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Laser--plasma interaction and hot electrons have been characterized in detail in laser irradiation conditions relevant for direct-drive inertial confinement fusion. The experiment was carried out at the Gekko XII laser facility in multibeam planar target geometry at an intensity of approximately 3x1015 W/cm². Experimental data suggest that high-energy electrons, with temperatures of 20-50 keV and conversion efficiencies ofη<1%, were mainly produced by the damping of electron plasma waves driven by two-plasmon decay (TPD). Stimulated Raman scattering (SRS) is observed in a near-threshold growth regime, producing a reflectivity of approximately 0.01%, and is well described by an analytical model accounting for the convective growth in independent speckles. The experiment reveals that both TPD and SRS are collectively driven by multiple beams, resulting in a more vigorous growth than that driven by single-beam laser intensity. [ABSTRACT FROM AUTHOR]

The shock ignition (SI) approach to inertial confinement fusion is a promising scheme for achieving energy production by nuclear fusion. SI relies on using a high intensity laser pulse (≈1016 W/cm2, with a duration of several hundred ps) at the end of the fuel compression stage. However, during laser–plasma interaction (LPI), several parametric instabilities, such as stimulated Raman scattering and two plasmon decay, nonlinearly generate hot electrons (HEs). The whole behavior of HE under SI conditions, including their generation, transport, and final absorption, is still unclear and needs further experimental investigation. This paper focuses on the development of an experimental platform for SI-related experiments, which simultaneously makes use of multiple diagnostics to characterize LPI and HE generation, transport, and energy deposition. Such diagnostics include optical spectrometers, streaked optical shadowgraph, an x-ray pinhole camera, a two-dimensional x-ray imager, a Cu Kα line spectrometer, two hot-electron spectrometers, a hard x-ray (bremsstrahlung) detector, and a streaked optical pyrometer. Diagnostics successfully operated simultaneously in single-shot mode, revealing the features of HEs under SI-relevant conditions. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Diamond capsule was fabricated by hot filament chemical vapor deposition. • Fabricated capsules were comprehensively characterized. • Laser experiment demonstrated successful implosion of the fabricated capsules. [ABSTRACT FROM AUTHOR]