Faculty Profiles - Nakai Mitsuo

Papers - Nakai Mitsuo

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Characterization of extreme ultraviolet emission from laser-produced spherical tin plasma generated with multiple laser beams Reviewed

Y Shimada, H Nishimura, M Nakai, K Hashimoto, M Yamaura, Y Tao, K Shigemori, T Okuno, K Nishihara, T Kawamura, A Sunahara, T Nishikawa, A Sasaki, K Nagai, T Norimatsu, S Fujioka, S Uchida, N Miyanaga, Y Izawa, C Yamanaka

APPLIED PHYSICS LETTERS 86 ( 5 ) 2005.1

Spherical solid tin targets were illuminated uniformly with twelve beams from the Gekko XII laser system to create spherical plasmas, and the extreme ultraviolet (EUV) emission spectra from the plasmas were measured. The highest conversion efficiency of 3% to 13.5 nm EUV light in 2% bandwidth was attained for an irradiance of around 5 x 10(10) W/cm(2). The experimental results were reproduced fairly well using a theoretical model taking the power balance in the plasma into consideration. (C) 2005 American Institute of Physics.

Petawatt-laser direct heating of uniformly imploded deuterated-polystyrene shell target

Yoneyoshi Kitagawa, Yasuhiko Sentoku, Shin Akamatsu, Wataru Sakamoto, Kazuo A. Tanaka, Ryosuke Kodama, Hiroaki Nishimura, Yuichi Inubushi, Mitsuo Nakai, Takeshi Watari, Takayoshi Norimatsu, Atsushi Sunahara

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 71 ( 1 ) 2005.1

A uniformly imploded deuterated polystyrene (CD) shell target is fast-heated by a Petawatt (PW) laser without cone guide. The best illumination timing is found to be in a narrow region around 80±20 picoseconds from the onset of the stagnation phase, where thermal neutrons are enhanced four to five times by the PW laser of energy less than 10% of the implosion laser. The timing agrees with the timings of enhancement of the x-ray emission from the core and reduction of the bremsstrahlung radiation from scattered hot electrons. The PW laser, focused to the critical density point, generates the energetic electrons within as narrow an angle as 30°, which then heats the imploded CD shell to enhance thermal neutrons. These results first demonstrate that the PW laser directly heats the imploded core without any conelike laser guide. © 2005 The American Physical Society.

Petawatt-laser direct heating of uniformly imploded deuterated-polystyrene shell target Reviewed

Y Kitagawa, Y Sentoku, S Akamatsu, W Sakamoto, KA Tanaka, R Kodama, H Nishimura, Y Inubushi, M Nakai, T Watari, T Norimatsu, A Sunahara

PHYSICAL REVIEW E 71 ( 1 ) 2005.1

A uniformly imploded deuterated polystyrene (CD) shell target is fast-heated by a Petawatt (PW) laser without cone guide. The best illumination timing is found to be in a narrow region around 80 20 picoseconds from the onset of the stagnation phase, where thermal neutrons are enhanced four to five times by the PW laser of energy less than 10% of the implosion laser. The timing agrees with the timings of enhancement of the x-ray emission from the coreand reduction of the bremsstrahlung radiation from scattered hot electrons. The PW laser, focused to the critical density point, generates the energetic electrons within as narrow an angle as 30degrees, which then heats the imploded CD shell to enhance thermal neutrons. These results first demonstrate that the PW laser directly heats the imploded core without any conelike laser guide.

Equation-of-state measurements for polystyrene at multi-TPa pressures in laser direct-drive experiments Reviewed

N. Ozaki, T. Ono, K. Takamatsu, K. A. Tanaka, M. Nakano, T. Kataoka, M. Yoshida, K. Wakabayashi, M. Nakai, K. Nagai, K. Shigemori, T. Yamanaka, K. Kondo

Physics of Plasmas 12 ( 12 ) 1 - 4 2005

Equation-of-state (EOS) measurements for polystyrene in TPa (10 Mbar) pressure regions are presented. Polystyrene Hugoniot data were obtained up to 2.7 TPa using impedance matching techniques with laser direct drive at the GEKKO/HIPER laser facility [N. Miyanaga, in Proceedings of the 18th International Conference on Fusion Energy (IAEA, Sorrento, Italy, 2001), IAEA-CN-77] The results were compared with theoretical models and previous experimental data and found to be in good agreement with the previous data obtained by different drive and diagnostic techniques. © 2005 American Institute of Physics.

Petawatt laser direct heating of imploded plasmas and thermal neutron enhancement

Yoneyoshi Kitagawa, Shin Akamatsu, Wataru Sakamoto, Kazuo A. Tanaka, Ryosuke Kodama, Hiroaki Nishimura, Yuichi Inubushi, Mitsuo Nakai, Takeshi Watari, Takayoshi Norimatsu, Atsushi Sunahara, Yasuhiko Sentoku

Inertial Fusion Sciences and Applications 2003 387 - 393 2004.12

To heat an uniformly imploded deuterated polystyrene shell target, Petawatt laser illumination timing is scanned from the implosion, through stagnation to expansion or disruption phases. After 80 ± 20 picosecond from the onset of the stagnation phase but yet within it, thermal neutrons are four times enhanced from l × 106. The timing exactly agrees with that of X-ray emission from the core. In the stagnation phase, beam fusion neutrons reduced to 1.5 times the thermal one. Temporal behaviours of Bremsstrahlung due to hot electrons and laser backscatterings support the the timing.

Two-dimensional ablation density measurement relevant to Rayleigh-Taylor instability with fresnel phase zone plate

Yohei Tamari, Hiroshi Azechi, Shinsuke Fujioka, Tatsuhiro Sakaiya, Hiroyuki Shiraga, Mitsuo Nakai, Keisuke Sigemori

Inertial Fusion Sciences and Applications 2003 182 - 185 2004.12

Fresnel phase zone plate (FPZP) can image hard x rays (>5 keV) with high spatial resolution of a few μm. In FPZP image, a structured background overlaps on an image. The image intensity profile cannot be obtained accurately. We have resolved this problem of structured background, and obtained plasma intensity profile within 10% error. Using FPZP, the plasma density measurement experiment has been demonstrated. The density profile of in-flight polystyrene plasma has been measured at several different times. The experimental density profiles have been compared with the calculated results by 1-D simulation code. The experiment data agreed with the simulation results.

Fast plasma heating in a cone-attached geometry - towards fusion ignition Reviewed

R Kodama, H Azechi, H Fujita, H Habara, Y Izawa, T Jitsuno, T Jozaki, Y Kitagawa, K Krushelnick, T Matsuoka, K Mima, N Miyanaga, K Nagai, H Nagatomo, M Nakai, H Nishimura, T Norimatsu, P Norreys, K Shigemori, H Shiraga, A Sunahara, KA Tanaka, M Tanpo, Y Toyama, K Tsubakimoto, T Yamanaka, M Zepf

NUCLEAR FUSION 44 ( 12 ) S276 - S283 2004.12

We have developed a PW (0.5 ps/500J) laser system to demonstrate fast heating of imploded core plasmas using a hollow cone shell target. Significant enhancement of thermal neutron yield has been realized with PW-laser heating, confirming that the high heating efficiency is maintained as the short-pulse laser power is substantially increased to a value nearly equivalent to the ignition condition. It appears that the efficient heating is realized by the guiding of the PW laser pulse energy within the hollow cone and by self-organized relativistic electron transport. Based on the experimental results, we are developing a 10kJ-PW laser system to study the fast heating physics of high-density plasmas at an ignition-equivalent temperature.

Suppression of the Rayleigh-Taylor instability and its implication for the impact ignition Reviewed

H Azechi, H Shiraga, M Nakai, K Shigemori, S Fujioka, T Sakaiya, Y Tamari, K Ohtani, M Murakami, A Sunahara, H Nagatomo, K Nishihara, N Miyanaga, Y Izawa

PLASMA PHYSICS AND CONTROLLED FUSION 46 B245 - B254 2004.12

The Rayleigh-Taylor (RT) instability with material ablation through an unstable interface is the key physics that determines the success or failure of inertial fusion energy (IFE) generation, as the RT instability potentially quenches ignition and burn by disintegrating the IFE target. We present two suppression schemes of the RT growth without significant degradation of the target density. The first scheme is to generate a double ablation structure in high-Z doped plastic targets. In addition to the electron ablation surface, a new ablation surface is created by x-ray radiation from the high-Z ions. Contrary to the previous thought, the electron ablation surface is almost completely stabilized by extremely high flow velocity. On the other hand, the RT instability on the radiative ablation surface is significantly moderated. The second is to enhance the nonlocal nature of the electron heat transport by illuminating the target with long wavelength laser light, whereas the high ablation pressure is generated by irradiating with short wavelength laser light. The significant suppression of the RT instability may increase the possibility of impact ignition which uses a high-velocity fuel colliding with a preformed main fuel.

Temporally resolved Schwarzschild microscope for the characterization of extreme ultraviolet emission in laser-produced plasmas

Y Tao, M Nakai, H Nishimura, S Fujioka, T Okuno, T Fujiwara, N Ueda, N Miyanaga, Y Izawa

REVIEW OF SCIENTIFIC INSTRUMENTS 75 ( 12 ) 5173 - 5176 2004.12

A temporally resolved monochromatic extreme ultraviolet (EUV) imager has been developed for use in EUV radiation source research. The imager consists of a Schwarzschild microscope, with near-normal-incident Mo/Si multilayer mirrors adjusted for 13.5 nm and 4% bandwidth, and an x-ray streak camera (XSC). The spatial resolution of the microscope was limited by the image detector's resolution to 3.5 mum for the CCD camera and 15 mum for the XSC, respectively, for a field of view of 1.2 mm. With the high photon collection efficiency, clear streak images could be obtained on a single-shot basis with laser pulse energy as low as 50 mJ at an intensity of 1x10(10) W/cm(2). Expansion behavior of the EUV emission region was successfully observed for laser-produced Sn plasmas. (C) 2004 American Institute of Physics.

Suppression of the Rayleigh-Taylor instability due to self-radiation in a multiablation target

Shinsuke Fujioka, Atsushi Sunahara, Katsunobu Nishihara, Naofumi Ohnishi, Tomoyuki Johzaki, Hiroyuki Shiraga, Keisuke Shigemori, Mitsuo Nakai, Tadashi Ikegawa, Masakatsu Murakami, Keiji Nagai, Takayoshi Norimatsu, Hiroshi Azechi, Tatsuhiko Yamanaka

Physical Review Letters 92 ( 19 ) 1 - 195001 2004.5

The suppression of the Rayleigh-Taylor instability that occurs due to self-Radiation in a multiablation target was investigated. It was demonstrated that in a high-Z doped plastic target, two ablation surfaces were formed separately, one driven by thermal radiation and the other driven by electron conduction. It was found that the growth of the Rayleigh-Taylor instability was significantly suppressed on the radiation-driven ablation surface inside the target becasue of a large ablation velocity and long density scale length. Using a brominated plastic target, a significant reduction in growth rate was observed.

Suppression of Rayleigh-Taylor instability due to radiative ablation in brominated plastic targets

S Fujioka, A Sunahara, N Ohnishi, Y Tamari, K Nishihara, H Azechi, H Shiraga, M Nakai, K Shigemori, T Sakaiya, M Tanaka, K Otani, K Okuno, T Watari, T Yamada, M Murakami, K Nagai, T Norimatsu, Y Izawa, S Nozaki, YW Chen

PHYSICS OF PLASMAS 11 ( 5 ) 2814 - 2822 2004.5

Suppression of hydrodynamic instabilities is very crucial for the ultimate goal of inertial fusion energy (IFE). A high-Z doped plastic of CHBr (brominated polystyrene) ablator is a very promising candidate to suppress the ablative Rayleigh-Taylor (RT) instability in a directly laser-driven IFE target. When a CHBr target is irradiated by intense laser beams, bromine atoms in the corona plasma emit strong radiation. The strong radiation drives the radiative ablation front inside the CHBr targets. This radiative ablation in the high-Z doped plastic target has many advantages for the suppression of the growth of the RT instability in analogy to the indirect-drive approach, i.e., large mass ablation rate, long density scale length and low peak density. Two-dimensional (2D) hydrodynamic simulation shows significant suppression of the RT instability in a CHBr target compared to an undoped polystyrene (CH) target. RT growth rate, calculated theoretically using the Betti-Goncharov procedure with a one-dimensional radiation-hydrodynamic simulation code, is in good agreement with the 2D calculations. Experiments were performed at the GEKKO XII- [C. Yamanaka , IEEE J. Quantum Electron. QE-17, 1639 (1981)] HIPER (High Intensity Plasma Experimental Research) laser facility. The trajectory of a laser-driven CHBr target observed in the experiment was reproduced fairly well by the simulation. The radiative ablation front formed inside a directly laser-driven CHBr target was clearly observed for the first time. The strong suppression of the RT instability in the CHBr target was confirmed using the face-on and side-on x-ray backlighting technique. The high-Z doped ablator can be applied to high density cryogenic deuterium-deuterium and deuterium-tritium compression, because the hydrogen-isotopes are nearly transparent to x rays, which are transmitted through the ablator from the laser-irradiation side. (C) 2004 American Institute of Physics.

GEKKO/HIPER-driven shock waves and equation-of-state measurements at ultrahigh pressures

N Ozaki, KA Tanaka, T Ono, K Shigemori, M Nakai, H Azechi, T Yamanaka, K Wakabayashi, M Yoshida, H Nagao, K Kondo

PHYSICS OF PLASMAS 11 ( 4 ) 1600 - 1608 2004.4

The GEKKO/HIPER-laser [N. Miyanaga et al., in Proceedings of the 18th International Conference on Fusion Energy (IAEA, Sorrento, Italy, 2001), IAEA-CN-77] driven shock experiments were characterized in detail for studies on equation-of-state (EOS) at ultrahigh pressures. High-quality shock waves were produced with the bundled 9 laser beams optically smoothed by spectral dispersion technique and Kinoform phase plates. The laser beams were directly focused on targets at up to an intensity of 10(14) W/cm(2) or higher with a wavelength of 351 nm and a duration of 2.5 us. Key issues on dynamic EOS research; the spatial uniformity and temporal steadiness of shock wave were estimated, and the preheating problem was also investigated by measurements of the self-emission and reflectivity from target rear surface. The experimental and analytical methods were validated by using double-step targets consisting of two Hugoniot standard metals. Extreme pressures only accessed in nuclear explosion experiments were generated with reproducibility and good accuracy using the laser direct-drive experimental system, for Al, Cu, polystyrene, and Ta. It was indicated that new and reliable EOS data at the ultrahigh pressures could be provided for materials ranging from low to high initial density by the laser direct-drive experimental technique. (C) 2004 American Institute of Physics.

Present status and future prospects of laser fusion research at ILE Osaka university Reviewed

K Mima, KA Tanaka, R Kodama, T Johzaki, H Nagatomo, H Shiraga, Y Sentoku, N Miyanaga, H Azechi, M Nakai, T Norimatu, K Nagai, J Sunahara, K Nishihara, T Taguchi, H Sakagami

PLASMA SCIENCE & TECHNOLOGY 6 ( 1 ) 2179 - 2184 2004.2

Reviewed are the present status and future prospects of the laser fusion research at the ILE Osaka. The Gekko XII and Peta Watt laser system have been operated for investigating the implosion hydrodynamics, fast ignition, and the relativistic laser plasma interactions and so on. In particular, the fast ignition experiments with cone shell target have been in progress as the UK and US-Japan collaboration programs. In the experiments, the imploded high density plasmas are heated by irradiating 500 J level peta-watt laser pulse. The thermal neutron yield is found to increase by three orders of magnitude by injecting the peta-watt laser into the cone shell target. The Rayleigh-Taylor instability experiment results are also reviewed is this paper.

Characterization of extreme UV radiation in laser produced plasma for use in lithography Reviewed

Nishimura H, Shigemori K, Nakai M, Fujioka S, Shimada Y, Hashimoto K, Yamaura M, Uchida S, Matsui R, Hibino T, Okuno T, Tao Y, Nagai K, Norimatsu T, Nagatomo H, Zhakhovckii V, Furukawa H, Sunahara A, Kawamura T, Murakami M, Nishihra K, Miyanaga N, Nakatsuka M, Izawa Y

Inertial Fusion Sciences and Applications 2003 1077 - 1080 2004

A new research project on extreme ultraviolet (EUV) source development has been started utilizing resources of laser fusion research. The main task is to provide a scientific basis for generating efficient, debris-free, high power EUV plasma source for production of semiconductor devices. Experimental data sets are obtained not only with Joule-class laser but also with GEKKO-XII to improve detailed atomic model included in a radiation hydrodynamic code and to find an optimum condition for source plasma generation. Spherical solid-tin and tin-oxide targets were illuminated uniformly with twelve beams from GEKKO XII to create one-dimensional (1-D) spherical plasmas. The highest conversion efficiency of a few % to 13.5 nm EUV light in 2% bandwidth was attained at irradiance of 1-2×1011 W/cm2. No significant difference was found between laser wavelengths of one μm and a half μm.

Estimation of emission efficiency for laser-produced EUV-plasmas Reviewed

T Kawamura, A Sunahara, K Gamada, K Fujima, F Koike, H Furukawa, T Nishikawa, A Sasaki, T Kagawa, R More, T Kato, M Murakami, Zhakhovskii, V, H Tanuma, T Fujimoto, Y Shimada, M Yamaura, K Hashimoto, S Uchida, C Yamanaka, T Okuno, T Hibino, N Ueda, R Matsui, YZ Tao, M Nakai, K Shigemori, S Fujioka, K Nagai, T Norimatsu, H Nishimura, K Nishihara, N Miyanaga, Y Izawa

EMERGING LITHOGRAPHIC TECHNOLOGIES VIII 5374 918 - 925 2004

Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the desired EUV light of 13 similar to 14 nm (about 90 eV) but also shorter x-rays. For example, emissions around. 4 similar to 8 nm (about 150 similar to 300 eV) and 1 similar to 2.5 nm (about 0.5 similar to 1.2 keV) are experimentally observed from Sn and/or SnO2 plasmas. These emissions are correspond to the N-shell and M-shell transitions, respectively. From the view point of energy balance and efficiency, these transitions should be suppressed. However, they may, to some extent, contribute to provide the 5p and 4f levels with electrons which eventually emit the EUV light and enhance the intensity. To know well about radiative properties and kinematic of the whole plasma, atomic population kinetics and spectral synthesis codes have been developed. These codes can estimate the atomic population with nl-scheme and spectral shapes of the EUV light. Radiation hydrodynamic simulation have been proceeding in this analysis. Finally, the laser intensity dependence of the conversion efficiency calculated by these codes agrees with that of the corresponding experimental results.

Characterization of GEKKO/HIPER-Driven Shock Waves for Equation-of-State Experiments in Ultra-High-Pressure Regime

Takamatsu Kikuo, Yamanaka Tatsuhiko, Azechi Hiroshi, Miyanaga Noriaki, Nakai Mitsuo, Ono Takatoshi, Nagai Keiji, Shigemori Keisuke, Ozaki Norimasa, Tanaka Kazuo A.

Journal of Plasma and Fusion Research vol. 80, pp. 486-491 ( 6 ) 486 - 491 2004

Experimental study on ablative stabilization of Rayleigh-Taylor instability of laser-irradiated targets Reviewed

K Shigemori, T Sakaiya, K Otani, S Fujioka, M Nakai, H Azechi, H Shiraga, Y Tamari, K Okuno, A Sunahara, H Nagatomo, M Murakami, K Nishihara, Y Izawa

HIGH-POWER LASER ABLATION V, PTS 1 AND 2 5448 634 - 641 2004

Hydrodynamic instabilities are key issues of the physics of inertial confinement fusion (ICF) targets. Among the instabilities, Rayleigh-Taylor (RT) instability is the most important because it gives the largest growth factor in the ICF targets. Perturbations on the laser irradiated surface grow exponentially, but the growth rate is reduced by ablation flow. The growth rate gamma is written as Takabe-Betti formula: gamma = (kg/(1+kL))(1/2). -betakm/rho(a), where k is wave number of the perturbation, g is acceleration, L is density scale-length, beta is a coefficient, m is mass ablation rate per unit surface, and rho(a), is density at the ablation front. We experimentally measured all the parameters in the formula for polystyrene (CH) targets. Experiments were done on the RIPER laser facility at Institute of Laser Engineering, Osaka University. We found that the beta value in the formula is similar to 1.7, which is in good agreements with the theoretical prediction, whereas the beta for certain perturbation wavelengths are larger than the prediction. This disagreement between the experiment and the theory is mainly due to the deformation of the cutoff surface, which is created by non-uniform ablation flow from the ablation surface. We also found that high-Z doped plastic targets have multiablation structure, which can reduce the RT growth rate. When a low-Z target with high-Z dopant is irradiated by laser, radiation due to the high-Z dopant creates secondary ablation front deep inside the target. Since, the secondary ablation front is ablated by x-rays, the mass ablation rate is larger than the laser-irradiated ablation surface, that is, further reduction of the RT growth is expected. We measured the RT growth rate of Br-doped polystyrene targets. The experimental results indicate that of the CHBr targets show significantly small growth rate, which is very good news for the design of the ICF targets.

Experimental study on basic properties of laser-produced EUV plasmas on GEKKO-XII laser facility Reviewed

K Shigemori, M Nakai, Y Shimada, M Yamaura, K Hashimoto, S Fujioka, H Nishimura, S Uchida, A Sunahara, K Nishihara, N Miyanaga, Y Izawa

EMERGING LITHOGRAPHIC TECHNOLOGIES VIII 5374 971 - 978 2004

Extremely ultraviolet (EUV) light at around 13.5 nm of wavelength is the most probable candidate of the light source for lithography for semiconductors of next generation. We have been studying about the EUV light source from laser-produced plasma. Detailed understanding of the EUV plasma is required for developments of modeling with simulation codes. Several parameters should be experimentally measured to develop the important issues in the simulation codes. We focused on density profile, properties of EUV emission, and opacity of the laser-produced plasmas. We present recent experimental results on these basic properties of the laser-produced EUV plasmas.

Rayleigh-taylor instability and its reduction in laser-accelerated targets Reviewed

Shiraga H, Shigemori K, Nakai M, Sakaiya T, Fujioka S, Tamari Y, Azechi H, Nishihara K, Sunahara A, Nagatomo H, Ikegawa T, Ohnishi N, Nagai K, Norimats T, Yamanaka T

Inertial Fusion Sciences and Applications 2003 115 - 120 2004

Rayleigh-Taylor (RT) instability is of great importance in laser fusion because its growth degrades integrity of the imploding shell and may lower the density of the imploded core plasma. The dispersion relation of RT growth rate described by the modified Bodner-Takabe formula have been intensively investigated with Gekko-XII-HIPER laser at ILE, Osaka University. The growth rate is strongly dependent on the density structure of the accelerated target, and hence, there is a possibility to reduce RT growth by controlling the energy transport in the laser-driven target. We recently demonstrated two types of technique to reduce RT growth: one is double ablation in which radiation transport is utilized, and the other is two-color laser irradiation in which induced nonlocal electron transport is enhanced. Both of them will be effective in reducing the RT instability growth in inertial fusion targets.

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

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

Inertial Fusion Sciences and Applications 2003 186 - 189 2004

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Papers - Nakai Mitsuo