion crystals, and approximately 1.8 times higher than that of fused silica.

A scheme to suppress the Rayleigh-Taylor instability has been investigated for a direct-drive inertial fusion target. In a high-Z doped-plastic target, two ablation surfaces are formed separately-one driven by thermal radiation and the other driven by electron conduction. The growth of the Rayleigh-Taylor instability is significantly suppressed on the radiation-driven ablation surface inside the target due to the large ablation velocity and long density scale length. A significant reduction of the growth rate was observed in simulations and experiments using a brominated plastic target. A new direct-drive pellet was designed using this scheme.

Intense proton and nitrogen beams were focused on various sample materials to observe the surface ablation thickness. The thickness change and the surface composition change with the beam dose were measured with various analysis methods.

To demonstrate the feasibility of the fast ignitor scheme, the 100 TW Petawatt Module laser PWM was illuminated on an imploded spherical CD shell, which enhanced neutron yield ~106 for the first time. This is due to the fusion reactions between the energetic deuteron beam and cold dense deuterons. Without PWM, we have observed no neutrons above the detection threshold ~105. Proton rear emission and possible gigagauss scale magnetic fields from 60 TW laser illuminated plastic target are observed in a good agreement with a two-dimensional PIC simulation. The PWM is up graded to the world biggest Petawatt Laser (say PW laser). The out put is 1 Petawatt, which is used also for ultra-intense laser matter interactions above the intensity of 1020 W/cm2.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An imploded plasma core is irradiated by a 100 ps laser pulse in a model experiments of fast ignition. Additional laser pulses for drilling and heating are introduced co- axially with the laser beams for the implosion. The preformed imploded core is created by the 12 beams of 0.53 micrometers laser with the total energy of 800 J. The additional heating pluses contain 100 ps pulses separated by 300 ps at the wavelength of 1.06 micrometers with the total energy of 320J. The first pulse is intended for drilling the coronal pulses surrounding the core and the second is for addition heating of the core. We measured the imploded core additionally heated with 100 ps pulses.

Ultra-fast x-ray imaging is of great importance for diagnosing laser-driven inertial confinement fusion (ICF) plasmas. Typical required spatial and temporal resolutions are 10 micrometer and 10 ps, respectively. We have developed variety of one- (1D) and two-dimensional (2D) image sampling technique for ultrafast time-resolved x-ray imaging with x-ray streak cameras. Moire imaging of an x-ray-backlit target has been developed as 1D image sampling of an objective with 1D repetitive structure with a spatial resolution of 5 micrometer for use in experiments on hydrodynamic instabilities in laser- accelerated targets. With 1D sampling of repetitive 2D images, a multi-imaging x-ray streak camera (MIXS) with temporal- and spatial-resolutions of 10 ps and 15 micrometer, respectively, has been developed and successfully utilized for diagnosing uniformity and heating process of the imploded core plasmas. Two types of spectroscopic applications of the MIXS have been developed. One is multi-channel MIXS (McMIXS) which has three MIXS channels with various spectral responses for time- resolved 2D temperature measurement. Another is monochromatic MIXS (M-MIXS) for temperature, density and material mixing measurement, in which monochromatic images with Bragg crystals are coupled to MIXS. Finally, 2D image sampling of a 2D image on an x-ray streak camera (2D-SIXS) was also developed.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An optical phase conjugation and laser pulse compression of laser light via stimulated Brillouin scattering at high energy are investigated. SBS operation at high-energy and high average power in fluorocarbon liquids is clarified. Fundamental nonlinear and SBS optics data on particle-free Fluorinert liquids are presented. High capacity SBS cells filed with particle-free Fluorinert liquids were used to demonstrate a very high energy SBS mirror in recent experiments at GEKKO XII laser facility. The pulses of energy as high as 73 Joules were phase conjugated using a SBS phase conjugating mirror. High energy SBS laser pulse compressor was operated at the level 30 Joules. Laser pulse with duration 13 ns FWHM was compressed to 0.6 ns with a high SBS internal efficiency.

This paper will discuss the laser conditions for producing efficient thrust on debris. In order to obtain high thrust for given laser energies, thermal energies dissipated into laser ablation and debris bulk have to be minimized. It was suggested that this minimization can be achieved by using ultrashort pulse interactions where laser pulse duration is too short for thermal conduction to take place deep into debris. The results of momentum coupling coefficients measured with ps and ns pulse duration will be presented.

This paper describes the widows for laser transport in the atmosphere. The limitations caused mainly by Rayleigh scattering, stimulated Raman scattering, thermal blooming and so on are discussed here. From these limitations, windows for laser space applications are found out. The widows have enough area in laser parameters for several applications of laser in the space.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have newly developed a two dimensionally (2D) space-resolved spectrometer for an ultra high intense laser plasma experiment. Output of this data, we can obtain four dimensional informations: space(2D), spectrum, and intensity. An optical fiber bundle is combined with a Cross Czerny-Turner type spectrometer to acquire 2D space-resolved spectrum. This has the spectral resolution 0.8 nm and the space 8 μm. Back-scattered light was measured from plasmas irradiated with an ultra-intense laser pulse at laser intensity 10 19 W/cm 2 . The obtained spectra show distinctive difference depending on both spatial and intensity profiles.

IN order to directly observe low-mode implosion nonuniformities, especially of l equals 1, which prevents stable formation of a hot spark in the compressed core plasma at the final stage of the implosion, a series of direct-drive implosion experiments has been performed at the Gekko-XII glass laser facility by using gas-filled plastic- shell targets. Partially coherent light (PCL) was used as a drive laser to suppress middle- to higher-modes of the irradiation nonuniformity down to approximately 1 percent. A clear shift from the center of the chamber and a slight crosswise structure were seen in the time- and space- resolved shape of the shell in the accelerating phase with an x-ray framing camera and absorbed laser intensity were also estimated by using a rocket equation. In order to confirm the property of these results, separate experiments under similar laser conditions were performed by using Au- coated sapphire spheres as surrogate target. The x-ray intensity distribution on the circumference of the target in the XFCs image, which is strongly dependent on the drive nonuniformity in the accelerating shell resulted from the l equals 1 drive nonuniformity. In our experiments, the l equals 1 drive nonuniformity due to some reproducible factors was found to be of the order of approximately 10-20 percent.

Reviewed are the progress in direct drive implosion researches with Gekko XII laser system. Precise observation of the growth rate of Rayleigh-Taylor instability and the suppression of imprints using indirect-direct hybrid implosion have been investigated. Theoretical and experimental researchers on the fast ignition scheme are also studied. Relativistic laser plasma interaction experiments with Peta-Watt Module and Gekko XII are also described. Finally, the future direction of the research including the development of solid state laser for fusion reactor is discussed.

We prepared optical thin films grown with surface chemical reactions using TiCl 4 and H 2 O for TiO 2 , Al(CH 3 )3 and H 2 O 2 for Al 2 O 3 as reactants. The uniformity of thickness distribution was good over 240 mm in diameter. The film thickness could be preciously controlled only by the number of cycles. Two-layer TiO 2 /Al 2 O 3 anti-reflection coating with less than 0.2 percent reflection loss at 532nm was fabricated. Laser induced threshold strongly depend on the growth temperature and the introduced gas pressure. They increasing with decreasing the growth temperature and with increasing the introduced gas pressure.

Laboratory and field experiments for laser triggered lightning were performed in order to induce so called triggered lightning, in which an electric leader initiates from tall objects on the ground to thunderclouds, using a plasma channel produced by CO2 lasers and Nd lasers. Significant milestone was achieved in technological developments and verification of a scientific feasibility of the techniques such as determination of laser trigger timing which lead to laser triggered natural lightning. In the laboratory experiments technologies to produced long plasma channels and characteristics of discharge process induced by plasma channels were extensively investigated. In the field experiments developed are an automatic laser trigger system using an RF burst emission by a preliminary breakdown, methods to measure lightning path, thundercloud movement and field strength, and to make a continuous plasma channel at the top of the lightning tower which acts effectively for leader initiation.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A series of field experiments for laser triggered lightning have been carried out targeting the winter thunderstorms in Fukui, Japan. A combination of a 2 kJ CO 2 laser system, a 600 J Nd-glass laser and a 4th harmonics 100 mJ YAG laser was used to produce a plasma channel which are effective to trigger and guide an electrical leader from the lightning tower. A new scheme of autotriggering laser system by preliminary breakdown (PB) has been developed. The PB triggering system demonstrated its capability of irradiating the laser at the right timing for triggering the leader. On February 11th 1997, in the field experiment, the electric leader was initiated by the laser plasma using the PB triggering system for the first time. A number of diagnostics including UHF interferometers, a tower current monitor, a capacitive antennas verified the initiation of leader triggered by the laser plasma both in time and space.

A novel laser architecture of laser-diode pumped eight pass 1064-nm Nd:YAG zig-zag slab laser amplifier was developed aiming to achieve high energy extraction efficiency and good beam quality. A high energy extraction efficiency of up to 73% for the laser mode volume has been achieved in this amplifier operating at an initial small signal gain of 3.36. The excellent beam quality factor M2 of 1.2 has been obtained. The compensation of thermal birefringence in this amplifier was successfully performed.

Heavy fluorocarbon liquids were found to be an ideal medium for high-power stimulated Brillouin scattering experiments. Fluorinert FC-75 gives high reflectivity, near 98%, and excellent fidelity in phase conjugation of 14-J, 18-ns optical pulses. The complete removal of microparticle impurities was found to be essential for avoiding laser damage in a high-energy operation. A method of purification based on ultrafiltration has been developed.

We report the high-sensitivity measurements of 2D thermal birefringence distribution in LD pumped Nd:glass (HAP4) laser under lasing at 1054 nm and non lasing conditions. The thermal birefringence was reduced under the lasing condition compared to the non lasing condition. By inserting a 90 degree(s) quartz rotator between two identically LD pumped Nd:glass rods, almost perfect birefringence compensation was achieved with very little residual birefringence. A considerable reduction of birefringence loss from over 20% before compensation to less than 1% after compensation was observed.

The design and construction of an efficient near-infrared holosteric optical parametric oscillator amplifier based on KTiOPO4 (KTP) crystal are reported. The system is tunable from 840 to 1451 nm and generates up to 0.45 mJ at 920 nm with 27% efficiency in 4 - 6 ns pulses. The 532 nm pump source is a frequency doubled laser-diode pumped Nd:YAG laser producing up to 2 mJ in 8 ns, 50 Hz pulses. We have used the system to study the fluorescence lifetimes of Yb:YAG and Yb:glass as a function of wavelength. The data reveal interesting variations in the reabsorption enhanced Yb-ion lifetime. Some Yb:YAG samples exhibiting a sharp increase in their lifetime near the peak absorption wavelengths. The longest lifetime measured in a few mm thick sample of Yb:YAG was 3.3 ms, which would be highly beneficial for energy storage applications using laser diodes.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An efficient laser‐diode (LD) array surface pumped Nd:YAG disk amplifier with an optical‐optical conversion efficiency of 12% was demonstrated. A double‐pass output energy of 67 mJ (input energy, 10 mJ) in a 1 ns pulse at 50 Hz rate was obtained. The two‐dimensional thermal stress‐induced birefringence in the disk amplifier was successfully measured using a newly designed polarimeter. An energy extraction efficiency of 65% was obtained which was well fitted to the Frantz‐Nodvik equations taking into account of the thermal birefringent loss.

Laser diodes may find an application as pump sources for high-power solid-state laser drivers in the inertial confinement fusion (ICF) reactor because of their high efficiency, good matching to absorption pump bands, and the lower amount of heat deposited in the lasant as compared to conventional flashlamp pumping. The potential of a diode-pumped high-power solid-state laser system for ICF reactor is described. The laser fusion reactor driver will require a few mJ output at the wavelength of 0.35 micrometers , a 10% overall efficiency, and a 10 Hz repetition frequency. Using a diode-pumped regenerative amplifier, it is shown that the diode-pumped solid-state laser system will have a sufficiently high efficiency (over 10%), a long lifetime (laser diodes have a lifetime of over three years at 10 Hz operation), and a high repetition frequency (over 10 Hz, owing to its small thermal load to laser materials). The most likely candidates for such laser materials of the system are Nd: phosphate-glass, Nd: silica- glass, Ho, Tm: YAG-crystal, and Tm: YAG-crystal. Technical items to be developed with the system are also discussed.

The initial imprint of mass perturbation due to spatial nonuniformity of laser intensity is one of the most important issues in laser fusion research. Several imprint mitigation schemes by the use of soft X-ray radiation have been proposed to enhance the thermal smoothing effect within the conduction region. One of the schemes uses external X-ray irradiation prior to laser incidence to produce preformed plasma. Another has a low-density foam layer and high-Z material to heat the foam radiatively and make it uniform in density.