Faculty Profiles - Nakai Mitsuo

Papers - Nakai Mitsuo

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Experimental Investigation of Aerosol Formation in Laser Fusion Reactor Chamber by Discharge Method Reviewed

T. Ohshige, T. Norimatsu, H. Furukawa, M. Nakai, K. Nagai, K. Mima

5TH INTERNATIONAL CONFERENCE ON INERTIAL FUSION SCIENCES AND APPLICATIONS (IFSA2007) 112 2008

Fast response neutron scintillation detector for FIRE-X Reviewed

Y. Arikawa, M. Nakai, T. Watari, H. Hosoda, K. Takeda, T. Fujiwara, Y. Furukawa, T. Norimatsu, H. Shiraga, N. Sarukura, H. Azechi

5TH INTERNATIONAL CONFERENCE ON INERTIAL FUSION SCIENCES AND APPLICATIONS (IFSA2007) 112 2008

We have been developing fast responding neutron detectors with a view to revealing the effect of additional laser heating in FIREX-I (Fast Ignition Realization Experiment) by measuring the burn time with a time resolution under 100 ps at the relatively low neutron yield (about10(6)). The detector is constructed with a thin plastic-scintillaotor (BC-422), a micro-channel-plate photomultiplier tube (MCP-PMT) and a bundle optical fiber as a light-guide. The rise time of a neutron signal from the detector is measured to be 220 ps. The time-determination error for measuring burn time is estimated to be less the +/- 46 ps from the data of characterization experiments measuring the transit time distribution of signals, and calculated values of Doppler broadening and the uncertainty of the scintillation time due to the thickness of the scintillator. In the future with more neutron yield, we will construct a scintillatiion-fiber-streak camera to detect burn history.

Fabrication and Characterization of Planar Cryogenic Targets for GEKKO-XII Reviewed

T. X. Huang, M. Nakai, K. Shigemori, H. Shiraga, T. Sakaiya, T. Watari, K. Ohtani, T. Shiota, K. Takeda, O. Maegawa, H. Azechi, M. Nagata, T. Norimatsu, K. Nagai, K. Mima, Y. Izawa, T. Mito, K. Iwamoto

5TH INTERNATIONAL CONFERENCE ON INERTIAL FUSION SCIENCES AND APPLICATIONS (IFSA2007) 112 2008

A planar cryogenic target system was built to simulate the ablator+fuel shell physics in ICF experimental studies. In the center of the target cell, a sandwich target of liquid deuterium was formed, which has open geometry for conventional side-on and face-on backlighting diagnostics. A set of double-flanges comprised a sealing flange and an extra flange, with which the target thickness could be changed using proper spacers. Furthermore, an interferometric method was used to characterize the cryogenic target before laser shot. The in situ target thickness and bulging shape could be obtained with acceptable accuracy. A typical planar cryogenic target was formed with diameter of 2 mm, which comprised three parts: a 7-m thick polyimide film, a layer of liquid deuterium with the thickness of 119 mu m at the center and 100 mu m at the edge, and another 7-mu m thick polyimide film.

Neutron generation from Impact Fast Ignition Reviewed

T. Watari, T. Sakaiya, H. Azechi, M. Nakai, H. Shiraga, K. Shigemori, H. Hosoda, H. Saito, Y. Arikawa, Y. Sakawa, S. Fujioka, Y. Hironaka, M. Murakami, M. Karasik, J. Gardner, J. Bates, D. Colombant, J. Weber, S. Obenschain, Y. Aglitsky, P. A. Norreys, S. Eliezer, K. Mima

5TH INTERNATIONAL CONFERENCE ON INERTIAL FUSION SCIENCES AND APPLICATIONS (IFSA2007) 112 2008

We have proposed a new ignition scheme of Fast Ignition, called "Impact Fast Ignition [1] (IFI)", in which a compressed fuel is ignited by impact collision of a fragment of separately imploded fuel. We have started experiments that used CD colliding foils as the fundamental experiment and integrated experiment of the IFI. In the fundamental experiment, we used targets which are consists of two CD foils (thickness of 20 m) with a 600 m separation. We have irradiated one of the CD foils by laser beams of the energy of 2 kJ in total and accelerated. The accelerated CD foil collides with another foil, and neutrons were generated by the nuclear fusion reaction on the heated foil. In this experiment, we measured the neutron yield of 10(6). In the integrated experiment, we used CD shell targets with a gold cone which had a hemispherical impactor (Fig. 1). The shell was imploded using 9 beams and the impactor was accelerated using 3 beams of the GEKKO XII laser system. The laser energy was 350 J per beam. Observed maximum neutron yield was 2x10(6). This yield was 80 times as large as that without impactor. We will present the experimental details and results.

Application of bubble detector in FIREX program Reviewed

Osamu Maegawa, Takashi Fujiwara, Mitsuo Nakai, Hiroyuki Shiraga

5TH INTERNATIONAL CONFERENCE ON INERTIAL FUSION SCIENCES AND APPLICATIONS (IFSA2007) 112 2008

In a Fast Ignition Realization Experiment (FIREX), measurement of the primary and the secondary neutron yield is one of the key diagnostics. However, gamma-ray background generated by an ultra-short pulse laser irradiation makes it difficult to measure the neutron yield in the fast ignition experiment. We adopted a bubble neutron detector as a promising gamma-insensitive detector to be developed and started to examine the feasibility of the detector using commercially available dosimeters provided from BTI inc. [1].

Solution viscosity adjustable phloroglucinolcarboxylic acid / formaldehyde applied in extremely thin shell fusion target fabrication Reviewed

H. Yang, K. Nagai, M. Nakai, T. Norimatsu

5TH INTERNATIONAL CONFERENCE ON INERTIAL FUSION SCIENCES AND APPLICATIONS (IFSA2007) 112 2008

Capsuliform fusion target was prepared by using the O(O) / W / O(I) emulsion process. Phloroglucinolcarboxylic acid / formaldehyde (PF) is expected to be used as the water phase (W) solution which then gelates to be the shell of the capsule. The density of the shell should be low to obtain high laser gain. Phloroglucinolcarboxylic acid had addition and condensation reaction with formaldehyde under basic condition to form linear polymer first. The nano-scale linear polymers were then linked via noncovalent interactions, such as hydrogen bonding, van der Waals, to continue the gelation process. The type and amount of the base catalyst used in the reaction can affect the polymerization of PF, including the gelation rate and gelation concentration. By changing the basic condition of the reaction, the proper viscosity of 9x10(-5) for capsuliform target fabrication was reached and extremely thin gel shell thickness of 17 mu m with low polymer concentration of 26.3mg/cm(3) was obtained.

Temperature Measurement of Preheated Planar-Cryogenic Targets Reviewed

K. Otani, K. Shigemori, D. Ichinose, T. Sakaiya, A. Sunahara, Y. Sakawa, M. Nakai, H. Shiraga, H. Azechi, K. Mima

5TH INTERNATIONAL CONFERENCE ON INERTIAL FUSION SCIENCES AND APPLICATIONS (IFSA2007) 112 2008

The temperature measurement of preheated target is reported. Inertial confinement fusion (ICF) targets should be imploded with a low adiabat for achievement of high density compression. The high energy photons and electrons from ablation plasma can preheat the uncompressed fuel and disturb the low adiabat compression. Preheat has to be evaluated and suppressed for ICF targets. The planar target of cryogenic deuterium was developed for measurement of the self emission from the rear surface. Experiment indicated that the x-ray preheating is more effective than that of non-local electron heat transport at the laser intensity of 10(14) W/cm(2).

Study on a fuel layering sequence of the foam target for the FIREX project Reviewed

Iwamoto, A, Fujimura, T, Nakai, M, Nagai, K, Norimatsu, T, Maekawa, R, Mito, T, Sakagami, H, Motojima, O, Azechi, H, Mima, K

5TH INTERNATIONAL CONFERENCE ON INERTIAL FUSION SCIENCES AND APPLICATIONS (IFSA2007) 112 32067 2008

Reduction of the Rayleigh-Taylor instability growth with cocktail color irradiation Reviewed

K. Otani, K. Shigemori, T. Sakaiya, S. Fujioka, A. Sunahara, M. Nakai, H. Shiraga, H. Azechi, K. Mima

PHYSICS OF PLASMAS 14 ( 12 ) 2007.12

A novel method for reducing the Rayleigh-Taylor instability (RTI) growth in inertial confinement fusion (ICF) targets is reported. It is well known that high-density compression of ICF targets is potentially prevented by the RTI. Previous studies [K. Shigemori , Phys. Rev. Lett. 78, 250 (1997), S. G. Glendinning , Phys. Rev. Lett. 78, 3318 (1997), and H. Azechi , Phys. Plasmas 4, 4079 (1997)] have indicated that nonlocal electron heat transport enhances the effect on the ablative stabilization of the RTI growth with long wavelength laser irradiation. Planar target experiments, using a small fraction of a long wavelength laser (lambda=0.53 or 1.05 mu m) in addition to the main drive laser (lambda=0.35 mu m), were conducted to verify the RTI reduction by inducing the effect of the nonlocal electron heat transport. The measured RTI growth rate for this "cocktail-color" laser irradiation was clearly reduced from that for the "single-color" short-wavelength laser irradiation. The experimental growth factors are in good agreement with the ablative RTI formula coupled with a one-dimensional Fokker-Planck simulation code. (c) 2007 American Institute of Physics.

Recent results and future prospects of laser fusion research at ILE, Osaka Reviewed

K. Mima, K. A. Tanaka, R. Kodama, T. Johzaki, H. Nagatomo, H. Shiraga, N. Miyanaga, M. Murakami, H. Azechi, M. Nakai, T. Norimatu, K. Nagai, T. Taguchi, H. Sakagami

EUROPEAN PHYSICAL JOURNAL D 44 ( 2 ) 259 - 264 2007.8

Reviewed are the present status of the fast ignition researches. Since 1997, the fast ignition experiment and theory researches have been extensively continued at the Institute of Laser Engineering of Osaka University. In particular, the cone-shell target experiments and simulation research have been progressing. In order to demonstrate heating of imploded high density plasma to the ignition temperature, in the April of 2003, the construction of heating laser of 10 kJ/10 ps/1.06 mu m (Laser for Fusion Experiment; LFEX), for FIREX-I (Fast Ignition Realization Experiment) has started. The fabrication of DT foam cryogenic cone target is also under development as a collaboration program between Osaka University and NIFS (National Institute for Fusion Science). The LFEX will be completed in 2008. After the completion of LFEX, the foam cryogenic cone shell target experiment will start in 2008. As a new approach toward a compact ignition, an impact fusion has been proposed, where the ablative acceleration to the order of 10(8) cm/s is the key issue. The ablation acceleration related to the impact fusion has been explored by experiments.

Foam structure of xerogel prepared via ring-opening reaction between epoxy groups attached on the side chain of polystyrene Reviewed

Kentaro Yamanaka, Keiji Nagai, Nobukatsu Nemoto, Kaori Nomura, Tomonori Shimoyama, Kei Tanji, Tomoya Tanji, Mitsuo Nakai, Takayoshi Norimatsu

FUSION SCIENCE AND TECHNOLOGY 51 ( 4 ) 665 - 672 2007.5

This paper deals with a new foam material containig polysytrene and its oxirane derivative. A monomer, 4-vinylphenyloxirane (MI), was prepared from 4-chlorostyrene. Polystyrene-based copolymers using styrene and M1 were prepared by free radical copolymerization using azo-bis(isobutyronitrile) (AIBN) as an initiator. The solutions of the obtainedpolystyrene-based copolymers in 4-chlorotoluene were gelated by the addition of a cationic initiator, which caused crosslinking via ring-opening polymerization of the pendant cyclic moieties. SEM images of the dried gel show various foam structures. The formation mechanism of the micro- and nano-structure was explained from the view point of the affinity of the monomer unit and the solvent. The homopolymer of 4-vinyphenylolxirane showed the finest and most uniform structure.

Laser machining of RF foam by second harmonics of Nd : Yag laser Reviewed

Takeshi Fujimura, Takayoshi Norimatsu, Mitsuo Nakai, Keiji Nagai, Akifumi Iwamoto, Kunioki Mima

FUSION SCIENCE AND TECHNOLOGY 51 ( 4 ) 677 - 681 2007.5

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.

Preliminary results of fuel layering on the cryogenic target for the firex project Reviewed

Iwamoto, A, Maekawa, R, Mito, T, Sakagami, H, Motojima, O, Nakai, M, Nagai, K, Fujimura, T, Norimatsu, T, Azechi, H, Mitna, K

FUSION SCIENCE AND TECHNOLOGY 51 ( 4 ) 753 - 757 2007.5

Comprehensive diagnosis of growth rates of the ablative Rayleigh-Taylor instability Reviewed

H. Azechi, T. Sakaiya, S. Fujioka, Y. Tamari, K. Otani, K. Shigemori, M. Nakai, H. Shiraga, N. Miyanaga, K. Mima

PHYSICAL REVIEW LETTERS 98 ( 4 ) 2007.1

The growth rate of the ablative Rayleigh-Taylor instability is approximated by gamma=root kg/(1+kL)-beta km/rho(a), where k is the perturbation wave number, g the gravity, L the density scale length, m the mass ablation rate, and rho(a) the peak target density. The coefficient beta was evaluated for the first time by measuring all quantities of this formula except for L, which was taken from the simulation. Although the experimental value of beta=1.2 +/- 0.7 at short perturbation wavelengths is in reasonably good agreement with the theoretical prediction of beta=1.7, it is found to be larger than the prediction at long wavelengths.

Encapsulation of low density materials for the first stage of fast ignition realization experiment (FIREX-I) - Control of microstructure and gelation process using a phase-transfer catalyst and tailored polymers Reviewed

Keiji Nagai, F. Ito, H. Yang, A. Iwamoto, M. Nakai, H. Homma, H. Shiraga, H. Azechi, K. A. Tanaka, R. Kodama, T. Norimatsu, K. Mima

22ND IEEE/NPSS SYMPOSIUM ON FUSION ENGINEERING 33 - + 2007

Development of foam capsule fabrication for cryogenically cooled fuel targets is overviewed in the present paper. The fabrication development was initiated as a part of the Fast Ignition Realization Experiment (FIREX) Project at the ILE, Osaka University in the way of bilateral collaboration between Osaka University and National Institute for Fusion Science (NIFS). For the first stage of FIREX (FIREX-1), a foam cryogenic target was designed where low-density foam shells with a conical fight guide will be cooled down to the cryogenic temperature and will be fueled through a narrow pipe. The required diameter and thickness of the capsule are 500 pm and 20 [km, respectively. The material should be low-density plastics foam. We have prepared such capsules using 1) new material of (phloroglucinolcarboxylic acid)/formalin resin to control kinematic viscosity of the precursor, 2) phase-transfer-catalyzed gelation process to keep density matching of three phases of the emulsion. 3) non-volatile silicone oil as outer oil of emulsion in order to prevent hazard halogenated hydrocarbon and flammable mineral oil. The obtained foam capsule had fine structure of 180 nm (outer surface) to 220 nm (inner surface) and uniform thickness reaching to resolution limit of optical analysis (similar to 0.5 mu m). A small hole was made before the solvent exchange and the drying process to prevent distortion due to volume changes. The density of dried foam was 0.29 g/cm(3). After attaching the petawatt laser guiding cone and fueling glass tube, poly([2,2]paracyclophane) was coated on the foam surface and supplied for a fueling test of cryogenic hydrogen.

Progress of impact fast ignition

M. Murakami, H. Nagatomo, T. Sakaiya, S. Fujioka, H. Saito, H. Shiraga, M. Nakai, K. Shigemori, H. Azechi, M. Karasik, J. Gardner, J. Bates, D. Colombant, J. Weaver, A. Schmitt, A. Velikovich, Y. Aglitskiy, J. Sethian, S. Obenshain

34th EPS Conference on Plasma Physics 2007, EPS 2007 - Europhysics Conference Abstracts 31 ( 1 ) 21 - 24 2007

Fabrication of low-density solid xenon as laser-produced plasma extreme ultraviolet source Reviewed

Mizuho Nagata, Takayoshi Norimatsu, Mitsuo Nakai, Keiji Nagai, Nobuyoshi Ueda, Shinsuke Fujioka, Tatsuya Aota, Hiroaki Nishimura, Katsunobu Nishihara, Noriaki Miyanaga, Yasukazu Izawa, Kunioki Mima

JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 45 ( 33-36 ) L884 - L886 2006.9

A low-density solid xenon (Xe) target has been developed as laser-produced plasma (LPP) extreme ultraviolet (EUV) source. Xe gas was cooled to below the condensation temperature, and liquid Xe was sprayed from a swirl atomizer into a low-pressure atmosphere. Microparticles of liquid Xe were rapidly cooled by their own latent heat of evaporation, and became a solid micrometer-sized powder. The most frequent particle diameter under an atomizing pressure of 0.3 MPa was 114 mu m, and the apparent density was 0.2 g/cm(3) 1/13 of the density of conventional solid Xe. The apparent production rate of the powder was 2.2 cm(3)/s.

Extreme ultraviolet emission from laser-irradiated low-density Xe targets Reviewed

Nobuyoshi Ueda, Mizuho Nagata, Hiroaki Nishimura, Shinsuke Fujioka, Tatsuya Aota, Yuzuri Yasuda, Yuichi Inubushi, Tsuyoshi Ando, Takayoshi Norimatsu, Mitsuo Nakai, Keiji Nagai, Katsunobu Nishihara, Atsushi Sunahara, Noriaki Miyanaga, Yasukazu Izawa, Kunioki Mima

JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 45 ( 7 ) 5951 - 5953 2006.7

The effect of the initial density of xenon (Xe) targets on extreme ultraviolet (EUV) emission has been investigated. With decreasing initial density, it was found that the spectral width around 11 nm becomes broad, and the intensity of 13.5 nm emission increases. The maximum conversion efficiency (CE) for solid Xe targets is approximately 0.6% at a 2 x 10(11) W/cm(2) of laser intensity, whereas that for the low-density targets is approximately 0.7% at a 4 x 10(11) W/cm(2). The spectral broadening with decreasing initial density can be attributed to the change in optical thickness of laser-produced plasmas. The enhancement of CE is attributed to optically thick plasma formation.

Conceptual design of laser fusion reactor KOYO-fast - Target design and the fueling system Reviewed

T. Norimatsu, T. Johzaki, H. Azechi, K. Mima, Y. Nakao, H. Sakagami, H. Shiraga, R. Kodama, H. Nagatomo, T. Endo, A. Iwamoto, H. Yoshida, M. Nakai

JOURNAL DE PHYSIQUE IV 133 841 - 843 2006.6

Preliminary results on the cryogenic target for FIREX project Reviewed

A. Iwamoto, R. Maekawa, T. Mito, M. Okamoto, O. Motojima, M. Nakai, T. Norimatsu, K. Nagai

JOURNAL DE PHYSIQUE IV 133 899 - 901 2006.6

Preliminary tests on the cryogenic target for the fast ignition realization experiment (FIREX) project has been conducted. A foam shell method is proposed to realize its target design. A foam target consists of three parts: a foam shell, a conical laser guide and a liquid or gas feeder made of glass. The shell is a hollow sphere (500 pm in diameter) with a uniform and thin foam layer (similar to 20 mu m in thickness). Epoxy resin is utilized to assemble the parts into the target. Regarding target fabrication, one of the concerns is the influence on various thermal contractions from the different materials when it is cooled down to cryogenic environment. This paper describes the result on the validity check at cryogenic environment and the demonstration of H-2 liquefaction using a dummy target instead of the foam target.

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Papers - Nakai Mitsuo