Takayoshi Sano

Stimulated Raman scattering (SRS) plays a crucial role in the energy conversion process from lasers to plasmas in the subrelativistic regime, where laser intensities range around 10 15 – 16 W/cm 2 . SRS induces excitations of Langmuir waves (LWs) and density fluctuations, such as plasma density cavities. These excitations modify laser absorption in underdense plasma regions and subsequently generate suprathermal hot electrons (HEs) that transport laser energy into overdense plasma regions, changing plasma dynamics, including shock wave formation and propagation. In this paper, we report saturations of SRS-induced HE generation within tens of picoseconds under continuous laser irradiation and a mechanism for HE generation driven by a stochastic process. One-dimensional particle-in-cell simulations with a large density-scale plasma ( ∼ 10 2 µ m ) over durations of up to 100 ps reveal the clear transition of the electron acceleration mechanism from previously reported electron trapping by SRS-excited LWs to a stochastic acceleration that produces fast electrons with mean energies greater than 100 keV as a result of random walks in fluctuating electric fields generated by SRS-induced density perturbations.

Recent observations by the IceCube Neutrino Observatory have revealed a significant excess of high-energy neutrinos from nearby Seyfert galaxies, such as NGC 1068, without a corresponding flux of high-energy gamma-rays. This suggests that neutrinos are produced via hadronic interactions in a region opaque to gamma-rays, likely a hot corona surrounding the central supermassive black hole. However, the mechanism responsible for accelerating the parent protons to the required energies (∼100 TeV) remains an open question. In this study, we investigate diffusive shock acceleration (DSA) in active galactic nucleus (AGN) coronae using a suite of one-dimensional particle-in-cell simulations spanning a broad range of plasma parameters. We find that DSA is a robust and efficient mechanism for proton acceleration, consistently channeling approximately 10% of the shock’s kinetic energy into nonthermal ions, even for shocks with sonic Mach numbers as low as M _s ≈ 2. In contrast, the efficiency of electron acceleration is highly variable and less efficient (<1%) in our parameter survey. These findings provide strong, first-principles support for the hadronic models of neutrino production in AGN, and offer quantitative constraints that can explain the observed gamma-ray deficit.

Low-density foams are of significant interest in inertial confinement fusion (ICF), with potential applications as fuel carriers, ablation layers, or as a hohlraum filling material. Despite their potential, the shock response of these materials remains poorly characterized, limiting the accuracy of hydrodynamic simulations. Here we report experimental measurements of the equation of state for 90 mg/cm 3 silica ( SiO 2 ) aerogel foam under laser-driven shock compression, conducted at the GEKKO XII laser facility. Shock pressures between 50 and 160 GPa were achieved, and the corresponding states were determined using standard impedance-matching techniques with a quartz reference material. Initial measurements appeared to underestimate the foam shock velocity relative to predictions by the Quotidian Equation of State (QEOS) model. Experimental diagnostics indicated the presence of a vacuum gap between the reference material and the foam. The vacuum gaps were characterized, and one-dimensional radiation-hydrodynamic simulations were conducted to estimate their impact on the measured shock velocity. After applying simulation-based corrections, the experimental Hugoniot aligns closely with QEOS predictions, supporting the model's applicability to low-density foams.

We propose a particle acceleration mechanism driven by large-amplitude Alfvén waves in a strong magnetic field. The acceleration process proceeds through multiple stages triggered by counterpropagating wave-particle resonant acceleration (CWRA) via decay instability. Initially, parent and daughter Alfvén waves resonantly accelerate particles perpendicular to the ambient magnetic field. The resultant modulational instability generates electrostatic fields within the wave packet, which are locally amplified by the ponderomotive force of the Alfvén wave packet. These fields subsequently drive further acceleration within a few relativistic gyroperiods via gyroresonant surfing acceleration (GRSA). During this, the v×B force facilitates momentum transfer from the perpendicular to the parallel direction. In the later stage, particles become trapped by the parent wave and gain additional energy through single wave resonant acceleration (SWRA). Furthermore, the accumulation of accelerated particles induces electrostatic trailing fields behind and at the tail of the wave packet, which drive trailing-field acceleration (TFA) of other electrons. The combined effects of these mechanisms, CWRA followed by GRSA and SWRA, result in highly relativistic electron energy. The electron energy accelerated through the above process is higher than that accelerated through TFA. This multistage acceleration process provides insights into the generation of high energy cosmic rays in astrophysical environments.

The phase transitions in minerals under shock are crucial for understanding meteorite impact history. Recent time-resolved x-ray diffraction (XRD) studies on silica shocked to 65 GPa proposed the formation of different high-pressure phases between fused silica and quartz. Furthermore, the dynamics of silica behavior under higher pressure need to be investigated, particularly during nonequilibrium superheating before melting. This study examines the time-dependent response of coesite, using laser-driven shock coupled with fast XRD and molecular dynamics simulations with our recently developed machine learning interatomic potential. Our results reveal a transient dense supercooled liquid crystallizes into a semi-disordered d-NiAs-type silica, followed by transforming into either seifertite or stishovite, depending on the pressure. Instead of thermodynamically stable quartz, a back-transformation to coesite phase is identified after release. The complicated phase evolution pathways in shocked coesite provide deeper insights into the high-pressure silica phases observed in the meteorite bombardments on the early Moon, Mars, and Earth.

The interaction between a thin foil target and a circularly polarized laser light injected along an external magnetic field is investigated numerically by particle-in-cell simulations. A standing wave appears at the front surface of the target, overlapping the injected and partially reflected waves. Hot electrons are efficiently generated at the standing wave due to the relativistic two-wave resonant acceleration if the magnetic field amplitude of the standing wave is larger than the ambient field. A bifurcation occurs in the gyration motion of electrons, allowing all electrons with nonrelativistic velocities to acquire relativistic energy through the cyclotron resonance. The optimal conditions for the highest energy and the most significant fraction of hot electrons are derived precisely through a simple analysis of test-particle trajectories in the standing wave. Since the number of hot electrons increases drastically by many orders of magnitude compared to the conventional unmagnetized cases, this acceleration could be a great advantage in laser-driven ion acceleration and its applications.

Experimental investigations of silica under high pressure and temperature offer crucial insights into modeling of Earth and super‐Earths’ interiors. Despite extensive studies on Hugoniots of silica polymorphs like fused‐silica (2.20 g/cm³), quartz (2.65 g/cm³) and stishovite (4.29 g/cm³) up to a terapascal, unexplored region of melting and liquid of silica at high pressures is leaved because of the Hugoniots dependence on ambient density. This emphasizes the urgence to supplement the phase diagram to constrain silica properties under extreme conditions. Here, the Hugoniot and shock temperature of coesite (2.92 g/cm³) were studied by laser shock compression experiments up to 950 GPa. Our findings confirm shock‐induced superheating in coesite, revealing a higher Grüneisen parameter and lower electrical conductivity compared to those of fused‐silica and quartz along an isothermal line (4 K). These results suggest unique properties of shocked coesite, which imply a warmer and longer‐lived silica magma ocean of earlier rocky‐planets.

Experimental investigations of silica under high pressure and temperature offer crucial insights into modeling of Earth and super-Earth’s interiors. Despite extensive studies on Hugoniots of silica polymorphs like fused-silica (2.20 g/cm3), quartz (2.65 g/cm3) and stishovite (4.29 g/cm3) up to tera-pascal, the initial density dependent Hugoniots leave an unexplored region of melting and liquid of silica at high pressures. This emphasizes the urgence to supplementing the phase diagram to constrain silica properties under extreme conditions. Here, the Hugoniot and shock temperature of coesite (2.92 g/cm3), were studied by laser shock compressions up to 950 GPa. Our findings confirm the shock-induced superheating in coesite, reveals a higher Grüneisen parameter and lower electrical conductivity compared to those of fused-silica and quartz along an isothermal line (< 2×104 K). These results suggest unique properties of shocked coesite, and implys a warmer and longer survival silica magma ocean in earlier rocky-planetary interior.

The interaction between a thin foil target and a circularly polarized laser light injected along an external magnetic field is investigated numerically by particle-in-cell simulations. A standing wave appears at the front surface of the target, overlapping the injected and partially reflected waves. Hot electrons are efficiently generated at the standing wave due to the relativistic two-wave resonant acceleration if the magnetic field amplitude of the standing wave is larger than the ambient field. A bifurcation occurs in the gyration motion of electrons, allowing all electrons with non-relativistic velocities to acquire relativistic energy through the cyclotron resonance. The optimal conditions for the highest energy and the most significant fraction of hot electrons are derived precisely through a simple analysis of test-particle trajectories in the standing wave. Since the number of hot electrons increases drastically by many orders of magnitude compared to the conventional unmagnetized cases, this acceleration could be a great advantage in laser-driven ion acceleration and its applications.