arXiv subjects

Atsushi Sunahara

Publications and source records attributed to Atsushi Sunahara.

9 recordsLinked to original sources

arXiv2026

40% boost in extreme ultraviolet conversion efficiency via simultaneous dual-beam 2-μm laser irradiation

Scaling extreme ultraviolet (EUV) source power for next-generation lithography demands higher conversion efficiency (CE) at reduced per-pulse energies. We demonstrated a 40% CE enhancement by simultaneous dual-beam irradiation of a planar Sn target with a 2090-nm, 20-ns Ho:YAG laser. Single-beam irradiation at 40 mJ yielded an EUV CE of 2.6%; splitting the same total energy equally into two beams of 20 mJ each - at identical peak intensity - raised the EUV CE to 3.6%, which was the highest reported for 2-μm-driven laser-produced plasma sources. The EUV source size (60-70 μm) and energetic-ion spectra were nearly identical across both configurations, confirming comparable plasma conditions. Because the scheme requires only passive beam splitting and scales readily to three or more beams, it offers a practical route toward multi-kW-class, energy-efficient EUV sources for high-NA and hyper-NA lithography.

physics.optics↗

arXiv2026

Optimization of EUV output by experimentally validated radiation-hydrodynamic simulations across a broad laser parameter space

Practical requirements such as improving wall-plug efficiency and reducing system footprint have become increasingly important with the introduction of extreme ultraviolet (EUV) lithography into high-volume semiconductor manufacturing. These demands motivate the development of solid-state mid-infrared lasers as alternatives to current CO2 lasers. Systematic exploration of laser-to-EUV conversion efficiency (EUV-CE) over a broad parameter space is essential when altering the drive laser's wavelength, because the EUV-CE depends on the laser parameters in a complex manner. In this work, we performed a large-scale grid search of more than 140,000 parameter combinations for laser-produced tin plasma EUV sources using the radiation-hydrodynamics code STAR-1D, which is validated against EUV source experiments. The systematic wavelength dependence of the optimum pulse width and target size is governed by the requirement to simultaneously achieve the electron temperature and density optimal for EUV emission, maintain efficient laser absorption, and suppress EUV self-absorption. The resulting CE map predicts a global maximum of 5.63% at 5.5 μm. For the practically relevant 2 μm solid-state driver, a maximum CE of 4.64% is obtained, in good agreement with recent experimental results. Multiple operating points are identified over a broad range of pulse parameters, providing guidance for 2 μm-driven EUV source development.

physics.plasm-ph↗

arXiv2026

Probing In-Solid Proton Energy Distributions in Laser-Driven Fusion via Nuclear Activation Diagnostics

The energy distribution of energetic protons inside a solid target is a key quantity governing nuclear reaction yields and energy deposition in high-intensity laser-driven fusion, including nonthermal proton--boron (p--B) schemes and proton fast ignition. Yet it has remained inaccessible to conventional particle diagnostics, which detect only ions escaping the target and are perturbed by intense plasma electromagnetic fields. Here we establish a quantitative diagnostic that uses nuclear activation reactions occurring within the target itself as an internal probe of the in-solid proton energy distribution. Applied to laser-driven p--B fusion experiments on the kJ-class laser, the method reconstructs an exponential-equivalent in-solid proton energy distribution from the absolute yields of $^{11}\mathrm{C}$ and $^{7}\mathrm{Be}$ produced via $\mathrm{^{11}B(p,n)^{11}C}$ and $\mathrm{^{10}B(p,α)^{7}Be}$, and yields the absolute number of $\mathrm{^{11}B(p,2α)^{4}He}$ reactions through a side-channel analysis with propagated cross-section uncertainties. This work opens a quantitative window onto the in-solid proton dynamics that drive nuclear reactions in laser-driven fusion experiments.

physics.plasm-ph↗

arXiv2025

Enhanced plasma heating via interaction with high-contrast laser and cone-shaped target

We investigated plasma heating enhancement using a high-intensity, high-contrast laser and a cone-attached target. Fast electron spectra and X-ray emission were measured with an electron spectrometer and a Bragg crystal spectrometer. The results were analyzed using PrismSPECT simulations with a two-component electron distribution model and empirical scaling laws. X-ray pinhole images showed that the cone effectively focused multi-spot laser light near its tip, enhancing local emission. While high-contrast laser irradiation reduced the fast electron slope temperature for flat targets, the use of a cone increased it by over threefold, corresponding to a fourfold rise in laser intensity. X-ray spectral analysis indicated an electron temperature of ~9~keV for the cone case, 17.5 times higher than that with a low-contrast laser. These findings demonstrate that combining high-contrast laser irradiation with cone-target geometry significantly improves laser energy coupling and plasma heating efficiency.

physics.plasm-ph↗

arXiv2019

Peta-Pascal Pressure Driven by Fast Isochoric Heating with Multi-Picosecond Intense Laser Pulse

Fast isochoric laser heating is a scheme to heat a matter with relativistic-intensity ($>$ 10$^{18}$ W/cm$^2$) laser pulse or X-ray free electron laser pulse. The fast isochoric laser heating has been studied for creating efficiently ultra-high-energy-density (UHED) state. We demonstrate an fast isochoric heating of an imploded dense plasma using a multi-picosecond kJ-class petawatt laser with an assistance of externally applied kilo-tesla magnetic fields for guiding fast electrons to the dense plasma.The UHED state with 2.2 Peta-Pascal is achieved experimentally with 4.6 kJ of total laser energy that is one order of magnitude lower than the energy used in the conventional implosion scheme. A two-dimensional particle-in-cell simulation reveals that diffusive heating from a laser-plasma interaction zone to the dense plasma plays an essential role to the efficient creation of the UHED state.

physics.plasm-ph↗

arXiv2018

Two-Dimensional Computation of Pulsed Magnetic Field Diffusion Dynamics in Gold Cone with Consideration of Inductive Heating and Temperature Dependence of Electrical Conductivity

Application of an external kilo-tesla-level magnetic field, which can be generated using high-intensity laser, to a target is a promising scheme to reduce spray angle of a laser-driven relativistic electron beam (REB) for enhancing the isochoric heating of a dense plasma with the laser-driven REB. Here we have developed a two-dimensional electro-magnetic dynamics (2D-EMD) simulation code to solve Maxwell equations with considerations of the inductive heating and temperature-dependence of electrical conductivity of a material for calculating temporally and spatially resolved two-dimensional profile of the externally applied magnetic field in a gold-cone-attached target.

physics.plasm-ph↗

arXiv2018

Super-ponderomotive electron acceleration in blowout plasma heated by multi-picosecond relativistic intensity laser pulse

The dependence of the mean kinetic energy of laser-accelerated electrons on the laser intensity, so-called ponderomotive scaling, was derived theoretically with consideration of the motion of a single electron in oscillating laser fields. This scaling explains well the experimental results obtained with high-intensity pulses and durations shorter than a picosecond; however, this scaling is no longer applicable to the multi-picosecond (multi-ps) facility experiments. Here, we experimentally clarified the generation of the super-ponderomotive-relativistic electrons (SP-REs) through multi-ps relativistic laser-plasma interactions using prepulse-free LFEX laser pulses that were realized using a plasma mirror (PM). The SP-REs are produced with direct laser acceleration assisted by the self-generated quasi-static electric field and with loop-injected direct acceleration by the self- generated quasi-static magnetic field, which grow in a blowout plasma heated by a multi-ps laser pulse. Finally, we theoretically derive the threshold pulse duration to boost the acceleration of REs, which provides an important insight into the determination of laser pulse duration at kilojoule- petawatt laser facilities.

physics.plasm-ph↗

arXiv2017

Magnetized Fast Isochoric Laser Heating for Efficient Creation of Ultra-High-Energy-Density States

The quest for the inertial confinement fusion (ICF) ignition is a grand challenge, as exemplified by extraordinary large laser facilities. Fast isochoric heating of a pre-compressed plasma core with a high-intensity short-pulse laser is an attractive and alternative approach to create ultra-high-energy-density states like those found in ICF ignition sparks. This avoids the ignition quench caused by the hot spark mixing with the surrounding cold fuel, which is the crucial problem of the currently pursued ignition scheme. High-intensity lasers efficiently produce relativistic electron beams (REB). A part of the REB kinetic energy is deposited in the core, and then the heated region becomes the hot spark to trigger the ignition. However, only a small portion of the REB collides with the core because of its large divergence. Here we have demonstrated enhanced laser-to-core energy coupling with the magnetized fast isochoric heating. The method employs a kilo-tesla-level magnetic field that is applied to the transport region from the REB generation point to the core which results in guiding the REB along the magnetic field lines to the core. 7.7 $\pm$ 1.3 % of the maximum coupling was achieved even with a relatively small radial area density core ($ρR$ $\sim$ 0.1 g/cm$^2$). The guided REB transport was clearly visualized in a pre-compressed core by using Cu-$K_α$ imaging technique. A simplified model coupled with the comprehensive diagnostics yields 6.2\% of the coupling that agrees fairly with the measured coupling. This model also reveals that an ignition-scale areal density core ($ρR$ $\sim$ 0.4 g/cm$^2$) leads to much higher laser-to-core coupling ($>$ 15%), this is much higher than that achieved by the current scheme.

physics.plasm-ph↗

arXiv2013

Prepulse and amplified spontaneous emission effects on the interaction of a petawatt class laser with thin solid targets

When a finite contrast petawatt laser pulse irradiates a micron-thick foil, a prepulse (including amplified spontaneous emission) creates a preplasma, where an ultrashort relativistically strong portion of the laser pulse (the main pulse) acquires higher intensity due to relativistic self-focusing and undergoes fast depletion transferring energy to fast electrons. If the preplasma thickness is optimal, the main pulse can reach the target generating fast ions more efficiently than an ideal, infinite contrast, laser pulse. A simple analytical model of a target with preplasma formation is developed and the radiation pressure dominant acceleration of ions in this target is predicted. The preplasma formation by a nanosecond prepulse is analyzed with dissipative hydrodynamic simulations. The main pulse interaction with the preplasma is studied with multi-parametric particle-in-cell simulations. The optimal conditions for hundreds of MeV ion acceleration are found with accompanying effects important for diagnostics, including high-order harmonics generation.

physics.plasm-ph↗