Faculty Profiles - Nakai Mitsuo

Papers - Nakai Mitsuo

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Density profile of the ablating plasma produced by soft x-ray irradiation

M Nishikino, H Shiraga, M Matsuoka, S Fujioka, K Shigemori, M Nakai, H Azechi, N Miyanaga, T Yamanaka, S Nakai

REVIEW OF SCIENTIFIC INSTRUMENTS 72 ( 1 ) 653 - 656 2001.1

Indirect/direct-hybrid irradiation has been proposed for suppressing the initial imprint of laser-irradiation nonuniformities. Indirect soft x-ray preirradiation of the target generates a plasma expansion layer on the target surface. When the drive laser beam comes later, the target has a substantial stand-off distance between the ablation front and the beam absorption region, in which the thermal smoothing effect is expected to take place, and then, the initial imprint of the laser irradiation nonuniformities can be significantly reduced. Stand-off distance is of great importance in the imprint mitigation. We measured the preformed plasma profile by using the x-ray side-on backlighting method. The preformed plasma was backlit by x rays and imaged onto an x-ray streak camera. The scale length of the preformed plasma was found to increase with duration of the soft x-ray irradiation. The expansion velocity was found to be 6 x 10(5) cm/s at an x-ray irradiation of 1 x 10(11) W/cm(2). (C) 2001 American Institute of Physics.

Monochromatic x-ray imaging with bent crystals for laser fusion research

K Fujita, H Nishimura, Niki, I, J Funakura, Uschmann, I, R Butzbach, E Forster, M Nakai, M Fukao, A Sunahara, H Takabe, T Yamanaka

REVIEW OF SCIENTIFIC INSTRUMENTS 72 ( 1 ) 744 - 747 2001.1

Monochromatic x-ray imaging with toroidally bent crystals has been applied to investigate critical issues in laser fusion research in collaboration with Jena University [for example, I. Uschmann , Rev. Sci. Instrum. 66, 734 (1995)]. Described in this article is the study on x-ray diagnostics to provide simultaneously time, space, and spectral resolutions for the laser fusion experiments by the use of toroidally bent crystals. A monochromatic x-ray imager was developed, which consists of two sets of identical five bent crystals and a fast x-ray framing camera. The attained spatial resolution of 6 mum, time resolution of 34 ps, and spectral resolution of 10 eV are high enough to observe temporal evaluation and temperature distribution of the laser fusion hot core. In addition, a new x-ray imager with bent crystal was developed to investigate hydrodynamic instabilities occurring in laser-driven planer targets as one of the most critical issues in laser fusion research. The imager was so designed as to match the experiments using rather thick, massive targets irradiated under the same condition as in the high-gain experiments. Configuration and specifications of the imager are presented. (C) 2001 American Institute of Physics.

Time-resolved ten-channel monochromatic imaging of inertial confinement fusion plasmas

Ingo Uschmann, Kazuhiza Fujita, Izam Niki, Randolf Butzbach, Hiroaki Nishimura, Jiro Funakura, Mitsuo Nakai, Eckhart Förster, Kunioki Mima

Applied Optics 39 ( 31 ) 5865 - 5865 2000.11

Feed-out of rear surface perturbation due to rarefaction wave in laser-irradiated targets

K Shigemori, M Nakai, H Azechi, K Nishihara, R Ishizaki, T Nagaya, H Nagatomo, K Mima

PHYSICAL REVIEW LETTERS 84 ( 23 ) 5331 - 5334 2000.6

We report experimental results on hydrodynamic perturbation transfer from the rear to the front of laser-irradiated targets. Flat polystyrene foils with rear-surface perturbations were irradiated by partially coherent light. We observed phase inversion of the rear surface after the shock breakout at the rear surface. Perturbations on the laser-irradiated surface arose due to the rippled rarefaction wave. Experimental results were well reproduced by a simple model with unperturbed hydrodynamic quantities calculated From the one-dimensional simulation.

Indirect-direct hybrid target experiments with the GEKKO XII laser Reviewed

H Nishimura, H Shiraga, H Azechi, N Miyanaga, M Nakai, N Izumi, M Nishikino, M Heya, K Fujita, Y Ochi, K Shigemori, N Ohnishi, M Murakami, K Nishihara, R Ishizaki, H Takabe, K Nagai, T Norimatsu, M Nakatsuka, T Yamanaka, S Nakai, C Yamanaka, K Mima

NUCLEAR FUSION 40 ( 3Y ) 547 - 556 2000.3

A new fusion capsule drive scheme was investigated. The capsule is illuminated by a low intensity thermal X ray pulse prior to the main drive pulse. This leads to a noticeable suppression of initial imprinting by the drive beam because of thermal smoothing in the preformed plasma. Of the several types of indirect-direct hybrid target, the authors investigated the hybrid effect for two types. One is a foam hybrid, in which the fuel capsule has a low density foam layer attached directly on its surface and where pulsed radiation generated from a thin high Z layer on the foam propagates through the foam, creating a preformed plasma. The other is an external hybrid, in which the capsule is illuminated by external X ray radiation generated using different beams from the capsule drive beams. The hybrid effect was demonstrated for both types by imposing an initial imprint on a planar target with an intensity modulated beam, and subsequent non-uniformity growth due to Rayleigh-Taylor instability was observed by means of face-on backlighting. The observed suppression due to the presence of the preformed plasma is interpreted by a cloudy day model for both hybrids. Capsule implosion experiments have also been started. The overall implosion dynamics observed is replicated by 1-D hydrocode simulations. Preliminary results from the implosion experiments are presented.

Hydrodynamic model experiment of the collision of supernova 1987A with its circumstellar ring using high power laser Reviewed

YG Kang, H Nishimura, H Takabe, H Azechi, T Norimatsu, M Nakai, H Nagatomo, A Sunahara, K Fujita, M Nakatsuka, K Mima, HG Kim, HJ Kong

HIGH-POWER LASERS IN ENERGY ENGINEERING 3886 489 - 495 2000

Laboratory simulation of the ejecta-ring collision of Supernova 1987A has been performed by using a high power laser system. Pure hydrodynamic modeling was applied using the invariance of the Euler equations and the experiment was designed with one-dimensional hydrodynamic simulations. Three laser beams of lambda = 0.53 mu m, at the intensity I = 2 x 10(14) W/cm(2) irradiated a CH foil to generate a strong shock wave and subsequent dense plasma flow in a low density CHO foam including a solid sphere at its center. Shock wave propagation and plasma flow with vortex ring - like structures were observed with gated x-ray radiography. Overall hydrodynamic behaviors are consistent with one- and two-dimensional hydrodynamic simulations. The collision dynamics is dominated by a complicated interplay of reflected shock, diffracted shock, transmitted shock waves, and plasma flow behind the main shock passage.

Indirect/direct-hybrid drive implosion experiments with x-ray pre-irradiation Reviewed

M Nishikino, H Shiraga, N Miyanaga, H Nishimura, K Shigemori, M Heya, H Azechi, M Nakai, K Mima, H Takabe, S Nakai, T Yamanaka

HIGH-POWER LASERS IN ENERGY ENGINEERING 3886 465 - 472 2000

Indirect/direct-hybrid drive scheme to suppress the initial imprint of the laser irradiation nonuniformities has been proposed and investigated as a new drive scheme for inertial fusion. In direct drive inertial confinement fusion, initial imprinting of laser irradiation nonunifornity is considered to cause seeding of the perturbation on target surface in the very beginning of the irradiation which may be amplified by Rayleigh-Taylor instability in the acceleration phase of the implosion and be deleterious to efficient heating of the hot spark at the center of the compressed fuel core plasma. In indirect/direct-hybrid drive scheme, the target is first irradiated very uniformly with low-intensity soft x-ray prepulse(indirect drive) from external sources apart from the target. Indirect x-ray pre-irradiation of the surface causes a pre-expansion layer of the plasma before the irradiation of the direct drive laser beam. When the drive beam(direct drive) comes later, the target has a substantial stand-off distance between the ablation front and the beam absorption region. Thus the thermal smoothing effect is expected to occur in this transport layer, and then, the initial imprint can be significantly reduced. We have demonstrated planar target experiments on the indirect/direct hybrid scheme and observed reduction of the initial imprint. Implosion experiments the indirect/direct hybrid drive spherical capsules with external x-ray sources has been started. Overall implosion was performed successfully.

Direct measurement of laser irradiation uniformity of fusion pellets by the use of X-ray frame images Reviewed

H Honda, H Nishimura, M Nakai, N Miyanaga, K Tsubakimoto, H Shiraga, H Azechi, H Takabe, K Nishihara, S Nakai, K Mima

FUSION ENGINEERING AND DESIGN 44 137 - 140 1999.2

Time-resolved 2-dimensional (2D) X-ray images were taken to evaluate laser irradiation non-uniformity of fusion pellets. X-ray distributions corresponding to the drive laser geometry were obtained on the circumference of the image of a gold-coated surrogate target. The experimental results were analyzed to derive the irradiances by using an experimentally-determine d relation between the laser irradiation intensity and the X-ray intensity. The analyzed results were compared with a laser illumination calculation in which temporally varying laser beams were projected onto a target sphere. A reasonable agreement between the measurement and calculation has been obtained. (C) 1999 Elsevier Science S.A. All rights reserved.

Effects of non-local electron thermal transport on ablative Rayleigh-Taylor instability Reviewed

M Honda, K Mima, K Shigemori, M Nakai, H Azechi, A Nishiguchi

FUSION ENGINEERING AND DESIGN 44 205 - 208 1999.2

A two-dimensional full Fokker-Planck (FP) simulation code coupled with an ideal fluid equation has been constructed. This code was applied for the numerical simulation of the Rayleigh-Taylor (RT) instability on the directly driven ablation front. The simulation results show that an accelerated thin foil is preheated by the non-local electron heat transport, the ablation front density is depleted, and the linear growth rate of the RT instability is suppressed strongly. By investigating the mode structure in the simulation, it is found that the peak of the eigen mode shifts toward the corona region by the non-local heat conduction effects. This is the reason why the growth rate is reduced. (C) 1999 Elsevier Science S.A. All rights reserved.

Hydrodynamic perturbation growth in the start-up phase Reviewed

R Ishizaki, K Nishihara, JG Wouchuk, K Shigemori, M Nakai, H Azechi, K Mima

FUSION ENGINEERING AND DESIGN 44 199 - 203 1999.2

A simple analytical model is presented to study hydrodynamic perturbation growth in the start-up phase in laser fusion, namely propagation of a rippled shock driven by non-uniform laser ablation induced by initial target roughness or non-uniform laser irradiation. These perturbation growths are very important because they seed the Rayleigh-Taylor (RF) instability in the subsequent acceleration and stagnation phases. We investigate temporal evolutions of the shock front and the ablation surface. In the result, we have found that the shock front ripples oscillate and decay in both cases of uniform laser irradiation on a target with a rippled surface and non-uniform laser irradiation on a smooth target surface. On the other hand, we obtain that there is the asymptotic value of the ablation surface deformation in the former case and the asymptotic growth rate of the ablation surface ripple in the latter case. Approximate formulas expressing both temporal evolutions of the shock front and the ablation surface are obtained in the weak shock limit. These formulas are also applicable in a relatively strong shock. We also investigate the case of oscillating non-uniform laser irradiation with time. The oscillation frequency dependences of the shock front ripple and the growth rate of the ablation surface ripple are discussed. (C) 1999 Elsevier Science S.A. All rights reserved.

Rippled shock propagation and hydrodynamic perturbation growth in laser implosion Reviewed

R Ishizaki, K Nishihara, JG Wouchuk, K Shigemori, M Nakai, N Miyanaga, H Azechi, K Mima

JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 85 ( 1-3 ) 34 - 38 1999.1

A simple analytical model is presented to study hydrodynamic perturbation growth in the start-up phase in laser fusion, namely propagation of a rippled shock driven by nonuniform laser ablation induced by initial target roughness or nonuniform laser irradiation. These perturbation growths are very important, because they seed the Rayleigh-Taylor instability in the subsequent acceleration and stagnation phases. We investigate temporal evolutions of the shock front and the ablation surface. As a result, we have found that the shock front ripples oscillate and decay in both the case of uniform laser irradiation on a target with a rippled surface and that of nonuniform laser irradiation on a smooth target. It was also found that there is an asymptotic value of the ablation surface deformation in the former case and an asymptotic growth rate of the ablation surface ripple in the latter case. Approximate formulae expressing both temporal evolutions of the shock front and the ablation surface are obtained in the weak shock limit. These formulae are also applicable in a relatively strong shock. We also investigate the case of oscillating nonuniform laser irradiation with time. The oscillation frequency dependences of the shock front ripple and the growth rate of the ablation surface ripple are discussed. (C) 1999 Elsevier Science S.A. All rights reserved.

Progress of direct drive laser fusion research at ILE, Osaka Reviewed

K Mima, H Azechi, H Fujita, N Izumi, T Jitsuno, Y Kato, T Kanabe, Y Kitagawa, R Kodama, N Miyanaga, M Nakai, M Nakatsuka, S Nakai, H Nagatomo, T Norimatsu, K Nishihara, H Nishimura, H Shiraga, H Takabe, T Yamanaka

THIRD INTERNATIONAL CONFERENCE ON SOLID STATE LASERS FOR APPLICATION TO INERTIAL CONFINEMENT FUSION, PTS 1 AND 2 3492 34 - 41 1999

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 researches 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.

Measurements of mass ablation rate of laser-irradiated target by the face-on x-ray backlighting technique

K Shigemori, H Azechi, M Nakai, K Mima

REVIEW OF SCIENTIFIC INSTRUMENTS 69 ( 11 ) 3942 - 3944 1998.11

We report a novel method to measure the value of mass ablation rate per unit surface (m(over dot))in laser-irradiated targets. The face-on x-ray backlighting technique was utilized for the measurement. Flat polystyrene foils were irradiated by the partially coherent light at the intensity of 0.7 or 1.4 x 10(14) W/cm(2). We observed the residual areal mass of the planar targets to deduce temporally resolved mass ablation rate. Observed mass ablation rates are well reproduced both by the steady-state ablation model and by one-dimensional simulation. (C) 1998 American Institute of Physics. [S0034-6748(98)02111-X].

Comment on "Measurements of Rayleigh-Taylor growth rate of planar targets irradiated directly by partially coherent light" - Reply Reviewed

K Shigemori, H Azechi, M Nakai, H Takabe, K Mima

PHYSICAL REVIEW LETTERS 80 ( 15 ) 3415 - 3415 1998.4

Recent progress on diagnostic developments for ICF research at IEE Osaka Reviewed

H Nishimura, H Azechi, M Nakai, N Miyanaga, H Shiraga, R Kodama, N Izumi, M Heya, H Honda, K Fujita, Y Ochi, H Takabe, T Yamanaka, Y Kato, K Mima

PLASMA PHYSICS REPORTS 24 ( 2 ) 114 - 122 1998.2

A variety of novel diagnostic instruments and methods have been developed for use in laser fusion research at the Institute of Laser Engineering (ILE), Osaka University. Our efforts are particularly concentrated on development of a diagnostic method capable of high spatial, temporal, and energy resolutions, which, at the same time, makes it possible to observe precisely implosion stability and hot spark formation at the center of compressed plasma. Present status of the X-ray and particle diagnostics development is overviewed.

Rippled Shock Propagation and Hydrodynamic Perturbation Growth in Laser Implosion

R. Ishizaki, K. Nishihara, J. G. Wouchuk, K. Shigemori, M. Nakai, N. Miyanaga, H. Azechi, K. Mima

Review of High Pressure Science and Technology/Koatsuryoku No Kagaku To Gijutsu 7 930 - 932 1998

A simple analytical model is presented to study hydrodynamic perturbation growths induced by nonuniform laser ablation in the start-up phase in laser fusion. Namely, propagation of a rippled shock and deformation of an ablation surface caused by nonuniform laser ablation are studied for cases of initial target roughness and nonuniform laser irradiation. The perturbation growth is very important because it seeds the Rayleigh-Taylor instability in the subsequent acceleration and stagnation phases. In result, it can be shown that high intensity of laser irradiation promotes the ablation surface to distort and short wavelength of laser inhibits its deformation. Those behaviors are also investigated in the case that nonuniformity of laser irradiation oscillates with time. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.

Direct-drive hydrodynamic instability experiments on the GEKKO XII laser Reviewed

H Azechi, M Nakai, K Shigemori, N Miyanaga, H Shiraga, H Nishimura, M Honda, R Ishizaki, JG Wouchuk, H Takabe, K Nishihara, K Mima, A Nishiguchi, T Endo

PHYSICS OF PLASMAS 4 ( 11 ) 4079 - 4089 1997.11

Hydrodynamic instabilities, such as the Rayleigh-Taylor (R-T) instability, play a critical role in inertial confinement fusion as they finally cause fuel-pusher mixing that potentially quenches thermonuclear ignition. Good understanding of the instabilities is necessary to limit the mixing within a tolerable level. A series of experiments has been conducted on the GEKKO XII laser facility [C. Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)] to measure hydrodynamic instabilities in planar foils directly irradiated by 0.53 mu m laser light. It has been found that (1) the imprint is reasonably explained by an imprint model based on the equation of motion with the pressure perturbation smoothed by the cloudy-day effect, and (2) the experimental R-T growth rate is significantly reduced from the classical growth rate due probably to ablative stabilization enhanced by nonlocal heat transport. (C) 1997 American Institute of Physics. [S1070-664X(97)0-2811-5].

Three-dimensional imaging of laser-imploded targets using X-ray computed tomography technique Reviewed

YW Chen, N Miyanaga, M Yamanaka, M Nakai, T Yamanaka, S Nakai

IEEE TRANSACTIONS ON NUCLEAR SCIENCE 44 ( 3 ) 890 - 893 1997.6

Irradiation uniformity measurement of laser fusion pellets by an X-ray imaging method Reviewed

H Honda, H Nishimura, S Miyamoto, M Nakai, K Kondo, N Miyanaga, K Tsubakimoto, H Azechi, S Nakai, K Mima

FUSION ENGINEERING AND DESIGN 34-35 197 - 200 1997.3

Time integrated X-ray emission images at an energy range of 1.8-3.6 keV were obtained to evaluate laser irradiation nonuniformity on fusion pellets. The experimental results were analyzed with a simple illumination calculation. A positive correlation was found between the nonuniformity obtained from the measured X-ray emission intensity and that from the calculated laser irradiation intensity, although the absolute values of the nonuniformities are discrepant. (C) 1997 Elsevier Science S.A.

Measurements of Rayleigh-Taylor growth rate of planar targets irradiated directly by partially coherent light

K Shigemori, H Azechi, M Nakai, M Honda, K Meguro, N Miyanaga, H Takabe, K Mima

PHYSICAL REVIEW LETTERS 78 ( 2 ) 250 - 253 1997.1

We report the first measurements of laser-driven Rayleigh-Taylor instability at the linear growth regime in an imprint-free condition. Initially rippled polystyrene foils were irradiated by the partially coherent light. We measured temporal evolution of areal-density perturbation by the face-on x-ray backlighting technique. The observed maximum growth factor was nearly 50. We also measured the acceleration of the irradiated foil. The growth rate was approximately 50% of the classical growth rate. These experimental results were well reproduced by a two-dimensional simulation coupled with the Fokker-Planck treatment of electron heat transport.

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Papers - Nakai Mitsuo