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For enhancing the core heating efficiency in fast ignition laser fusion, we proposed the fast electron beam by externally-applied the kilo-tesla (kT) class longitudinal magnetic field. We evaluated the imploded core and the magnetic field profiles formed through the implosion dynamics by resistive MHD radiation hydro code. Using those profiles, the guiding effect was evaluated by fast electron transport simulations, which shows that in addition to the feasible field configuration (moderate mirror ratio), the kT-class magnetic field is required at the fast electron generation point. In this case, the significant enhancement in heating efficiency is expected.

The dependence of high-energy electron generation on the pulse duration of a high intensity LFEX laser was experimentally investigated. The LFEX laser (λ = 1.054 and intensity = 2.5 – 3 x 1018 W/cm2) pulses were focused on a 1 mm3 gold cubic block after reducing the intensities of the foot pulse and pedestal by using a plasma mirror. The full width at half maximum (FWHM) duration of the intense laser pulse could be set to either 1.2 ps or 4 ps by temporally stacking four beams of the LFEX laser, for which the slope temperature of the high-energy electron distribution was 0.7 MeV and 1.4 MeV, respectively. The slope temperature increment cannot be explained without considering pulse duration effects on fast electron generation.

We propose a new scheme for heating an imploded core in the fast-ignition scheme. In this method, a heating laser irradiates an imploded core plasma directly. The accelerated fast-ions as well as fast-electrons heat the core. Two-dimensional particle in cell (PIC) simulation confirmed that carbon C6+ and deuteron D+ ions were accelerated as well as fast electrons when ultra-intense laser irradiates the CD plasma. In order to estimate the temperature scaling of the heated core in this scheme, we conducted transport simulations in the one-dimensional conical geometry. Our results show that 5 keV of ignition temperature can be achieved at the intensity of 1021 W/cm2, and 1.5 ps pulse for the compressed CD plasma with 10g/cm3 density.

We demonstrate a magnetic thrust chamber system, in which an expanding plasma is controlled by an external magnetic field to produce a thrust. The plasma structure and energy dependences are discussed in terms of the drive laser energy and magnetic field strength. The density distribution from two different experiments show identical structure despite the laser energy is different by two order of magnitude when the ratio of magnetic field to plasma energy is more or less same. The experimental results indicate that this ratio is one of the essential factors to extrapolate the plasma dynamics for much larger energy such as inertial confinement fusion plasmas.

Compression of solid spherical target under the strong external magnetic field is studied using two dimensional radiative magneto-hydrodynamic (MHD) simulation code for fast ignition. The simulation results show that a compression of a solid sphere target is stable, and it is possible to achieve a high areal density core plasma. Assuming the GXII scale laser, it will be ρR=60-80mg/cm2. Due to the magnetic diffusion in the solid target, the magnetic mirror ratio is less than 4, which does not reflect most of the hot electrons for heating core. These properties are preferable for fast ignition scheme.

Hot electrons which are generated from targets irradiated by a high-intense laser are measured by two electron spectrometers (ESMs). However, total electron energy observed by the ESM is only less than 1%. Hot electrons are confined by self-fields due to the huge current. When an external magnetic field of several hundred Tesla is applied during the laser irradiation on targets, the ESM signals always increase. In the simulation, the same result can be obtained. The reason is that the Alfven limit can be mitigated due to the external longitudinal magnetic field.

Fast Ignition Realization Experiment project phase-I (FIREX-I) is being performed at Institute of Laser Engineering, Osaka University. In this project, the four-beam bundled high-energy Petawatt laser (LFEX) is being operated. LFEX laser provides great multi-beam irradiation flexibility, with the possibility of arrange the pulses in temporal sequence, spatially separate them in distinct spots of focus them in a single spot. In this paper, we study the two- beam interference effects on high-intensity picosecond laser-plasma interaction (LPI) by twodimensional relativistic Particle-In-Cell simulations. The interference causes surface perturbation, which enhances laser absorption and underdense plasma generation, increasing the accelerated electron number and their slope temperature. The laser-to-electron energy conversion efficiency for two-beam interference case is suitable for Fast Ignition (FI) compared to the single beam case, but the increment of fast electron divergence leads to lower energy coupling. To optimize the target design for FI, these interference effects should be taken into consideration.

As the heating efficiency by fast electrons in the fast ignition scheme is estimated to be very low due to their large divergence angle and high energy. To mitigate this problem, low-density plastic foam, which can generate not only proton (H+) but also carbon (C6+) beams, can be introduced to currently used cone-guided targets and additional core heating by ions is expected. According to 2D PIC simulations, it is found that the ion beams also diverge by the static electric field and concave surface deformation. Thus structured targets are suggested to optimize ion beam characteristics, and their improvement and core heating enhancement by ion beams are confirmed.

The traditional fast ignition scheme is that a compressed core created by an imploding laser is auxiliary heated and ignited by the hot electrons (produced by a short pulse laser guided through the cone and guided under-dense plasma). However sufficient heating has not be achieved because the hot electron energy is too high and dissipated in the cone, the angular divergence of the hot electron is too large, and the distance from the generation point to the core is too long. Here we clarify the problems of fast ignition by observation of the hot-electron spectra.

The energy coupling efficiency (CE) in Fast Ignition (FI) laser fusion was studied at GEKKO XII and LFEx laser facility by using newly developed targets and plasma diagnostic instruments. The gated-liquid scintillator neutron detectors had been upgraded by using neutron collimators for intense background fluxes of γ-rays and neutrons in the FI experiment. Clear fusion neutron signal was successfully recorded in the sub-kJ heating FI experiment. Up to 5 times neutron yield enhancement was observed, and the CE of the heating laser to core plasma was estimated to be 1.6% for cone-in-shell target implosion by 9 beams and core heating by LFEX pulse 115 ps before bang time. The laser-to-electron energy conversion efficiency was separately diagnosed using a newly developed target and resulted to be 45%. The fast electron energy spectrum was estimated to be 2.3 MeV slope temperature by hard x-ray spectroscopy. Monte Carlo simulations demonstrate the consistency of the data set.

We propose the direct heating of an imploded plasma core by ultra-intense lasers in inertial confinement fusion, to increase the heating coupling efficiency. In this scheme, both fast-electrons and fast-ions heat the plasma core. Experiments using this direct heating scheme has been carried out at GXII and LFEX laser facility at Osaka Univeristy. To model this direct heating scheme, we developed the 1D simulation model and carried out simulations using the experimental conditions. Comparison between results of the simulation and the experimental observations validates the simulation model. We show that even in the unoptimized experimental conditions used in simulations, our calculations show that the maximum temperature, 1.6 keV, of the CD plasma.

In this paper, magnetized fast ignition (MFI) is proposed for improving the coupling efficiency of a heating laser to a core plasma. In the MFI, the external magnetic field is applied to reduce the hot electron energy and focus the dense hot electron flux to the core. The external magnetic field higher than 100T is generated by the laser driven coil and it is amplified by the implosion. The magnetic field at the tip of the cone is expected to reach higher than 10kT and the laser plasma interaction and the hot electron transport are modified. As the results of applying the external magnetic field, hot electron energy is reduced to less than 5MeV for the laser intensity of 1020W/ cm2 and the Weibel instability is suppressed to collimate the hot electron beam to the core.

Although the energy conversion efficiency from the heating laser to fast electrons is high, the coupling efficiency from fast electrons to the core is estimated to be very low due to large divergence angle of fast electrons in fast ignition experiments at ILE, Osaka University. To mitigate this problem, a plastic thin film or low-density foam, which can generate not only proton (H+) but also carbon (C6+) beams, is combined with currently used cone-guided targets and additional core heating by ions is expected. According to integrated simulations, it is found that these ion beams can enhance the core heating by 20~60% and it shows a possibility of ion beam assisted fast ignition.

Absolute Ka line spectroscopy is proposed for studying laser-plasma interactions taking place in the cone-guided fast ignition targets. X-ray spectra ranging from 20 to 100 keV were quantitatively measured with a Laue spectrometer. The absolute sensitivities of the Laue spectrometer system 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 open code for x-ray diffraction. The energy transfer efficiency from incident laser beams to hot electrons, as the energy transfer agency, is derived as a consequence of this work. The absolute yield of Au and Ta Ka lines were measured in the fast ignition experimental campaign performed at Institute of Laser Engineering, Osaka University. Applying the hot electron spectrum information from the electron spectrometer, an energy transfer efficiency of the incident LFEX [1], a kJ-class PW laser, to hot electrons was derived for a planar and cone-guided geometry.

Recently, generation of external magnetic fields of a few kT has been reported [Fujioka et al. Scientific Reports 2013 3 1170]. These fields can be used in fast ignition to mitigate the large fast electron divergence. In this summary, two fast ignition applications are briefly outlined. The first one deals with electron guiding by external B-fields applied at the end of the shell implosion of a re-entrant cone target. Preliminary results show that the B-field strength at the time of peak ρR may be sufficiently high for fast electron guiding. The second application deals with guiding of fast electrons in magnetized wires surrounded by plasma. Results show a significant enhancement of electron energy deposition at the end of the wire, which is particularly important for low-Z wires.

The core heating properties of a Au cone-attached CD shell target in FIREX-I were investigated by the integrated simulations with FI3 code systems. The importance of fat electron transport in the deformed cone tip was shown. In addition to the collisional scattering and drag in the Au cone, the strong resistive magnetic field is generated at the top of the cone tip when the tip shape is deformed. Due to this field, the fast electrons are strongly scattered, and then the core heating efficiency becomes smaller compared with the case neglecting the magnetic field.

In fast ignition, the intensity of an unavoidable pre-pulse of a heating laser is still high to generate plasmas even if the contrast ratio is high. So, a preformed plasma (pre-plasma) is created before a main pulse of the heating laser irradiates a target. The recent research suggests that coating an inner surface of the cone of the cone-guided target with the low-density foam enhances fast electron generation. But the pre-plasma and the foam structure, which consists of dense CH plasmas and voids, are ignored in that research. In this paper, effects of the pre-plasma and the foam structure on the fast electron generation are investigated with the use of an one-dimensional Particle-In-Cell code. It is found that effects of the pre-plasma on the fast electron generation are small and the foam structure weakens the generation of fast electrons with the moderate energy.

We have conducted preliminary experiments to prove the vacuum-shielding effect using an Al-Cu double-foil (both Al and Cu foils are 1 mm × 1 mm × 10 μmt) target with a vacuum-gap. Particle-in-cell simulation results have shown that the double-cone confines the electrons for hundreds of femtoseconds by the sheath electric field inside the vacuum-gap [T. Nakamura, et al., Phys. Plasmas 14, 103105 (2007)]. Cu-Kα intensity decreased with the vacuum-gap, i.e., the number of fast electrons that reached the Cu foil decreased with the gap. In the stack measurement, the number of electrons detected in the target surface direction increased with the gap. These results indicate that when the double-cone target is used, fast electrons created by an ignition laser can be reflected from the vacuum-gap, move along the cone surface, and be transferred towards the cone tip.

We simulated the plasma expansion of the cone wall due to the pre-pulse irradiation in fast-ignition experiments. Our radiation hydrodynamic simulations show the temporal evolution of the pre-formed plasma inside the cone. At the onset of the pre-pulse, collision within the expanding plasma near the cone axis generate a plasma jet. At the later stage of the pre-pulse the large density shelf above the critical density of 1.06 μm wavelength laser is formed near the tip inside the cone, and subsequently this critical density surface moves away from the inner surface of the cone tip. Our simulation gives roughly 80μm shift of the critical density surface under the typical pre-pulse condition of LFEX laser at Osaka University for 45 deg open angle cone. This shift of the critical density can reduce the energy coupling between the heating laser and the imploded core.

In the Fast ignition, formation of highly compressed core plasma is one of critical issue. In this work, the effect hydrodynamic instability in cone-guided shell implosion is studied. Two-dimensional radiation hydrodynamic simulations are carried out where realistic seeds of Rayleigh-Taylor instability are imposed. Preliminary results suggest that the instability reduces implosion performance, such as implosion velocity, areal density, and maximum density. In perturbed target implosion, the break-up time of the tip of the cone is earlier than that of ideal unperturbed target implosion case. This is crucial matter for the Fast ignition because the pass for the heating laser is filled with plasma before the shot of heating laser. A sophisticated implosion design of stable and low in-flight aspect ratio is necessary for cone-guided shell implosion.

X-ray polarization spectroscopy was studied to derive directly the velocity distribution function (VDF) of hot electrons propagating in plasma created with a high intensity laser pulse. Polarization measurement was made at around 1018 W/cm2 using a laser pulse (~10 J in ~1 ps) from Alise facility at CEA/CESTA. Chlorinated triple-layer targets were irradiated, and C1 Heα line was observed with an x-ray polarization spectrometer. Polarization degrees were measured as a function of the target overcoat thickness. It is found that the polarization is weakly negative for thin coating, but becomes positive, and finally zero for thick coating. This result is consistent with predictions made with a time-dependent atomic kinetics code developed to gain an insight into the generation of polarized C1 Heα radiation. The de-polarization on the surface is attributed to excessive bulk electron temperature and that in the deep region to elastic-scattering processes by the isotropic bulk electrons in dense region.

Polarized Heα of chlorine from ultra-intense laser produced plasmas was calculated. The experiment with 800 nm laser pulses of 100 — 130 mJ in 130 fs was also carried out, and the resultant polarization of time- and space-integrated Heα is so high (≥ 10%). A new time-dependent atomic kinetics code was developed to gain insight into the generation of polarized Heα by fast electron transport relevant to fast ignition. The polarized feature is due to anisotropic electron impacts by fast electrons, and the aspect ratio of fast electron temperatures associated with longitudinal and transverse directions is found to be essential with the numerical calculation. In the calculation, so small polarization (≤ 2 ~ 3%) is predicted in dense plasma region (≥ 100 times the critical density), which is due to frequent elastic atomic transitions between magnetic atomic sublevels, while high polarization (≥ 10%) is observable in low density region (≤ 10 times the critical density). The collimated fast electrons are generated by electromagnetic instability, resulting in large anisotropy along the propagation direction in low density region.

The Breit-Pauli R-matrix method is used to calculate integral cross sections between magnetic sublevels for He-like Cu ions by electron impact for x-ray polarization spectroscopy of ultrahigh-intensity laser-produced plasma. The integral cross sections for inelastic scattering that produce aligned states are calculated. And, the integral cross sections for elastic scattering that destroy the alignment of excited states are also calculated for transitions between magnetic sublevels. The data is useful to analyze the anisotropic plasmas using the anisotropic collisional-radiative atomic-kinetics model.

X-ray polarization spectroscopy is a useful diagnostic tool for measuring the velocity distribution of electrons inside plasma. A new polarization measurement was performed at relativistic laser intensity using a laser pulse (10 J in ~1 ps) from Alise facility at CEA/CESTA. Chlorinated triple-layer targets were irradiated, and polarization degrees of C1-Heα line were measured. Obtained polarization degrees are negative in shallow region from the target surface and positive in deep region. This result is qualitatively consistent with the experimental results at non-relativistic intensity. Moreover, de-polarizations due to isotropic excitation and elastic collision, which were predicted by a model calculation and a time-dependent atomic kinetic code, were observed.

Two-dimensional coupled Fokker-Planck and radiation-hydro simulations showed that for efficient ion heating in FIREX-I class small-sized DT cores (ρR ~ 0.2g/cm2), because of short confinement time, short duration and low temperature fast electron beam is required compared with high-gain class targets. In the case of fast electron beam with the energy of 4kJ, duration of 5ps, slop temperature of 0.5MeV and the intensity of 1.1×1020W/cm2, the energy coupling from fast electron beam to dense core of 49%, heated bulk electron and ion temperatures of 8.0keV and 4.6keV and neutron yield of 1.1×1014 are obtained. The fuel material dependence of heating properties are also discussed.

A high ρR core which is formed by non-spherical cone-guided implosion is required in fast ignition. Contrary to the central-hot-spot scheme, high temperature is not necessary in the implosion process. Therefore, relatively slow implosion design could be effective using a massive shell target. In this work, two-dimensional simulations of such slow implosions are performed to achieve high ρR core. In the result, the higher ρRmax can be obtained if the target mass is increased. In this study, an implosion with a foot pulse laser is examined also. In the initial phase, during the foot pulse irradiation, ablated plasma affects the gold cone surface, and the gold plasma expands, which leads pernicious effect on the implosion process.

The possibility of igniting D3He plasma in the fast-ignition, inertial confinement fusion scheme is discussed. Use of a small amount of DT fuel as an igniter is indispensable in order to mitigate the requirement on driver energy. Simulations have been made for a DT/D3He fuel compressed to 4000 times the liquid density by incorporating a newly-developed neutron diffusion code into FIBMET, a 2D fusion ignition and burning code. The DT igniter is placed at the edge of the compressed fuel. The work shows that it is possible to obtain sufficient pellet gains (~60) with realistic driver energy below 10 MJ. The essential role of DT fusion neutron is clarified. The possibility to reduce the amount of DT fuel is discussed.

The potentiality of electron degeneracy diagnostics based on γ-ray measurement is discussed. For this purpose, the nuclear reactions D (α, γ)6Li and T (α, γ)7Li are considered. These reactions are induced in DT plasma by fusion-born energetic α-particles and proceed through the excited nuclei 6Li* and 7Li* emitting monochromatic γ-quanta with 2.186-MeV and 0.478-MeV energies, respectively, in their decay to the ground state. If the plasma is being highly compressed and the electrons are in degenerate state, then the stopping range of the α-particles is lengthened compared with the case of non-degenerate electron plasma. As a result, the probability that a fusion-born α-particle will undergo D (α, γ) or T(α, γ) reaction during slowing down would be enhanced. In this paper we will discuss the feasibility of detecting the 2.186-MeV and 0.478-MeV γ-ray in ICF environment.

Electron energy characteristics generated by the irradiation of ultra-intense laser pulses onto solid targets are controlled by using cone targets. Two important parameters characterizing the laser-cone interaction are introduced, which are cone angle and the ratio of the laser spot size to cone tip. By changing these parameters, the electron energy characteristics are controlled. They are optimized for fast ignition with the aid of 2D Particle-in-Cell (PIC) simulations.

On the basis of the integrated simulations for ultra-intense laser-matter interactions, we demonstrated the long-scale (~20μm) low-density plasma generation by the irradiation of low intensity pre-pulse and its effects on the fast electron generation. It is also found that the resistivity jump at the material contact surface in the low-temperature multi-layer solid target generates the strong magnetic field when the direction of gradient of resistivity is nearly perpendicular to the fast electron beam direction. The fast electron beam is scattered by this field, which strongly affects the target heating properties.

FI3 Integrated simulations with the heating laser whose pulse duration is 10[ps] have been performed to investigate the core heating property of FIREX-I experiments. As the preformed plasma was pushed by the Ponderomotive force, the electron density increased in time and the fast electron beam intensity decreased. It was found that the fast electron beam intensity well scaled as the inverse square root of the electron density at the laser front. If the preformed plasma had the short scale length profile, the average core temperature quickly rose but shortly saturated because the fast electron beam intensity decreased. With the longer scale length preformed plasma, the beam intensity of fast electrons was maintained and the core heating was sustained for a longer time, the core reached higher average temperature.