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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.

The effect of laser ignition on the deflagration-to-detonation transition (DDT) was experimentally investigated. Explosive gas, which was 0.87[(1/4)C2H4+(3/4)O2]+0.13N2 contained in a smooth-wall tube at 100 kPa and approximately 20 °C, was ignited by a 1064-nm 12-ns laser at either 8 or 88.8 mm from the closed tube end connected to the gas-feeding pipe, where the incident laser energy was either 40, 80, or 120 mJ. When the gas was ignited at 8 mm from the closed tube end, although laser ignition promoted DDT, the run-up distance to DDT was shortened by approximately 10% only. This was because the DDT behavior was essentially the same as the one observed typically in ordinary spark-plug ignition, in which a flame is accelerated approximately to be a choked flame before the detonation onset. When the gas was ignited at 88.8 mm from the closed tube end, two DDT scenarios were observed. The first scenario was the typical one mentioned above. In the second scenario, the detonation onset was induced by the collision between the flame and a shock wave following the flame before the flame became the choked flame. Significant DDT promotion was realized in the second scenario by the use of laser ignition with a larger energy. The shock wave inducing the detonation onset was likely created by the end-gas autoignition near the tube end or inside the gas-feeding pipe. As a result of the analysis on the shock wave, the mixing of the unburned and burned gasses induced by the vortex generation at the flame front by the baroclinic torque (Richtmyer-Meshkov instability) is the most plausible mechanism for initiating the micro-explosions that trigger the detonation onset.

The self-similar propagation of expanding spherical flames in lean hydrogen–air mixtures at elevated pressure was experimentally investigated using a dual-chamber apparatus with the quartz windows that enabled observing a flame radius of up to 110 mm. The flame images of the fully developed cellular structure were recorded, and the largest values of r/rc and Pe for the dual-chamber apparatus were estimated. The acceleration exponent, α, corresponding to the fractal dimension in lean hydrogen–air mixtures at elevated pressure was evaluated by plotting the experimental flame radius as a function of time. In this study, the experimental values of α (α = 1.25–1.43) occur within a wide range of r/rc (r/rc = 2–15). The observed trend of α to increase with a decrease in equivalence ratio, φ, and an increase in initial pressure, Pi, is owing to the effects of the diffusional–thermal and Darrieus–Landau instabilities. The value of α increased with an increase in r/rc and saturated at α = 1.43 at r/rc > 10, and the saturated values of the hydrogen–air flame are within α = 1.4–1.5. The results demonstrated that the flame propagation in lean hydrogen–air mixtures was classified as laminar (r/rc 1), transition (1 10) regimes. Consequently, the practical three-dimensional fractal dimensions for accidental hydrogen–air explosions are D3 = 2.24–2.33.

A counter-propagating laser-beam platform using a spherical plasma mirror was developed for the kilojoule-class petawatt LFEX laser. The temporal and spatial overlaps of the incoming and redirected beams were measured with an optical interferometer and an x-ray pinhole camera. The plasma mirror performance was evaluated by measuring fast electrons, ions, and neutrons generated in the counter-propagating laser interaction with a Cu-doped deuterated film on both sides. The reflectivity and peak intensity were estimated as ∼50% and ∼5 × 1018 W/cm2, respectively. The platform could enable studies of counter-streaming charged particles in high-energy-density plasmas for fundamental and inertial confinement fusion research.

In this study, two critical Peclet numbers, Pec, for the onset of cracks on the flame surface, and, Pecl, for the onset of fully developed cellular flame leading to flame acceleration, of hydrogen–air flames were experimentally evaluated. The dependency of Pecl on Mab, the Markstein number for flame speed, and Kacl, the critical Karlovitz number, were discussed and compared with previous experiments for various fuel-air mixtures. In this study, we investigated the correlation between Pecl and Mab rather than that between Pecl and Masr, Markstein number for the burning velocity strain rate. The results demonstrated that the values of Pec and Pecl increased with an increase in Mab, because the transitions to cellularity were gradually affected by the intensity of the diffusional-thermal instability. The linear correlation observed between Pecl and Mab qualitatively agreed with the results of previous works on other types of fuels. The dependency of Kacl on Mab was estimated. The values of Kacl rapidly decreased with an increase in Mab. Empirical correlations were derived to estimate the critical conditions for the onset of flame acceleration due to cellular flame instabilities. These empirical expressions approximatively correlated well with the experimental data.

In this study, Minimum Ignition Energy (MIE) and Minimum Explosible Concentration (MEC) of L-isoleucine and glycine powder with two different particle size distributions were experimentally investigated. The experimental results showed that the values of MIE and MEC of L-isoleucine powder were lower than those of glycine powder. The pyrolysis and the combustible products of glycine and L-isoleucine powders were measured. The results showed that a number of flammable and combustible materials are formed during the pyrolysis of L-isoleucine, although few flammable compounds are generated during the pyrolysis of glycine due to polymerization. This result indicated that the L-isoleucine powder has a high risk of dust explosion as compared to the glycine powder. Consequentially, an appropriate risk assessment for the L-isoleucine powder with fine particles is necessary in the powder handle processes.

Heat transport properties in a kilotesla transverse magnetic field are compared for the two propagation modes, so-called ordinary (O) and extraordinary (X) modes, of laser using one-dimensional collisional particle-in-cell simulations with ablation-plasma-like settings. The evanescent wave of the X-mode laser is expected to cause an upper hybrid resonance from its dispersion relation, as opposed to the O-mode. Our simulations revealed that significantly hot electrons can be generated by the resonance even if the evanescent wave is initially unreachable to the resonance position, because the radiation pressure steepens the density profile and shortens the cutoff-to-resonance distance. The hot electrons are found to produce a significant amount of heat flux while keeping the transport by the bulk electrons, which results in an enhanced total heat transport.

Fast heating efficiency when a pre-imploded core is directly heated with an ultraintense laser (heating laser) was investigated. ‘Direct heating’ means that a heating laser hits a pre-imploded core without applying either a laser guiding cone or an external field. The efficiency, η, is defined as the increase in the internal core energy divided by the energy of the heating laser. Six beams (output of 1.6 kJ) from the GEKKO XII (GXII) green laser system at the Institute of Laser Engineering (ILE), Osaka University were applied to implode a spherical deuterated polystyrene (CD) shell target to form a dense core. The DD-reacted protons and the core x-ray emissions showed a core density of 2.8 ± 0.7 g cm−3, or 2.6 times the solid density. Furthermore, DD-reacted thermal neutrons were utilized to estimate the core temperature between 600 and 750 eV. Thereafter, the core was directly heated by a laser for fast-ignition experiments (LFEX, an extremely energetic ultrashort pulse laser) at ILE with its axis lying along or perpendicular to the GXII bundle axis, respectively. The former and latter laser configurations were termed ‘axial’ and ‘transverse modes’, respectively. The η was estimated from three independent methods: (1) the core x-ray emission, (2) the thermal neutron yield, and (3) the runaway hot electron spectra. For the axial mode, 0.8% < η < 2.1% at low power (low LFEX energy) and 0.4% < η < 2.5% at high power (high LFEX energy). For the transverse mode, 2.6% < η < 7% at low power and 1.5% < η < 7.7% at high power. Their efficiencies were compared with that in the uniform implosion mode using 12 GXII beams, 6% < η < 12%, which appeared near to the η for the transverse mode, except that the error bar is very large.

In the counter-irradiation, which is one of the fast ignition schemes, higher core energy coupling can be expected when there are two hot electron flows in counter directions. Two plasma mirrors were installed for the counter irradiation at about 180 degrees. The hot electron effective temperatures (Teff) were measured by using electron energy spectrometers. Teff vs the laser intensity on a foil target followed Wilkes' scaling law. The energy incident on the target could be calculated by estimating the laser intensity on the target from Teff and estimating the focusing radius from the X-ray pinhole camera image. As a result, the reflectivity could be estimated to be 17 ± 3%.

A laser inertial fusion energy (IFE) reactor requires repetitive injection of fuel pellets and laser engagement to fuse fusion fuel beyond a few Hz. We demonstrate 10 Hz free-fall bead pellet injection and laser engagement with γ-ray generation. Deuterated polystyrene beads with a diameter of 1 mm were engaged by counter illuminating ultra-intense laser pulses with an intensity of 5 × 1017 W cm−2 at 10 Hz. The spatial distribution of free-fall beads was 0.86 mm in the horizontal direction and 0.18 mm in the vertical direction. The system operated for more than 5 min and 3500 beads were supplied with achieved frequencies of 2.1 Hz for illumination on the beads and 0.7 Hz for γ-ray generation; these frequencies were three times greater than with the previous 1 Hz injection system. The duration of operation was limited by the pellet supply. This injection and engagement system could be used for laser IFE research platforms.

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 inertial confinement fusion (ICF) ignition sparks. Laser-produced relativistic electron beam (REB) deposits a part of kinetic energy in the core, and then the heated region becomes the hot spark to trigger the ignition. However, due to the inherent large angular spread of the produced REB, only a small portion of the REB collides with the core. Here, we demonstrate a factor-of-two enhancement of laser-to-core energy coupling with the magnetized fast isochoric heating. The method employs a magnetic field of hundreds of Tesla that is applied to the transport region from the REB generation zone to the core which results in guiding the REB along the magnetic field lines to the core. This scheme may provide more efficient energy coupling compared to the conventional ICF scheme., It is desirable to deposit more energy in the dense plasma core to trigger the fusion ignition. Here the authors demonstrate enhanced energy coupling from laser to plasma core by using solid targets and guiding the transport of relativistic electron beam with external magnetic field.

The development of cellularity for expanding spherical hydrogen-air flames at elevated pressure, owing to hydrodynamic and diffusional-thermal instabilities was experimentally investigated using high-speed schlieren images. The critical flame radii and critical Peclet numbers at which cracks begin to propagate and for the onset of flame acceleration were obtained. The dependency of Pecl on Ma for hydrogen-air flames was compared with that for other fuels. The results showed that cellular development depends on the intensity of the diffusional-thermal instability. The acceleration exponent and fractal excess for the acceleration regime of a hydrogen-air flame were evaluated. The values show that the transition regime and self-similar regime exist in the acceleration regime of expanding spherical hydrogen-air flames.

Laser ignition and spark-plug ignition were experimentally compared in high-speed ethylene-oxygen mixture flows of up to approximately 100 m/s. Nd:YAG laser of 12-ns pulse duration and a semi-surface-discharge-type spark plug of 1.8-ms discharge duration were used to conduct the experiments with deposited energy of approximately 24 mJ in both cases. The self-emission was observed by a high-speed camera. The flame-spread behavior and ignition ability were examined in lean-fuel conditions. The study findings revealed that laser ignition was superior to the spark-plug ignition in the aspect of the early-stage rapid flame spread, although it showed lower probability of successful ignition than that by the spark plug near the lean-fuel ignitable limit. These findings suggest that the ignition in high-speed flows is significantly influenced by the turbulence via the enhancement of heat transport in particular.

For the fast ignition scheme of laser fusion, a highly compressed fuel core is necessary using a cone-inserted spherical solid target. We design an optimal compression method with a multistep laser pulse shape using a 2-D radiation hydrodynamics simulation. In the design, the maximum areal density is ρRmax =0.46 g/cm2 with 8 kJ of implosion laser. Considering the scaling law of hydrodynamic, we obtained 82 kJ and 1.3 MJ for the ignition-scale-target (ρRmax =1.1 g/cm2) and for burning-scale-target (ρRmax =2.5 g/cm2), respectively. In a preliminary study of hydrodynamic instability in the optimized cone-inserted implosion, there is no significant growth of the instabilities that affect the implosion performance. A parameter search to investigate the feasibility and robustness of a high areal density design for changes in the laser pulse shape is performed. As a result, a delay of several tens of pico-second of the rising laser pulse is acceptable.

For examining the ability of laser ignition, a quiescent lean-fuel propane–air mixture was ignited in a constant-volume chamber by a spark induced by a pair of electrodes and an automotive spark driver, a spark induced by focused high-power laser, or a spark induced by focused high-power laser between a pair of dummy electrodes, where the deposited energy into the gas was the same in all cases. In the experiments, the mole fraction of the fuel in the mixture was varied, and the pressure history on the inner wall of the chamber, probability of successful ignition, and the high-speed multi-frame schlieren images of an ignition process were obtained. The results showed that the ignition ability of a laser-induced spark was superior to that of a conventional electrical spark in lean-fuel conditions near the ignitable limit, and that the higher ignition ability of a laser-induced spark was due not only to the lack of heat loss to electrodes but also to a large initial flame kernel whose effective energy was augmented by rapid heat release from the combustible mixture sucked into the kernel by a non-spherical inward flow created by a laser-induced spark. In addition, reinjection of the transmitted laser light into plasma strengthened the ignition ability of the laser. This was predominantly due to the increase of the total absorbed energy.

We want to provide the scientific community with a new soft x-ray microscope (SXRM), that: Provides 50 nm spatial resolution, Is powered by a commercial laser and Comes at convinient tabletop size. To realize such a system, a bright short-pulsed soft x-ray source must be developed., The 11th International Conference on Inertial Fusion Sciences and Applications (IFSA 2019)

To generate bright water-window (WW) soft x rays (2.3–4.4 nm), gold slab targets were irradiated with laser pulses (1064 nm, 7 ns, 1 J). Emission spectroscopy showed that the introduction of low-pressure nitrogen enhanced the soft x-ray yield emitted from the laser-produced Au plasma. The intensity of the WW x-ray transported in a 400-Pa N2 atmosphere from the laser-produced plasma increased by 3.8 times over that in vacuum. Considering a strong x-ray absorption, the x-ray yield emitted directly from the Au plasma in the N2 gas was evaluated to be 13 times higher than that in vacuum. Although similar measurements were made for various gases, only N2 gas causes an increase in a soft x-ray yield. The processes leading to this enhancement mechanism were revealed by using hydrodynamic simulation and atomic structure codes.

To develop laser-ignition technology, transparent glass plates were artificially sooted and irradiated repetitively by laser from the front (sooted) and back sides separately. Generally, the back-side irradiation was more effective at soot removal. However, the cleaning effect was saturated after thousands of laser shots. Although the saturated soot quantity was a decreasing function of the laser fluence per pulse, its magnitude remained the same for both front-side and back-side irradiations. In examining several soot-removal mechanisms proposed so far, it was found that the aerodynamic force produced by the flow induced by the laser heating of the soot was the most plausible mechanism.

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., 8 pages, 4 figures, 1 table

We present a systematic approach how to characterize the dynamics of a laser-produced Au plasma, generated with a commercial Joule-class laser system. By means of our diagnostic methods, we identify the dominant ionic species within the water-window spectral region, as well as the radiated energy, the x-ray emission duration and the total dimensions of the x-ray source. We present additional analysis through radiation hydrodynamic simulations to provide details about the electron temperature and density profiles and further show that the plasma exhibits nearly 20% self-absorption, while the electron temperature is limited to 400 eV. A comparison with the experimental data further allows us to verify the applied numerical code.

We conducted fast heating of a core plasma produced by counter-illumination of two beam bundles of implosion lasers for two incident directions of the heating laser. For the axial heating configuration, wherein the heating laser was incident along the coaxial direction of the implosion lasers, the size and density of the counter-imploded core plasma were estimated. The incident timing of the heating laser was determined. Although the heating laser was incident during the stagnation of the core plasma, there were no considerable increases in the intensity of X-rays with energies beyond 1 keV and the yield of the thermonuclear fusion neutrons. Effective core heating did not occur due to the interaction of the heating laser with the low-density ablated plasma far from the core. On the other hand, for the transverse heating configuration, wherein the heating laser was incident from the transverse direction of the implosion laser axis, optimization of the incident timing of the heating laser was not achieved. However, two-dimensional radiation-hydrodynamic simulation revealed that a heating laser can directly interact with a core plasma and effective heating is expected. In our scheme, transverse heating configuration appears to be significant.

A Monte Carlo (MC) code simulating the kinetics of two-particle collisions has shown to have certain advantages in the spectral analysis of neutrons from fast-ignition inertial confinement fusion (ICF). The MC code quickly displays neutron spectra produced in the expected fusion plasma kinetics (incl. ion energy spectrum, anisotropy, and collision geometry). This allows to explore various potential mechanisms of neutron production without using time-consuming hydrodynamic or particle simulations and makes it possible to give a feedback to ongoing experiments. The validity of this model was demonstrated in the ”cone-free-shell” fast ignition experiment at the GEKKO-XII - LFEX laser facility, where the model provided a clear explanation for the significant spectral modulation of D(d, n)3He neutrons generated through the interaction between a high-power laser and a long-scale plasma.

The kilo-tesla-class converging magnetic field effects on the core heating properties and the energy and duration of relativistic electron beam (REB) required for ignition in fast ignition laser fusion has been evaluated by parametric 2D hybrid simulations where a uniformly compressed DT core with ρ = 300 g/cm3 is heated by REB with TREB = 3 MeV, IREB = 0.69 × 1020 W/cm2 and θREB = 40° under sufficiently strong magnetic field (Bext = 3 kT at the REB injection position). The kilo-tesla-class converging magnetic field can intensify REB and then enhances the core heating power drastically. As the results, the fusion ignition can be initiated by the REB with EREB ≧ 50 kJ and τREB = 10 ~ 30 ps. With increasing mirror ratio RM, the negative effect of converging field, i.e., mirror reflection, becomes significant, which enhances the REB energy required for ignition (EREB,ig = 49 kJ for RM = 3 → EREB,ig =77 kJ for RM = 20).

Experiments on thermal spray by pulsed detonations at 150 Hz were conducted. Two types of pulse detonation combustors were used, one operated in the inert gas purge (GAP) mode and the other in the liquid-purge (LIP) mode. In both modes, all gases were supplied in the valveless mode. The GAP mode is free of moving components, although the explosive mixture is unavoidably diluted with the inert gas used for the purge of the hot burned gas. In the LIP mode, pure fuel-oxygen combustion can be realized, although a liquid-droplet injector must be actuated cyclically. The objective of this work was to demonstrate a higher spraying temperature in the LIP mode. First, the temperature of CoNiCrAlY particles heated by pulsed detonations was measured. As a result, the spraying temperature in the LIP mode was higher than that in the GAP mode by about 1000 K. Second, the temperature of yttria-stabilized zirconia (YSZ) particles, whose melting point was almost 2800 °C, heated by pulsed detonations in the LIP mode was measured. As a result, the YSZ particles were heated up to about 2500 °C. Finally, a thermal spray experiment using YSZ particles was conducted, and a coating with low porosity was successfully deposited.

The first demonstration of target acceleration by a strong magnetic field is conducted. Two miniature targets, a gold ball and a neodymium permanent magnet, are accelerated by a magnetic field. A magnetic-field strength as high as 1 kT is produced using a capacitor-coil in which two copper disks are connected by a single-turn coil, and the capacitor was driven by two beams from the GEKKO-XII high-power laser. The targets are accelerated by the interaction force between two magnetic moments: the target and the capacitor-coil. The velocity of the accelerated gold ball was estimated as 10 m/s from shadowgraphy images. In the case of the magnet, although the image could not be observed clearly due to the plasma generated around the magnet, it was confirmed that the magnet interacts with the magnetic field of the capacitor-coil.

Solid-density plasmas driven by intense x-ray free-electron laser (XFEL) radiation are seeded by sources of non-thermal photoelectrons and Auger electrons that ionize and heat the target via collisions. Simulation codes that are commonly used to model such plasmas, such as collisional-radiative (CR) codes, typically assume a Maxwellian distribution and thus instantaneous thermalization of the source electrons. In this study, we present a detailed description and initial applications of a collisional particle-in-cell code, PICLS, that has been extended with a self-consistent radiation transport model and Monte-Carlo models for photoionization and KLL Auger ionization, enabling the fully kinetic simulation of XFEL-driven plasmas. The code is used to simulate two experiments previously performed at the Linac Coherent Light Source investigating XFEL-driven solid-density Al plasmas. It is shown that PICLS-simulated pulse transmissions using the Ecker-Kr\"oll continuum-lowering model agree much better with measurements than do simulations using the Stewart-Pyatt model. Good quantitative agreement is also found between the time-dependent PICLS results and those of analogous simulations by the CR code SCFLY, which was used in the analysis of the experiments to accurately reproduce the observed K{\alpha} emissions and pulse transmissions. Finally, it is shown that the effects of the non-thermal electrons are negligible for the conditions of the particular experiments under investigation., Comment: 12 pages, 11 figures

Using one of the world most powerful laser facility, we demonstrate for the first time that high-contrast multi-picosecond pulses are advantageous for proton acceleration. By extending the pulse duration from 1.5 to 6 ps with fixed laser intensity of 1018 W cm−2, the maximum proton energy is improved more than twice (from 13 to 33 MeV). At the same time, laser-energy conversion efficiency into the MeV protons is enhanced with an order of magnitude, achieving 5% for protons above 6 MeV with the 6 ps pulse duration. The proton energies observed are discussed using a plasma expansion model newly developed that takes the electron temperature evolution beyond the ponderomotive energy in the over picoseconds interaction into account. The present results are quite encouraging for realizing ion-driven fast ignition and novel ion beamlines.

Laser ignition and spark-plug ignition were compared in high-speed ethylene-oxygen mixture flows of up to approximately 100 m/s. In the experiments, an Nd:YAG laser of 12-ns pulse duration and a semi-surface-discharge-type spark plug of 1.8-ms discharge duration were used, where the deposited energy was approximately 24 mJ in both cases. Observing the self-emission using a high-speed camera, the flame-spread behavior and ignition ability were examined in lean-fuel conditions. The laser ignition was superior to the spark-plug ignition in the aspect of rapid flame spread, although the probability of successful ignition for the laser ignition was a little lower than that for the spark-plug ignition near the lean ignitable limit.

For the fast ignition of laser fusion, a reliable target design is required for an ignition scale target. This paper shows the first optimized target design for the implosion phase of fast ignition, which is scalable to larger targets. The requirements from the heating process are taken account. In conclusion, a target can be highly compressed using multi-step laser pulse irradiation to a solid spherical target with an inserted gold cone. In an 8 kJ scale implosion, the maximum areal density of deuterium–tritium fuel reaches 0.39 g cm−2 according to 2D simulation results, which is 62% of the case without the cone. Based on the similarity rule, we estimate that the requirement of the implosion laser energy for ignition scale targets (ρR max =1.0 g cm−2) is 135 kJ.

Absolute Kα line spectroscopy is proposed for studying laser–plasma interactions taking place in the Au cone-guided fast ignition targets. X-ray spectra ranging from 20 to 100 keV were quantitatively measured with a Laue spectrometer composed of a cylindrically curved crystal and a filter-absorption method for Bremsstrahlung continuum emission. The absolute sensitivities of the Laue spectrometer systems were calibrated using pre-characterized laser-produced X-ray sources and radioisotopes. The integrated reflectivity for the crystal is in good agreement with predictions by an X-ray diffraction code. The energy transfer efficiency from incident laser beams to hot electrons, as the energy transfer mechanism, is derived from this work. The absolute yield of Au and Ta Kα lines were measured in the fast ignition experimental campaign performed at Institute of Laser Engineering, Osaka University. Applying the hot electron spectrum information from electron spectrometer and scaling laws, the energy transfer efficiency from the incident LFEX, a kJ-class PW laser, to hot electrons was derived for the first time.

In fast ignition, optimization of laser profile for core heating is one approach to ignite the core. However, the profile is not so optimized and its effects on fast electron characteristics are not fully clarified yet. The laser profile is optimized under the condition of same laser energy because laser energy is restricted in experiments. Therefore, we investigate effects of laser profile on fast electron generation under the condition of same laser energy. In this paper, each effect of laser temporal and spatial profile is estimated independently using two-dimensional Particle-In-Cell simulations. We conclude that lower intensity laser suitable for fast ignition under the limit of simulated parameters when energy of laser is same because efficient core-heating electrons are much generated and divergence angle is smaller in low-intensity case compared to high-intensity case.

The FIREX-1 project, the goal of which is to demonstrate fuel heating up to 5 keV by fast ignition scheme, has been carried out since 2003 including construction and tuning of LFEX laser and integrated experiments. Implosion and heating experiment of Fast Ignition targets have been performed since 2009 with Gekko-XII and LFEX lasers. A deuterated polystyrene shell target was imploded with the 0.53- μm Gekko-XII, and the 1.053- μm beam of the LFEX laser was injected through a gold cone attached to the shell to generate hot electrons to heat the imploded fuel plasma. Pulse contrast ratio of the LFEX beam was significantly improved. Also a variety of plasma diagnostic instruments were developed to be compatible with harsh environment of intense hard x-rays (γ rays) and electromagnetic pulses due to the intense LFEX beam on the target. Large background signals around the DD neutron signal in time-of-flight record of neutron detector were found to consist of neutrons via (γ,n) reactions and scattered gamma rays. Enhanced neutron yield was confirmed by carefully eliminating such backgrounds. Neutron enhancement up to 3.5 × 107 was observed. Heating efficiency was estimated to be 10-20% assuming a uniform temperature rise model. © Owned by the authors, published by EDP Sciences, 2013.

To enhance the core heating efficiency in fast ignition laser fusion, the concept of relativistic electron beam guiding by external magnetic fields was evaluated by integrated simulations for FIREX class targets. For the cone-attached shell target case, the core heating performance is deteriorated by applying magnetic fields since the core is considerably deformed and the most of the fast electrons are reflected due to the magnetic mirror formed through the implosion. On the other hand, in the case of cone-attached solid ball target, the implosion is more stable under the kilo-tesla-class magnetic field. In addition, feasible magnetic field configuration is formed through the implosion. As the results, the core heating efficiency becomes double by magnetic guiding. The dependence of core heating properties on the heating pulse shot timing was also investigated for the solid ball target., Comment: 9pages, 12 figures, submitted to Plasma Physics and Controlled Fusion

Implosion and heating experiments at the Institute of Laser Engineering, Osaka University on Fast Ignition (FI) targets for the FIREX-1 project have been performed with Gekko-XII laser for implosions and LFEX laser for heating. We tried to reduce the prepulse level in the LFEX laser system and have improved the plasma diagnostics to observe the plasma in the harsh hard X-ray environment. A plastic (CD) shell target, 7-μm thick and 500 μm in diameter with a hollow gold cone was used in this experiment to guide the short-pulse laser at the time of the maximum compression. The shell target was imploded with 9 or 12 beams of Gekko-XII laser (527 nm) with energy of 300 J/beam in a 1.5 ns pulse. Two of the four LFEX laser (1053 nm) beams were injected into the inside bottom of the cone with an energy up to 0.7 kJ/beam in a 1.5 ps pulse at the time around the maximum implosion. We have observed neutron enhancement up to 3.5 × 107 with total heating energy of 300 J, which is higher than the yield obtained in the previous experiment in 2002 [R. Kodama et al. Nature 418, 933 (2002)]. We found the estimated heating efficiency is at a level of 10–20%. Fuel heating to 5 keV is expected when the full output of LFEX is used.

Creation of a preformed plasma (preplasma) by heating laser prepulse is crucial to fast ignition. Because it is difficult to control the prepulse and preplasma, control of fast electron beam generation by low-density foam was recently reported. However, this simulation study ignored the foam preplasma. Therefore, we calculated foam preplasma formation using a hydrodynamic code and investigated the effects of the preplasma on fast ignition by using integrated simulations, including radiation hydrodynamic, Particle-In-Cell (PIC) and Fokker–Planck simulations. We conclude that the average core temperature decreased by approximately 10% in the integrated simulations of the foam preplasma case.

We investigated generation of pre-formed plasma on plates and inside cone targets due to a pre-pulse before the arrival of the main ultra-intense laser pulse in the fast-ignition scheme of the inertial confinement fusion. We estimated the pre-pulse level to be 130 mJ for LFEX laser used in the 2009 FIREX experiment, and the density gradient scale length of the pre-formed plasma inside the cone target to be 27–47 microns between the critical and 1/10 of the critical density, based on the two-dimensional radiation hydrodynamic simulations. In order to reduce the generation of pre-formed plasma, we investigated a thin CH foil pre-pulse absorber, and proposed using a cone target with a pointed tip. We simulated CH plasma expansion to show that the CH foils works as a pre-pulse absorber. We also show the aluminum pointed tip of the cone target can delay the shock arrival time by 20 ps, much longer than the delay for the 10 micron thickness gold tip used in the typical implosion of GXII at Osaka University.

Long-scale preformed plasmas are generated inside the cone by the pre-pulse of the heating laser in the cone-guided fast ignition scheme and it is found that coupling efficiency from the heating laser to fast electrons especially suitable for core heating is drastically reduced by the preformed plasmas. To mitigate this serious problem, an extremely thin film is suggested to cover the entrance of the cone. This method, however, introduces long rarefied plasmas around the entrance of the cone and the main pulse must propagate through these plasmas. Therefore, fast electron characteristics produced by the main pulse could be affected, and effects of long rarefied plasmas on fast electron generation are investigated. It is found that the electron beam intensity becomes larger than that without the rarefied plasma, but the energy coupling rate from the heating laser to the core decreases due to lack of appropriate electrons for core heating. To achieve less than 10% degradation of the core electron temperature, the thin film must be expanded by irradiation of the pre-pulse so that the length and the density of rarefied plasmas become less than 500 µm and one-tenth of the critical density. A thickness of the thin film can be determined by these criteria and the intensity of the pre-pulse.