Mitsuo Nakai | Osaka University (original) (raw)
Papers by Mitsuo Nakai
The Review of Laser Engineering, 2011
Scintillation properties of Pr 3-doped 20Al (PO 3) 3-80LiF glasses melted in N 2 were investigate... more Scintillation properties of Pr 3-doped 20Al (PO 3) 3-80LiF glasses melted in N 2 were investigated in order to seek a candidate for scattered neutron scintillator in nuclear fusion diagnostics. Time-resolved measurements yielded approximately 20 ns decay times for ultraviolet and x-ray excitation while much faster decay times of approximately 6 ns were observed for alpha particle and neutron excitation. Actual time-of-flight data in laser fusion experiments at the GEKKO XII facility of the Institute of Laser Engineering, Osaka University reveal that it can clearly discriminate fusion neutrons from the much stronger x-rays signals. Based on our material design strategy, we have successfully developed the fast response time praseodymium-doped 6 Li glass scintillator for scattered neutron diagnostics.
New Journal of Physics, 2013
We report new results from the intense laser target interaction experiment that produces relativi... more We report new results from the intense laser target interaction experiment that produces relativistic electron-positron pairs. Laser to electron energy transfer, inferred using x-ray and neutron measurements, was found to be consistent with the measured positrons. To increase the number of positrons, one needs to deliver a greater number of relativistic electrons from the laser-plasma interaction to the high Z gold target. A large preplasma was found to have a negative impact for this purpose, while the laser could produce hotter electrons in such preplasma. The peak energy shift in the positron spectrum is confirmed as the post-acceleration in the sheath potential behind the target. The results were supported by a collisional one-dimensional particle-in-cell code. This experiment was performed using the high-power LFEX laser at the Institute of Laser Engineering at Osaka University using a suite of diagnostics measuring electrons, positrons, x-rays and neutrons from the laser-target interaction at the relativistic regime.
![Research paper thumbnail of Erratum: “Efficient and Repetitive Neutron Generation by Double-Laser-Pulse Driven Photonuclear Reaction” [Plasma Fusion Res. 13, 2404009 (2018)]](https://attachments.academia-assets.com/88145067/thumbnails/1.jpg)
](Plasma and Fusion Research, 2018
In Fast Ignition experiments, targets are irradiated with ultra-intense short-pulse laser for hea... more In Fast Ignition experiments, targets are irradiated with ultra-intense short-pulse laser for heating of the compressed fuel plasma. Intense hard x rays (γ rays) and electromagnetic pulse (EMP) are generated in such experiments from hot electrons generated by the heating laser. Those bring serious background problems to x-ray imaging diagnostics and neutron diagnostics as well as malfunctioning of electronic devices. Furthermore, intense γ rays causes some nuclear reactions producing neutrons which are obstacles to the measurement of fusion neutrons. Plasma diagnostic instruments for Gekko-XII/LFEX laser facility have been improved to be compatible with such hard x-ray and EMP harsh environment.
Review of Scientific Instruments, 2020
The nuclear burn history provides critical information about the dynamics of the hot-spot formati... more The nuclear burn history provides critical information about the dynamics of the hot-spot formation and high-density fuel-shell assembly of an Inertial Confinement Fusion (ICF) implosion, as well as information on the impact of alpha heating, and a multitude of implosion failure mechanisms. Having this information is critical for assessing the energy-confinement time τE and performance of an implosion. As the confinement time of an ICF implosion is a few tens of picoseconds, less than 10-ps time resolution is required for an accurate measurement of the nuclear burn history. In this study, we propose a novel 1-ps time-resolution detection scheme based on the Pockels effect. In particular, a conceptual design for the experiment on the National Ignition Facility and OMEGA are elaborated upon herein. A small organic Pockels crystal "DAST" is designed to be positioned ∼5 mm from the ICF implosion, which is scanned by a chirped pulse generated by a femtosecond laser transmitted through a polarization-maintained optical fiber. The originally linearly polarized laser is changed to an elliptically polarized laser by the Pockels crystal when exposed to neutrons, and the modulation of the polarization will be analyzed. Our study using 35-MeV electrons showed that the system impulse response is 0.6 ps. The response time is orders of magnitude shorter than current systems. Through measurements of the nuclear burn history with unprecedented time resolution, this system will help for a better understanding of the dynamics of the hot-spot formation, high-density fuel-shell assembly, and the physics of thermonuclear burn wave propagation.
AIP Conference Proceedings, 1994
ABSTRACT Physics investigation of implosion and relevant technology development at ILE, Osaka Uni... more ABSTRACT 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.
Conference on Lasers and Electro-Optics 2010, 2010
Experimental results are presented on the properties of a custom-designed fast-response lithium-6... more Experimental results are presented on the properties of a custom-designed fast-response lithium-6 glass scintillator for inertial confinement fusion diagnostics. This newly developed scintillator promises as an indispensable tool in the realization of scattered neutron diagnostics.
Hugoniot data of diamond was obtained using laser-driven shock waves in the terapascal range of 0... more Hugoniot data of diamond was obtained using laser-driven shock waves in the terapascal range of 0.5-2 TPa. Strong shock waves were generated by direct irradiation of a 2.5 ns laser pulse on an Al driver plate. The shock wave velocities in diamond and Al were determined from optical measurements. Particle velocities and pressures were obtained using an impedance matching method and known Al Hugoniot. The obtained Hugoniot data of diamond does not show a marked difference from the extrapolations of the Pavlovskii Hugoniot data in the TPa range within experimental errors.
ABSTRACT Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the... more ABSTRACT Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the desired EUV light of 13 ~ 14 nm (about 90 eV) but also shorter x-rays. For example, emissions around 4 ~ 8 nm (about 150 ~ 300 eV) and 1 ~ 2.5 nm (about 0.5 ~ 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.
Journal of Plasma and Fusion Research, 2004
A new research project on extreme ultraviolet(EUV) source development has been started utilizing ... more A new research project on extreme ultraviolet(EUV) source development has been started utilizing resources of laser fusion research. The main task of the project is to provide a scientific basis for generating efficient, debris-free, high power EUV plasma source for production of semiconductor devices. Spherical solid-tin targets were illuminated uniformly with twelve beams from GEKKO XII to create spherical plasmas,
Review of Scientific Instruments, 2010
The characteristics of an APLF80+3Ce scintillator are presented. Its sufficiently fast decay prof... more The characteristics of an APLF80+3Ce scintillator are presented. Its sufficiently fast decay profile, low afterglow, and an improved light output compared to the recently developed APLF80+3Pr, were experimentally demonstrated. This scintillator material holds promise for applications in neutron imaging diagnostics at the energy regions of 0.27 MeV of DD fusion down-scattered neutron peak at the world's largest inertial confinement fusion facilities such as the National Ignition Facility and the Laser Mégajoule.
Plasma and Fusion Research, 2009
Cryogenically cooled foam shells with deuterium and tritium fuels are expected to be utilized in ... more Cryogenically cooled foam shells with deuterium and tritium fuels are expected to be utilized in the fast ignition realization experiment project of ILE, Osaka University, to demonstrate efficient heating of properly compressed fuel plasmas. These targets consist of a foam shell with solid fuel and a conical light guide for additional heating with an ultra-high intensity laser beam, in accordance with previous preliminary experiments [R. Kodama et al., Nature 412, 798 (2001).] Recent theoretical predictions and elemental experiments have suggested some advanced modifications to enhance the coupling efficiency of fast heating and improve implosion performance. The five major points of these improvements are as follows: 1) use of a low-Z foam layer on the inner surface of the cone; 2) use of a double-layered cone as a light guide; 3) use of a low-Z plastic layer on the outer surface of the cone; 4) adding a Br-doped plastic ablator to the fuel capsule; and 5) evacuation of the target center.
Physics of Plasmas, 1997
Hydrodynamic instabilities, such as the Rayleigh-Taylor ͑R-T͒ instability, play a critical role i... more Hydrodynamic instabilities, such as the Rayleigh-Taylor ͑R-T͒ 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 ͑1981͔͒ to measure hydrodynamic instabilities in planar foils directly irradiated by 0.53 m laser light. It has been found that ͑1͒ 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 ͑2͒ the experimental R-T growth rate is significantly reduced from the classical growth rate due probably to ablative stabilization enhanced by nonlocal heat transport.
Physics of Plasmas, 2002
ABSTRACT Flat plastic targets were directly irradiated and accelerated by partially coherent ligh... more ABSTRACT Flat plastic targets were directly irradiated and accelerated by partially coherent light from the GEKKO XII laser [Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)] with the wavelength of 0.53 μm in order to investigate initial laser imprinting. The growth of the perturbation imprinted on the target by an initial foot pulse modulated with a single spatial frequency was observed by the face-on x-ray backlight technique. Imprint levels produced by the foot beam with a stationary intensity modulation of the illumination profile and with a dynamically changing modulation were successfully obtained by an image relay technique and the improved two-wavelength Young’s interference method. Simple analytic models are proposed and compared with the experimental results. Stationary imprinting with perturbation wavelength longer than the target thickness is found to be well described by a simple incompressible model. The dynamic dependence of the imprint on the time scale of the temporal illumination profile is found to be qualitatively explained by linear perturbation analysis. © 2002 American Institute of Physics.
Physical Review Letters, 2004
A scheme to suppress the Rayleigh-Taylor instability has been investigated for a direct-drive ine... more 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.
Physical Review Letters, 2002
Optical Materials, 2010
A novel custom-developed scintillator for laser fusion diagnostics is reported. With the aim of d... more A novel custom-developed scintillator for laser fusion diagnostics is reported. With the aim of diagnosing scattered neutrons from the imploded high-density fusion plasma, a fast response lithium-6 glass scintillator has been developed. Praseodymium, instead of the more widely-used cerium was chosen as the dopant. Sufficiently fast luminescence lifetime for scattered neutron diagnostics (<20 ns) was experimentally observed, and the feasibility was successfully shown in laser-fusion experiments at the GEKKO XII facility of the Institute of Laser Engineering, Osaka University.
Journal of Plasma and Fusion Research, 2004
Journal of Non-Crystalline Solids, 2011
Optical and scintillation properties of Pr-doped Li-glass, 20Al(PO 3) 3-80LiF:Pr 3%, have been st... more Optical and scintillation properties of Pr-doped Li-glass, 20Al(PO 3) 3-80LiF:Pr 3%, have been studied for applications in neutron detection systems. Based on optical transmission and reflectivity, the absorption coefficient and refractive index were calculated from the Beer Lambert law. The absorption edge was apparently shifted to the longer wavelength from 160 nm to 240 nm due to 4f → 5d transitions of Pr ions. The strong absorption peaks of praseodymium 4f → 4f transitions were observed from 420 nm to 500 nm and around 590 nm. The radio-luminescence spectrum excited by 241 Am 5.5 MeV α source was measured. Strong emission peaks were observed around 250 nm. The α-ray excited pulse height spectrum and decay kinetics were also examined. Light yield was estimated to be 400 ± 40 photons/5.5 MeV α and the main component of the decay time was evaluated to be about 12 ns. Furthermore, the pulse height spectrum of the glass excited by 252 Cf neutrons was also measured, and the light yield was estimated to be 140 ± 10 photons/neutron.
IEEE Transactions on Nuclear Science, 2010
20Al(PO3)380 LiF glasses melted in N2 were investi-gated to seek a candidate for a scattered neu... more 20Al(PO3)380 LiF glasses melted in N2 were investi-gated to seek a candidate for a scattered neutron scintillator in nuclear fusion diagnostics. The fluorescence lifetime of the sample with 217 nm ultraviolet femtosecond pulse excitation was measured to be 19.5 ns. More ...
The Review of Laser Engineering, 2011
Scintillation properties of Pr 3-doped 20Al (PO 3) 3-80LiF glasses melted in N 2 were investigate... more Scintillation properties of Pr 3-doped 20Al (PO 3) 3-80LiF glasses melted in N 2 were investigated in order to seek a candidate for scattered neutron scintillator in nuclear fusion diagnostics. Time-resolved measurements yielded approximately 20 ns decay times for ultraviolet and x-ray excitation while much faster decay times of approximately 6 ns were observed for alpha particle and neutron excitation. Actual time-of-flight data in laser fusion experiments at the GEKKO XII facility of the Institute of Laser Engineering, Osaka University reveal that it can clearly discriminate fusion neutrons from the much stronger x-rays signals. Based on our material design strategy, we have successfully developed the fast response time praseodymium-doped 6 Li glass scintillator for scattered neutron diagnostics.
New Journal of Physics, 2013
We report new results from the intense laser target interaction experiment that produces relativi... more We report new results from the intense laser target interaction experiment that produces relativistic electron-positron pairs. Laser to electron energy transfer, inferred using x-ray and neutron measurements, was found to be consistent with the measured positrons. To increase the number of positrons, one needs to deliver a greater number of relativistic electrons from the laser-plasma interaction to the high Z gold target. A large preplasma was found to have a negative impact for this purpose, while the laser could produce hotter electrons in such preplasma. The peak energy shift in the positron spectrum is confirmed as the post-acceleration in the sheath potential behind the target. The results were supported by a collisional one-dimensional particle-in-cell code. This experiment was performed using the high-power LFEX laser at the Institute of Laser Engineering at Osaka University using a suite of diagnostics measuring electrons, positrons, x-rays and neutrons from the laser-target interaction at the relativistic regime.
Plasma and Fusion Research, 2018
In Fast Ignition experiments, targets are irradiated with ultra-intense short-pulse laser for hea... more In Fast Ignition experiments, targets are irradiated with ultra-intense short-pulse laser for heating of the compressed fuel plasma. Intense hard x rays (γ rays) and electromagnetic pulse (EMP) are generated in such experiments from hot electrons generated by the heating laser. Those bring serious background problems to x-ray imaging diagnostics and neutron diagnostics as well as malfunctioning of electronic devices. Furthermore, intense γ rays causes some nuclear reactions producing neutrons which are obstacles to the measurement of fusion neutrons. Plasma diagnostic instruments for Gekko-XII/LFEX laser facility have been improved to be compatible with such hard x-ray and EMP harsh environment.
Review of Scientific Instruments, 2020
The nuclear burn history provides critical information about the dynamics of the hot-spot formati... more The nuclear burn history provides critical information about the dynamics of the hot-spot formation and high-density fuel-shell assembly of an Inertial Confinement Fusion (ICF) implosion, as well as information on the impact of alpha heating, and a multitude of implosion failure mechanisms. Having this information is critical for assessing the energy-confinement time τE and performance of an implosion. As the confinement time of an ICF implosion is a few tens of picoseconds, less than 10-ps time resolution is required for an accurate measurement of the nuclear burn history. In this study, we propose a novel 1-ps time-resolution detection scheme based on the Pockels effect. In particular, a conceptual design for the experiment on the National Ignition Facility and OMEGA are elaborated upon herein. A small organic Pockels crystal "DAST" is designed to be positioned ∼5 mm from the ICF implosion, which is scanned by a chirped pulse generated by a femtosecond laser transmitted through a polarization-maintained optical fiber. The originally linearly polarized laser is changed to an elliptically polarized laser by the Pockels crystal when exposed to neutrons, and the modulation of the polarization will be analyzed. Our study using 35-MeV electrons showed that the system impulse response is 0.6 ps. The response time is orders of magnitude shorter than current systems. Through measurements of the nuclear burn history with unprecedented time resolution, this system will help for a better understanding of the dynamics of the hot-spot formation, high-density fuel-shell assembly, and the physics of thermonuclear burn wave propagation.
AIP Conference Proceedings, 1994
ABSTRACT Physics investigation of implosion and relevant technology development at ILE, Osaka Uni... more ABSTRACT 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.
Conference on Lasers and Electro-Optics 2010, 2010
Experimental results are presented on the properties of a custom-designed fast-response lithium-6... more Experimental results are presented on the properties of a custom-designed fast-response lithium-6 glass scintillator for inertial confinement fusion diagnostics. This newly developed scintillator promises as an indispensable tool in the realization of scattered neutron diagnostics.
Hugoniot data of diamond was obtained using laser-driven shock waves in the terapascal range of 0... more Hugoniot data of diamond was obtained using laser-driven shock waves in the terapascal range of 0.5-2 TPa. Strong shock waves were generated by direct irradiation of a 2.5 ns laser pulse on an Al driver plate. The shock wave velocities in diamond and Al were determined from optical measurements. Particle velocities and pressures were obtained using an impedance matching method and known Al Hugoniot. The obtained Hugoniot data of diamond does not show a marked difference from the extrapolations of the Pavlovskii Hugoniot data in the TPa range within experimental errors.
ABSTRACT Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the... more ABSTRACT Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the desired EUV light of 13 ~ 14 nm (about 90 eV) but also shorter x-rays. For example, emissions around 4 ~ 8 nm (about 150 ~ 300 eV) and 1 ~ 2.5 nm (about 0.5 ~ 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.
Journal of Plasma and Fusion Research, 2004
A new research project on extreme ultraviolet(EUV) source development has been started utilizing ... more A new research project on extreme ultraviolet(EUV) source development has been started utilizing resources of laser fusion research. The main task of the project is to provide a scientific basis for generating efficient, debris-free, high power EUV plasma source for production of semiconductor devices. Spherical solid-tin targets were illuminated uniformly with twelve beams from GEKKO XII to create spherical plasmas,
Review of Scientific Instruments, 2010
The characteristics of an APLF80+3Ce scintillator are presented. Its sufficiently fast decay prof... more The characteristics of an APLF80+3Ce scintillator are presented. Its sufficiently fast decay profile, low afterglow, and an improved light output compared to the recently developed APLF80+3Pr, were experimentally demonstrated. This scintillator material holds promise for applications in neutron imaging diagnostics at the energy regions of 0.27 MeV of DD fusion down-scattered neutron peak at the world&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s largest inertial confinement fusion facilities such as the National Ignition Facility and the Laser Mégajoule.
Plasma and Fusion Research, 2009
Cryogenically cooled foam shells with deuterium and tritium fuels are expected to be utilized in ... more Cryogenically cooled foam shells with deuterium and tritium fuels are expected to be utilized in the fast ignition realization experiment project of ILE, Osaka University, to demonstrate efficient heating of properly compressed fuel plasmas. These targets consist of a foam shell with solid fuel and a conical light guide for additional heating with an ultra-high intensity laser beam, in accordance with previous preliminary experiments [R. Kodama et al., Nature 412, 798 (2001).] Recent theoretical predictions and elemental experiments have suggested some advanced modifications to enhance the coupling efficiency of fast heating and improve implosion performance. The five major points of these improvements are as follows: 1) use of a low-Z foam layer on the inner surface of the cone; 2) use of a double-layered cone as a light guide; 3) use of a low-Z plastic layer on the outer surface of the cone; 4) adding a Br-doped plastic ablator to the fuel capsule; and 5) evacuation of the target center.
Physics of Plasmas, 1997
Hydrodynamic instabilities, such as the Rayleigh-Taylor ͑R-T͒ instability, play a critical role i... more Hydrodynamic instabilities, such as the Rayleigh-Taylor ͑R-T͒ 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 ͑1981͔͒ to measure hydrodynamic instabilities in planar foils directly irradiated by 0.53 m laser light. It has been found that ͑1͒ 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 ͑2͒ the experimental R-T growth rate is significantly reduced from the classical growth rate due probably to ablative stabilization enhanced by nonlocal heat transport.
Physics of Plasmas, 2002
ABSTRACT Flat plastic targets were directly irradiated and accelerated by partially coherent ligh... more ABSTRACT Flat plastic targets were directly irradiated and accelerated by partially coherent light from the GEKKO XII laser [Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)] with the wavelength of 0.53 μm in order to investigate initial laser imprinting. The growth of the perturbation imprinted on the target by an initial foot pulse modulated with a single spatial frequency was observed by the face-on x-ray backlight technique. Imprint levels produced by the foot beam with a stationary intensity modulation of the illumination profile and with a dynamically changing modulation were successfully obtained by an image relay technique and the improved two-wavelength Young’s interference method. Simple analytic models are proposed and compared with the experimental results. Stationary imprinting with perturbation wavelength longer than the target thickness is found to be well described by a simple incompressible model. The dynamic dependence of the imprint on the time scale of the temporal illumination profile is found to be qualitatively explained by linear perturbation analysis. © 2002 American Institute of Physics.
Physical Review Letters, 2004
A scheme to suppress the Rayleigh-Taylor instability has been investigated for a direct-drive ine... more 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.
Physical Review Letters, 2002
Optical Materials, 2010
A novel custom-developed scintillator for laser fusion diagnostics is reported. With the aim of d... more A novel custom-developed scintillator for laser fusion diagnostics is reported. With the aim of diagnosing scattered neutrons from the imploded high-density fusion plasma, a fast response lithium-6 glass scintillator has been developed. Praseodymium, instead of the more widely-used cerium was chosen as the dopant. Sufficiently fast luminescence lifetime for scattered neutron diagnostics (<20 ns) was experimentally observed, and the feasibility was successfully shown in laser-fusion experiments at the GEKKO XII facility of the Institute of Laser Engineering, Osaka University.
Journal of Plasma and Fusion Research, 2004
Journal of Non-Crystalline Solids, 2011
Optical and scintillation properties of Pr-doped Li-glass, 20Al(PO 3) 3-80LiF:Pr 3%, have been st... more Optical and scintillation properties of Pr-doped Li-glass, 20Al(PO 3) 3-80LiF:Pr 3%, have been studied for applications in neutron detection systems. Based on optical transmission and reflectivity, the absorption coefficient and refractive index were calculated from the Beer Lambert law. The absorption edge was apparently shifted to the longer wavelength from 160 nm to 240 nm due to 4f → 5d transitions of Pr ions. The strong absorption peaks of praseodymium 4f → 4f transitions were observed from 420 nm to 500 nm and around 590 nm. The radio-luminescence spectrum excited by 241 Am 5.5 MeV α source was measured. Strong emission peaks were observed around 250 nm. The α-ray excited pulse height spectrum and decay kinetics were also examined. Light yield was estimated to be 400 ± 40 photons/5.5 MeV α and the main component of the decay time was evaluated to be about 12 ns. Furthermore, the pulse height spectrum of the glass excited by 252 Cf neutrons was also measured, and the light yield was estimated to be 140 ± 10 photons/neutron.
IEEE Transactions on Nuclear Science, 2010
20Al(PO3)380 LiF glasses melted in N2 were investi-gated to seek a candidate for a scattered neu... more 20Al(PO3)380 LiF glasses melted in N2 were investi-gated to seek a candidate for a scattered neutron scintillator in nuclear fusion diagnostics. The fluorescence lifetime of the sample with 217 nm ultraviolet femtosecond pulse excitation was measured to be 19.5 ns. More ...