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 ^11C and ^7Be produced via ^11B(p,n)^11C and ^10B(p,α)^7Be, and yields the absolute number of ^11B(p,2α)^4He 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.

The burn characteristics of multi-shock-compressed DT fuel under fast ignition conditions are investigated using two-dimensional burn simulations. In solid-sphere implosions, high fuel areal density can be achieved through multi-shock compression with reduced susceptibility to hydrodynamic instabilities. However, the resulting compressed-fuel density structure is highly nonuniform, consisting of a localized high-density core surrounded by a low-density fuel region that contains most of the fuel mass. In such a configuration, ignition can be initiated in the central high-density core, but the burn wave rapidly attenuates as it propagates into the surrounding low-density region, preventing self-sustained burn propagation. Consequently, the burn-up ratio is significantly lower than that of uniformly compressed fuel with comparable total fuel areal density. By examining the effects of implosion timing and artificially modified surrounding-density profiles, we show that the burn-up ratio is governed not only by the total fuel areal density, but also by the density structure around the ignition region. These results indicate that achieving a high burn-up ratio requires a sufficiently dense fuel region along the burn-wave propagation path, rather than a localized high-density core alone.

We present an experimental investigation demonstrating that the suppression of parametric instabilities in laser-plasma interactions under conditions relevant to direct-drive inertial confinement fusion, specifically backward stimulated Raman scattering (SRS) and two-plasmon decay (TPD), shows the effects of spatial beam coherence and plasma-density scale length. Experiments were performed with the GEKKO-XII kilojoule laser, both with and without random phase plates. Results indicate that SRS suppression is enhanced under conditions of initial spatial incoherence, whereas TPD is influenced primarily by the plasma scale length. A comparison of the relationship between the SRS and TPD signals and hot-electron measurements suggests that TPD is the primary source of hot electrons across all experimental conditions. These findings underscore the importance of beam coherence in determining instability dominance and carry implications for mitigating hot-electron preheat in ignition-scale inertial-confinement implosions as well as for developing hot-electron-driven schemes such as shock ignition.

We investigated plasma-heating enhancement for the efficient production of high-energy-density plasma by irradiating a high-intensity, high-contrast short pulse from a kilojoule-class laser system onto a cone-attached target. Irradiating a cone-attached planar target with a high-contrast laser causes specular reflection of the laser pulse on the inner cone wall, aligning the laser pointing direction with the cone axis and focusing the laser near the tip. Using a simplified cone-attached target configuration, we directly demonstrate this guiding/focusing mechanism while isolating it from other complexities inherent to integrated fast-ignition experiments. The high contrast was achieved using a plasma mirror. Electron energy analyzer, X-ray spectrometer, and atomic kinetics modeling with a two-component electron distribution were applied to three cases: (i) flat target irradiated by low contrast pulse, (ii) flat target irradiated by high contrast pulse, and (iii) cone target irradiated by high contrast pulse. The cone irradiated by the high-contrast pulse case achieved an electron temperature of 9.9 +4.1-2.5 key, that is roughly 17.5 x higher than in the low-contrast flat case. X-ray pinhole images show a multi-spot laser being guided to the cone tip, yielding localized X-ray emission. While a high-contrast laser reduced the fast-electron slope temperature for flat targets, adding the cone boosted the slope temperature by more than threefold, equivalent to a fourfold rise in local laser intensity. These results demonstrate that combining a high-contrast pulse with cone geometry markedly improves laser-to-plasma energy coupling for the fast-ignition inertial confinement fusion research.

The atomic processes in laser-produced tin plasmas for the extreme ultraviolet light sources are investigated. The level population of complex tin ions is calculated using the collisional-radiative (CR) model, and then spectral emissivity and opacity are calculated, taking the spectral structure of unresolved transition array into account. A rule-based method for developing a large-scale CR model is discussed, which enables the simulation of the emission spectrum using a relatively compact model. The effect of configuration interaction on the wavelength of the emission and broadening of the main peak at lambda = 13.5 nm by the emission from multiply excited states is discussed. (c) 2026 The Japan Society of Plasma Science andNuclear Fusion Research

Laser-driven capacitor-coil targets provide a compact platform for generating strong magnetic fields and are widely used in magnetized high-energy-density (HED) plasma experiments. In addition to magnetic-field generation, these targets also produce plasma in the coil region, which can influence the subject physical processes and interact with secondary targets or external plasmas in their applications. However, direct, time-resolved measurements of the plasma density surrounding the coil remain limited. Here, we report interferometric measurements of the plasma density evolution in laser-driven capacitor-coil targets irradiated by the University of Osaka LFEX laser. Two-dimensional electron density maps reveal two distinct plasma sources loading the coil region: plasma generated in the coil itself and plasma produced by laser ablation of the target plates. These results provide quantitative information on plasma loading and evolution in capacitor-coil targets and are directly relevant to the design and modeling of magnetized HED plasma experiments.

Opacity data is one of the fundamental data for modeling laser plasma extreme-ultraviolet (EUV) source. This includes the spectral emissivity and opacity of tin plasmas across a possible range of temperatures and densities. The opacity data is obtained using the collisional radiative model based on the calculated atomic data, such as energy levels and rates of collisional and radiative excitation and ionization processes, and applying the spectral profile for atomic transitions in the EUV wavelength region, for 4 to 17 times ionized tin. The emission from multiply excited states is also considered. The opacity data is useful for investigating the pumping condition of EUV source to obtain high output power and efficiency. The modeling method will also be useful for investigating the shorter-wavelength sources.

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

In direct-drive inertial-confinement fusion, understanding and controlling laser–plasma instabilities (LPIs) is crucial to optimizing energy coupling and achieving high-gain fusion. Herein, we report the experimental investigation of the effects of density scale-length on LPIs. The experiment was performed at the GEKKO-XII Laser facility [C. Yamanaka et al., IEEE J. Quantum Electron. 17, 1639 (1981)], specifically to characterize stimulated Raman scattering (SRS) and two-plasmon decay (TPD), and the effects of density scale-lengths on the relationship between these LPIs and hot-electron generation. The experimental results consistently indicate that the reduction in hot-electron generation with increasing density scale-length is strongly correlated with decreases in both SRS and TPD in high-density regions. Rosenbluth gain analysis implies that pump depletion by stimulated Brillouin scattering is not responsible for the observed reduction in SRS and TPD. Instead, spatial and temporal incoherence of propagating laser beams, driven by filamentation, could have suppressed SRS and TPD, as evaluated by filamentation figure of merit (FFOM). On the other hand, the thermally corrected FFOM suggests that hot spots by random phase plates are the origin of the early growth of SRS in low-density regions.

Optical generation of energetic particle bunches requires high-power laser facilities operating in picosecond or femtosecond temporal domain. It is therefore preferable to use short laser pulses in all-optical platforms designed for guiding and focusing of such particle beams, increasing their brightness and decreasing their angular divergence. We propose and discuss theoretical and experimental results for a novel electromagnetic guiding setup based on a shaped spiral-like 'snail' target with a relatively large useful aperture. Due to the diameter increased to a sub-mm scale, a wider particle bunches may be efficiently focused, as we show in theoretical modelling with a model field distributions, supported by the experimental data.

Efficient generation of multi-hundred-keV electrons is essential for isochoric heating and can influence ion acceleration. We investigated electron acceleration from copper-oleate foil targets, either planar or coated with a gold mesh structure (bar width 5,μm, spacing 7.5,μm, thickness ∼6,μm), irradiated by 1.5-ps, 350-J LFEX laser pulses. Two laser-contrast conditions were examined: high (∼10^10, with a plasma mirror) and low (∼10^8, without a plasma mirror). Using Cu-K_α emission mapping, we found that under high-contrast irradiation the micro-structured target enhanced the laser-to-electron conversion efficiency from 4.9% to 14%, attributed to multiple internal reflections that strengthen J× B acceleration. In contrast, under low-contrast conditions the structures were filled with pre-plasma before the main pulse, and no enhancement was observed. These results demonstrate that both fine-scale structuring and high contrast are crucial for maximizing J× B-driven electron generation in laser-plasma interactions. Our findings suggest a practical approach to improving laser-plasma coupling efficiency by exploiting micro-structured surfaces and contrast-controlled irradiation.

In pursuing a carbon-neutral society, inertial fusion energy is a pivotal technology, offering a sustainable and virtually limitless energy source. Enhancing plasma heating efficiency is crucial for inertial fusion energy, following the ignition success using the laser-indirect-drive central-ignition approach at the U.S. National Ignition Facility. Fast ignition is a potentially high-efficiency approach among inertial confinement fusion methods. However, it remains significantly challenging due to the large divergence angle of relativistic electron beams (REBs) produced by high-intensity laser pulses. Here, we demonstrate the spontaneous collimation of REBs facilitated by an ultrahigh contrast of the heating laser in conjunction with a cone-shaped target. This collimation doubles the plasma heating efficiency compared to that achieved with the lower-contrast laser, as conducted with LFEX laser at Osaka University. This finding enables the use of a larger spot laser under ignition conditions, which is crucial for achieving the heating efficiency required for fusion ignition by reducing laser intensity and the mean kinetic energy of REBs. This collimation method significantly advances the field of inertial confinement fusion, elevating fast ignition to a practical level.

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.

One of the critical issues in lithography using extreme ultraviolet (EUV) light is tin contamination of the EUV collector mirrors in the tin-based LPP-EUV light source. The contamination can be removed by the reaction of tin atoms with hydrogen radicals producing stannane (SnH4), which is gaseous at the normal temperature. Hydrogen radicals can be produced from hydrogen molecules through photo-dissociation and photo-ionization induced by broadband radiation emitted from the EUV light source. In this work, an efficient production of hydrogen radical using vacuum ultraviolet (VUV) light emitted from laser-produced high-Z plasma is experimentally demonstrated. A two-dimensional radiation hydrodynamic simulation, coupled with photoionization and photo-dissociation cross sections, also shows the efficient hydrogen radical production by increased VUV light emission, as observed in the experiment.

Efficient terahertz sources with controllable characteristics such as frequency range and polarization state are being rapidly researched and developed to suit various practical applications. To address this need, we realized the idea of combining micro- and nano-sized materials by fabricating micrometer-scale split-ring resonators made of a metal nanostructured film. We found that the peak frequencies of the emitted terahertz waves are in good correspondence with the terahertz resonance frequencies of the split-ring resonators. A possible mechanism is that a surge current was induced inside the split-ring resonators as a result of photoexcitation with the help of plasmon resonance around nanostructures, and the induced current emitted terahertz waves reflecting the resonance properties of the split-ring resonators. Although the silver nanostructures constituting the rings are random and homogeneous, the induction of the current parallel to the sample surface is enabled by the oblique incidence excitation, which breaks the symmetry along the sample surface. The present study shows the possibility of making compact terahertz emitters with flexibly tunable spectral shape, potentially leading to the development of terahertz sources optimized for specific spectroscopic uses.

The temperature measurement of material inside of an object is one of the key technologies for control of dynamical processes. For this purpose, various techniques such as laser-based thermography and phase-contrast imaging thermography have been studied. However, it is, in principle, impossible to measure the temperature of an element inside of an object using these techniques. One of the possible solutions is measurements of Doppler brooding effect in neutron resonance absorption (NRA). Here we present a method to measure the temperature of an element or an isotope inside of an object using NRA with a single neutron pulse of approximately 100 ns width provided from a high-power laser. We demonstrate temperature measurements of a tantalum (Ta) metallic foil heated from the room temperature up to 617 K. Although the neutron energy resolution is fluctuated from shot to shot, we obtain the temperature dependence of resonance Doppler broadening using a reference of a silver (Ag) foil kept to the room temperature. A free gas model well reproduces the results. This method enables element(isotope)-sensitive thermometry to detect the instantaneous temperature rise in dynamical processes.

In inertial confinement fusion using central ignition, the ignition hot spot is generated through self-heating during fuel compression. In contrast, fast ignition creates the hot spot through external heating. This difference allows the fast ignition approach to use a solid sphere as the fusion fuel shape. The implosion of a solid sphere is one form of laser-direct-drive slow implosion. Solid sphere fuel exhibits tolerance to hydrodynamic instability and can be mass-produced relatively easily, offering significant advantages for developing inertial fusion energy. Achieving high fuel peak and areal densities of with a solid sphere requires quasi-isentropic compression, which involves multiple shock waves. Our results show the critical role of uniform laser irradiation in initiating weak shock waves in the early phase, which is essential for forming a uniform and dense fuel core with solid spheres. Furthermore, dynamically adjusting the laser spot diameter could be crucial in optimizing the effectiveness of laser-direct-drive and fast ignition techniques when using solid sphere fuel.

We report on the third harmonic generation (THG) in InSb semiconductor irradiated by a terahertz (THz) free electron laser (FEL). The conversion of 4 THz (wavelength 70 µm) FEL outputs into its third harmonic 12 THz was observed. We found that by tuning the sample temperature to 360 K, high conversion efficiency up to 1% can be obtained and is the highest in the THz and FIR regions below 10 THz. We also discuss the observed intensity dependence of the THG with the nonlinear order lower than 3 when the pumping intensity was high.