Image-based deep learning (IBDL) is an advanced technique for predicting the surface irradiation conditions of laser surface processing technology. In pulsed-laser surface processing techniques, the ...number of superimposed laser shots is one of the fundamental and essential parameters that should be optimized for each material. Our primary research aims to build an adequate dataset using laser-irradiated surface images and to successfully predict the number of superimposed shots using the pre-trained deep convolutional neural network (CNN) models. First, the laser shot experiments were performed on copper targets using a nanosecond YAG laser with a wavelength of 532 nm. Then, the training data were obtained with the different superimposed shots of 1 to 1024 in powers of 2. After that, we used several pre-trained deep CNN models to predict the number of superimposed laser shots. Based on the dataset with 1936 images, VGG16 shows a high validation accuracy, higher sensitivity, and more than 99% precision than other deep CNN models. Utilizing the VGG16 model with high sensitivity could positively impact the industries' time, efficiency, and overall production.
Using one of the world most powerful laser facility, we demonstrate for the first time that high-contrast multi-picosecond pulses are advantageous for proton acceleration. By extending the pulse ...duration from 1.5 to 6 ps with fixed laser intensity of 10
W cm
, the maximum proton energy is improved more than twice (from 13 to 33 MeV). At the same time, laser-energy conversion efficiency into the MeV protons is enhanced with an order of magnitude, achieving 5% for protons above 6 MeV with the 6 ps pulse duration. The proton energies observed are discussed using a plasma expansion model newly developed that takes the electron temperature evolution beyond the ponderomotive energy in the over picoseconds interaction into account. The present results are quite encouraging for realizing ion-driven fast ignition and novel ion beamlines.
To enhance the core heating efficiency in electron-driven fast ignition, we propose fast-electron beam guiding using externally applied longitudinal magnetic fields. Based on particle-in-cell ...simulations for FIREX-class experiments, we demonstrate sufficient beam guiding performance in collisional dense plasma by kilotesla-class external magnetic fields for the case with moderate mirror ratio ( 10). Boring of the mirror field was found through the formation of magnetic pipe structure due to the resistive effects, which indicates a possibility of beam guiding in high mirror field for higher laser intensity and/or longer pulse duration.
Fast ignition (FI) is a promising approach for high-energy-gain inertial confinement fusion in the laboratory. To achieve ignition, the energy of a short-pulse laser is required to be delivered ...efficiently to the pre-compressed fuel core via a high-energy electron beam. Therefore, understanding the transport and energy deposition of this electron beam inside the pre-compressed core is the key for FI. Here we report on the direct observation of the electron beam transport and deposition in a compressed core through the stimulated Cu Kα emission in the super-penetration scheme. Simulations reproducing the experimental measurements indicate that, at the time of peak compression, about 1% of the short-pulse energy is coupled to a relatively low-density core with a radius of 70 μm. Analysis with the support of 2D particle-in-cell simulations uncovers the key factors improving this coupling efficiency. Our findings are of critical importance for optimizing FI experiments in a super-penetration scheme.
Direct-drive inertial confinement fusion (ICF), where a fuel capsule is imploded by high-power laser beams to cause ignition, is a promising energy production method that does not emit any greenhouse ...gases. Diamond is an ideal capsule material for direct-drive ICF. Its low compressibility and high density, which exceed those of conventional capsule materials (e.g., plastic), are advantageous for the robust implosion of fuel capsules. We are developing polycrystalline diamond capsules using the hot-filament chemical vapor deposition (HF-CVD) technique. HF-CVD is ideal for capsule fabrication for power plants of the future because it is inexpensive and amenable to mass production, as its deposition area can be extended by only increasing the number of filaments. In this study, we improved upon our previous fabrication process and obtained nanocrystalline diamond capsules of better quality. The capsules were comprehensively characterized, employing both the typical parameters (i.e., surface morphology and sp2 content) as well as parameters important for direct-drive ICF application (i.e., tungsten and hydrogen contents, capsule thickness, density, and mode amplitudes of surface roughness). The characterized hollow nanocrystalline diamond capsules were introduced into the laser experiment at the GEKKO laser facility at Osaka University, where a capsule can be spherically irradiated by 12 high-power laser beams. The experimental results showed successful implosion, and the implosion dynamics were well reproduced by radiation hydrodynamic simulation code calculations. These results verify the accurate characterization and quality of the nanocrystalline diamond capsules as being applicable to laser irradiation experiments.
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•Diamond capsule was fabricated by hot filament chemical vapor deposition.•Fabricated capsules were comprehensively characterized.•Laser experiment demonstrated successful implosion of the fabricated capsules.
To enhance the core heating efficiency in fast ignition laser fusion, the concept of relativistic electron beam guiding by external magnetic fields was evaluated by integrated simulations for FIREX ...class targets. For the cone-attached shell target case, the core heating performance deteriorates by applying magnetic fields since the core is considerably deformed and most of the fast electrons are reflected due to the magnetic mirror formed through the implosion. On the other hand, in the case of a cone-attached solid ball target, the implosion is more stable under the kilo-tesla-class magnetic field. In addition, feasible magnetic field configuration is formed through the implosion. As a result, the core heating efficiency doubles by magnetic guiding. The dependence of core heating properties on the heating pulse shot timing was also investigated for the solid ball target.
In implosion experiments of a cone-guided target using Gekko XII laser, the lasers on the cone side are not irradiated to avoid the irradiation of the cone. In such condition, the implosion process ...is done highly asymmetrically. Thus we evaluated the effects of the asymmetric implosion on the compression ratio of the fuel in Gekko XII irradiation orientation by three-dimensional hydro simulations. In this paper, we discuss the degradation of the compression ratio by asymmetric implosion and show that the compression ratio can be enhanced by adjusting the laser intensity between each beam to reduce the asymmetry of the implosion.
The effect of pre-plasma on core heating in cone-guiding fast ignition is evaluated by two-dimensional particle-in-cell (PIC) and Fokker–Planck (FP) simulations. If the long-scale pre-plasma exists ...in the cone, the generated fast electron energy becomes too high for effective core heating. As a result, the energy coupling from laser to core η
L→core
is reduced by 80% compared with the case without a pre-plasma. Even for the case without a pre-plasma, η
L→core
obtained in the simulation is smaller than that required for 5 keV heating in FIREX-I. In order to enhance η
L→core
, we propose a new target design ‘extended double cone with short inner cone wall’ for fast electron guiding to imploded core and show sufficient improvement of heating efficiency using this new cone on the basis of PIC and FP hydro-simulations.
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