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.

In recent years, laser-driven neutron sources have attracted attention for their applications such as nondestructive analysis and the production of medical radioisotopes. One of the efficient neutron production methods is the use of the 9Be(d, n)10B reaction on a beryllium target with deuterons accelerated by laser-plasma interactions, since this is an exothermic reaction. For efficient deuteron acceleration, we have developed a formation system for solid deuterium targets. A millimeter thick solid deuterium target can be formed with the system. Before the laser shot, the solid deuterium target in the laser chamber can be mechanically moved to the laser irradiation point. We have demonstrated deuteron acceleration by the LFEX laser, and a highly pure deuteron pulse with energies of up to 6.2 MeV was measured with a Thomson parabola ion spectrometer.

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.

There is a strong demand for efficient second harmonic generation (SHG) in ultra-intense short-pulse lasers. This paper demonstrates the generation of an unconverted fundamental (1ω)+second harmonics (2ω) mixed laser on the LFEX laser system. The experimental setup utilizes 0.5 mm-thick LBO crystal plates in a focusing beams implemented after an off-axis parabola, the design reduces the size and cost of the SHG system. The LFEX laser beams with four-beams combined energy of 222 J and a pulse duration of 1.5 ps, is successfully converted to 102 J of 2ω light and 100 J of unconverted 1ω light, 20 J is lost through surface reflections, and they are mixed at the focal point. Verification of successful SHG is confirmed through X-ray pinhole imaging and electron spectrometry. This novel technique is not limited to LFEX lasers and holds applicability for various ultra-intense lasers. Consequently, this accomplishment significantly contributes to expanding the capability for high-energy density laser-plasma experiments.

We evaluated the neutron bang time in fast-ignition inertial confinement fusion and the response function of deuterium-deuterium (DD) neutrons for burn history monitoring applications with a single-crystal CVD diamond detector. Signals were successfully obtained for the first time with a single-crystal diamond detector for DD neutrons of above 10(8) neutrons/shot.

For high repetition ultrahigh-intensity laser system, automatic alignment of structured target is key to achieving consistent particle acceleration and plasma heating. In this work, we demonstrate efficient electron acceleration with two sequential steps of laser processing using a high repetition rate, 30-fs ultrahigh-intensity laser. The first pulse does laser machining and creates a steep cylindrical crater on the surface of a flat stainless -steel target. The crater is formed by the hydrodynamic expansion of the heated surface and by spallation of the inner, deeper material by nonthermal relativistic electrons. The crater shape is well controlled and reproducible with 200 mu m width and 350 mu m depth. The second pulse irradiates deeply inside the crater and interacts with the crater wall, efficiently accelerating electrons via surface plasmonic, without need for target realignment. The laser absorption efficiency increases from 32.5 to 97.5% by the process.

Abstract We predict the production yield of a medical radioisotope ${}^{67}$ Cu using ${}^{67}$ Zn(n, p) ${}^{67}$ Cu and ${}^{68}$ Zn(n, pn) ${}^{67}$ Cu reactions with fast neutrons provided from laser-driven neutron sources. The neutrons were generated by the p+ ${}^9\mathrm{Be}$ and d+ ${}^9$ Be reactions with high-energy ions accelerated by laser–plasma interaction. We evaluated the yield to be (3.3 $\pm$ 0.5) $\times$ 10 ${}^5$ atoms for ${}^{67}$ Cu, corresponding to a radioactivity of 1.0 $\pm$ 0.2 Bq, for a Zn foil sample with a single laser shot. Using a simulation with this result, we estimated ${}^{67}$ Cu production with a high-frequency laser. The result suggests that it is possible to generate ${}^{67}$ Cu with a radioactivity of 270 MBq using a future laser system with a frequency of 10 Hz and 10,000-s radiation in a hospital.

Abstract The enhancement of plasma heating efficiency is important for realizing inertial fusion energy, which follows the ignition achieved with the laser-indirect-drive central-ignition approach. An increase in the relativistic electron beam (REB) generation efficiency, an improvement in REB directivity, and the optimization of REB energy distribution are key for enhancing the heating efficiency in the fast ignition approach using laser-produced REBs. Here, we show that an extreme ultra-high contrast of the heating laser pulse is essential for efficiently heating high-density plasma using the fast-ignition approach of inertial confinement fusion. A heating efficiency enhancement of more than 2 x was obtained with a high-contrast heating laser compared to that obtained with the lower-contrast LFEX laser. The enhanced heating efficiency is ascribed to the significant reduction of the REB divergence and the increased number of electrons injected into the plasma. The interaction between the high-contrast laser and a plasma suppresses excessive acceleration of the REB The high contrast shortens the distance between the REB generation point and the high-density plasma. However, it is clarified that ultra-high contrast reduces the laser absorption fraction, future research on improving laser absorption for high-contrast lasers is required.

In this study, we experimentally evaluate the ion transportation through a cone guide target, which accelerates ions up to MeV energies via target normal sheath acceleration, and transports them onto the position of imploding fuel in the fast ignition scenario of nuclear fusion. We measured the electric and magnetic fields (EM-fields) induced by return current streaming along the cone wall by proton radiography, and we report that the EM-fields are predominantly induced within a temporal window up to 30 ps after the laser injection. The magnitude of the electric field is maximized around 13 ps, reaching $4.0\times 10^{10} \mathrm {V}\ \mathrm {m}^{-1}$ , when the magnetic field is below 200 T. The present scheme provides insights on the EM-fields evaluation in the time region that is difficult to treat with simulations due to the computing resources.

We produced high-energy neutrons by p + Be and d + Be reactions with protons and deuterons with continuous energies of up to 30 and 10 MeV/u, respectively, provided from laser-plasma interactions. Thermal neutrons were subsequently generated through deceleration by a small moderator with conjunction on the Be target. Here, we report the measurements on the thermal neutron fluence using the activation method with/without Cd filters. The unstable isotopes of Au-198, Mn-56, Co-60, and Hf-181 were produced through (n, gamma) reactions by a single laser shot. The nuclear reaction fractions were evaluated by measuring the gamma-rays emitted from these unstable isotopes. The thermal neutron fluences from the four nuclides are consistent within their experimental uncertainties. The fluence is evaluated to be (2.2 +/- 0.4) x 10(5) neutrons cm(-2) at the distance of approximately 9 mm at 90 degrees against the laser axis. The present result shows that the method to generate thermal neutrons from a compact neutron source with a diameter of 44 mm and length of 46 mm is expected to be a useful tool for various nuclear experiments and applications.

A test experiment to polarize tritium nuclei to develop a polarized deuterium-tritium (D-T) laser fusion concept is proposed in which a ferromagnetic complex with a high internal magnetic field is used to polarize tritium nuclei on physisorbed D-T molecules with an internal beta-decay heat load in a D-T target. Heteronuclear hydrogen deuteride (HD) is used to conduct the measurements herein instead of as in typical D-T-based experiments. As proof-of-concept experimentation, the adsorption and desorption characteristics of HD are examined on Prussian blue ferromagnetic analogue Ni-3[Fe(CN)(6)](2) at temperatures of 77 K and around 23 K. Nuclear magnetic resonance (NMR) analysis of the ferromagnetic complex-mediated adsorption of HD onto activated carbon pellets at 10 K is conducted step by step using a multilocular probe cell that had been simplified to give a single-tube probe cell. The resulting H-1 NMR spectra are compared with F-19 NMR spectra obtained for reference on a Kel-F probe cell wall. Slight differences between the calculated NMR frequency from the gyromagnetic ratio and the actually observed NMR frequency are also discussed.

The thermal and epithermal neutrons are powerful probes to inspect water or high-pressure hydrogen gas because of their large scattering cross-sections with protons. Laser-driven neutron source, which is able to simultaneously emit different types of radiations such as x rays, can be used for neutron and x-ray radiography in the same laser shot. In this paper, we report the demonstration of non-destructive inspection for H2O contained within a stainless steel pipe using a laser-driven thermal neutron source, where water and stainless containers are detected by neutrons and x rays, respectively. The simulation result indicates that this method can also provide the capability to measure the hydrogen density in high-pressure hydrogen gas in metal containers.

We demonstrate that $(n,2n)$ reactions are induced by a high-flux pulse of fast neutrons ($\ensuremath{\sim}{10}^{10}$ neutrons in $\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$) provided from a laser-driven neutron source (LDNS). The maximum energy of the broadband neutrons reaches a few tens MeV. Several kinds of metal targets are exposed to the fast neutrons. As a result, unstable isotopes, such as $^{54}\mathrm{Mn}, ^{58}\mathrm{Co}, ^{175}\mathrm{Hf}$, and $^{196}\mathrm{Au}$ are produced by $(n,2n)$ reactions and $^{180}\mathrm{Hf}^{m}, ^{181}\mathrm{Hf}, ^{56}\mathrm{Mn}, ^{198}\mathrm{Au}$, and $^{60}\mathrm{Co}$ are produced by $(n,\ensuremath{\gamma})$ reactions. We evaluate the neutron fluence and energy spectrum using the activation method in conjunction with a time-of-flight measurement. The neutron fluence is determined to be $(4.3\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{8}\phantom{\rule{0.16em}{0ex}}\mathrm{neutrons}/\mathrm{c}{\mathrm{m}}^{2}$ in the energy range from approximately 8 to 20 MeV at 8-mm downstream of the neutron source. The present scheme provides a method to evaluate high-density neutrons seen in stellar environments, which are expected to be generated from future LDNSs.

Thermal neutron (∼25 meV) beam is a powerful tool for investigating the structure and properties of materials used in science and technology. A laser-driven neutron source generating 10 10 neutrons within 1 ns duration is utilized to a single shot radiography with a dual beam of thermal neutrons and X-rays. As a proof of principle, we show the non-destructive inspection of hazardous substances (Cadmium) contained in a typical battery, when the cadmium anode thickness is evaluated from the transmittance of thermal neutrons. The fact that the neutron inspection above is performed with a single laser shot, i.e. with a single bunch of neutrons leads to a safer neutron source that is optically controlled on/off, and provides a novel tool for science and engineering.

The infrared (IR) heating method for a central ignition target with spherical symmetry is modified for the axisymmetric Fast Ignition Realization EXperiment (FIREX) target. The challenge is that the FIREX target pretends to be a thermally spherical shell. Our previous simulation studies (A. Iwamoto et al., Fusion Sci. Technol. 56, 427 (2009), A. Iwamoto et al., J. Phys.: Conf. Ser. 244, 032039 (2010)) have shown that the combination of volumetric heating in a fuel and cone temperature control has the potential to finish a uniform fuel layer. We have developed the IR heating system, dedicated to the FIREX target, with exclusive cone temperature control. The ability of solid fuel layering was examined by using an 826 mu m polystyrene (PS) shell with a gold cone of 1.2 mm in length instead of the 500 mu m FIREX target for easy observation. The system could control the profile of a solid fuel layer in the PS shell target. Eventually, the solid layer with the best sphericity of 92% was formed, and the RMS roughness of the inner surface was 44-49 mu m in modes 1 to 100 and 14-26 mu m in modes 5 to 100. (C) 2021 The Japan Society of Plasma Science and Nuclear Fusion Research

In inertial confinement fusion (ICF), a fuel target containing deuterium and tritium is used. In recent ICF experiments on the Gekko XII LFEX facility at the Institute of Laser Engineering at Osaka University (ILE-Osaka), a target comprised of a polystyrene capsule filled with D2O liquid and a solution of X-ray tracer materials, such as copper, titanium, or chlorine, was developed. In this study, an additional T2O doping technique by which tritium can be mixed uniformly has been developed. The T2O is synthesized by T-2 gas using a CuO oxidation catalyst. The T2O is agglutinated by cold trap and transferred to a target cell in which a D2O-solution-filled target is placed. Because polystyrene is slightly permeable for T2O and D2O, D2O is exchanged by T2O and completely mixed. Thus, a uniform tritium-doped ICF target with various materials can be fabricated. The T2O synthesizing and doping system is developed and tested using H-2 as a cold run. The H2O is successfully doped to a D2O prefilled target at approximately 50% doping. This scheme will be utilized in future fast ignition experiments at ILE-Osaka.

Fast isochoric laser heating is a scheme to heat matter with a relativistic intensity (>10^{18}  W/cm^{2}) laser pulse for producing an ultrahigh-energy-density (UHED) state. We have demonstrated an efficient fast isochoric heating of a compressed dense plasma core with a multipicosecond kilojoule-class petawatt laser and an assistance of externally applied kilotesla magnetic fields for guiding fast electrons to the dense plasma. A UHED state of 2.2 PPa 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 confirmed 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.

Here we report on the effects of material strength factors on the generation of surface structure due to nonuniform laser irradiation. The influence of material strength on the generation of perturbation on a diamond surface subjected to nonuniform laser irradiation was experimentally investigated. Our previous investigations suggested that stiffer and denser materials reduce surface perturbation due to spatially nonuniform laser irradiation, which was reproduced well by calculations with multi-dimensional hydrodynamic simulation code. In this work, we found that local fractures due to yield strength failure are generated by high degrees of irradiation non-uniformity. A characteristic crack-like surface structure was observed, which was not reproduced by the 2D simulation code calculations at all. The 2D simulations showed that the pressure at the diamond surface locally exceeds the Hugoniot elastic limit due to nonuniform irradiation, implying the potential for development of surface perturbations. We also measured the areal-density distribution of perturbations for single-crystal diamond and diamond with a thin high atomic number (high-Z) coating on its surface. The experimental results imply that the combination of a stiff material and thin high-Z coating can suppress the solid-strength effects caused by large irradiation non-uniformity. The knowledge given here is applicable to inertial confinement fusion target design, laser material processing, and universal problems involving solids and high-energy-density plasmas.

The nuclear burn history provides critical information about the dynamics of the hot-spot formation and high-density fuel-shell assembly of an Inertial Confinement Fusion (ICF) implosion, as well as information on the impact of alpha heating, and a multitude of implosion failure mechanisms. Having this information is critical for assessing the energy-confinement time τE and performance of an implosion. As the confinement time of an ICF implosion is a few tens of picoseconds, less than 10-ps time resolution is required for an accurate measurement of the nuclear burn history. In this study, we propose a novel 1-ps time-resolution detection scheme based on the Pockels effect. In particular, a conceptual design for the experiment on the National Ignition Facility and OMEGA are elaborated upon herein. A small organic Pockels crystal “DAST” is designed to be positioned ∼5 mm from the ICF implosion, which is scanned by a chirped pulse generated by a femto-second laser transmitted through a polarization-maintained optical fiber. The originally linearly polarized laser is changed to an elliptically polarized laser by the Pockels crystal when exposed to neutrons, and the modulation of the polarization will be analyzed. Our study using 35-MeV electrons showed that the system impulse response is 0.6 ps. The response time is orders of magnitude shorter than current systems. Through measurements of the nuclear burn history with unprecedented time resolution, this system will help for a better understanding of the dynamics of the hot-spot formation, high-density fuel-shell assembly, and the physics of thermonuclear burn wave propagation.

The dependence of the mean kinetic energy of laser-accelerated relativistic electrons (REs) on the laser intensity, so-called ponderomotive scaling, explains well the experimental results to date; however, this scaling is no longer applicable to multi-picosecond (multi-ps) laser experiments. Here, the production of REs was experimentally investigated via multi-ps relativistic laser–plasma-interaction (LPI). The lower slope temperature shows little dependence on the pulse duration and is close to the ponderomotive scaling value, while the higher slope temperature appears to be affected by the pulse duration. The higher slope temperature is far beyond the ponderomotive scaling value, which indicates super-ponderomotive REs (SP-REs). Simulation and experimental evidence are provided to indicate that the SP-REs are produced by LPI in an under-critical plasma, where a large quasi-static electromagnetic field grows rapidly after a threshold timing during multi-ps LPI.