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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.

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.

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.

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).

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.

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.

The fast ignition realization experiment (FIREX) project is progressing. The new short pulse laser system, LFEX laser, has been completely assembled and one of the four beamlets is now in operation. A fast-ignition experiment was performed using this single short pulse combined with the Gekko XII implosion laser. The energy of the GXII implosion laser was about 2 kJ and the pulse width was 1.5 ns. The energy of the LFEX laser was increased upto 800 J and two pulse durations 5 and 1.6 ps were compared. Targets were deuterated plastic shells with gold cones. It was found that the neutron yield was increased by a factor of 30 as a result of the fast electron-induced heating in LFEX 1.6 ps shot. The estimated coupling efficiency between the LFEX laser pulse and the compressed fuel was low (less than 5%). This may be due to pre-plasma formed by light arriving at the target before the main laser pulse. Further investigations and attempts to overcome these problems are now in progress. © 2011 Elsevier B.V.

Interactions of cone targets with different shapes with laser pulses are studied numerically. Two important parameters which characterize the laser-cone interaction in 2006 are introduced, which are cone angle and ratio of laser spot and cone tip. By changing these two parameters, energy coupling from laser to electrons is controlled. Some fraction of high energy electrons generated at side wall and cone tip are not freely propagating out from the target, but confined around the cone tip due to the disturbed electric field.

In fast ignition research, the clarification of core heating mechanism is one of the most critical issues. To understand and identify the crucial physics in fast heating, we developed the fast ignition integrated interconnecting code FI3 and carried out the core heating simulations for fast heating experiments with cone-guided targets. It was found that the scale length of the pre-plasma at the inner-surface of the cone and the density gap at the contact surface between the cone tip and the imploded core plasma strongly affect the efficiency of core heating. In the case of heating laser with intensity of 1020 W/cm2 and duration of 1 ps, the pre-plasma scale length of 1.5 µm is optimum for the core heating; the dense core is heated up to 0.86 keV. In the double scale length case (long scale of ~5 µm in underdense region and short scale of ~ 1 µm in overdense region), of which generation due to the pre-pulse irradiation of heating pulse is observed at the radiation–hydro simulations, the dense core is heated more efficiently than single short scale length cases. The contribution of fast ions to the core heating is also discussed.

Theoretical study of energy transfer in ultrahigh intensity laser plasma was performed using a computer code to analyze the polarization of the Cl–He α line. The Breit–Pauli R -matrix method was used to calculate elastic- and inelastic-scattering cross-sections with magnetic sublevels by electron impacts for He-like and Li-like Cl ions, respectively. The dependence of polarization on the electron density was investigated by using a time-dependent, collisional-radiative atomic kinetics model for ion densities from intermediate region to near solid. It is found that the polarization degree decreases with increase in electron density due to dominance of elastic collisional process.

A series of experiments were carried out to evaluate the energy-coupling efficiency from heating laser to a fuel core in the fast-ignition scheme of laser-driven inertial confinement fusion. Although the efficiency is determined by a wide variety of complex physics, from intense laser plasma interactions to the properties of high-energy density plasmas and the transport of relativistic electron beams (REB), here we simplify the physics by breaking down the efficiency into three measurable parameters: (i) energy conversion ratio from laser to REB, (ii) probability of collision between the REB and the fusion fuel core, and (iii) fraction of energy deposited in the fuel core from the REB. These three parameters were measured with the newly developed experimental platform designed for mimicking the plasma conditions of a realistic integrated fast-ignition experiment. The experimental results indicate that the high-energy tail of REB must be suppressed to heat the fuel core efficiently.

For the improvement of the laser-absorption efficiency in laser-induced gaseous breakdown, a corner cube was used to return the transmitted light to the laser-produced plasma. We demonstrated that the laser-absorption efficiency was considerably improved.

Evaluation of the energy deposition region of a laser-produced relativistic electron beam in dense core plasma is essential in the fast ignition approach to ICF. Recently, we have developed a two-dimensional relativistic Fokker-Planck code “RFP-2D” for fast electron transport in dense plasmas to investigate the energy deposition profiles. In this paper, we compare the sizes of the energy deposition region derived from RFP-2D and another RFP-type calculation model with stochastic treatments. It is found to be considerably important to take into account the influence of the self-generated electromagnetic field when evaluating the energy deposition region.

To investigate the core plasma heating in fast ignition targets, we recently developed a two-dimensional relativistic Fokker-Planck code "RFP-2D" for fast electron transport in dense plasmas. In the present study, we have coupled RFP-2D with a 2D hydrodynamics code for bulk plasma and analyzed the core plasma heating by assuming a condition similar to that in the GXII-PW laser experiments at ILE. It was found that when the source beam electrons enter the dense core region with "low" (< 1 MeV) average energy and a Super-Gaussian spatial distribution, the core plasma can be heated above 0.8 keV at maximum, which is almost the same temperature as measured in the experiments.

The hot electron generation by irradiation of intense short laser pulses onto cone targets is studied by using 2D PIC simulation. It is shown that the high energy electrons from the cone target are characterized with three effective temprature, while two-temperature for plane target. The electrons composing most energetic component are generated at the cone tip where laser field is intensified by cone-guiding, and electrons for middle component are generated at cone wing which is not observed for plane target. The acceleration processes at cone wing, cone tip, and rear surface are explained.

Observation is reported of the polarization of the helium-like Cl–He α line (1s 2 1 S 0 –1s2p 1 P 1 ), at 2.79 keV emitted from a triple-layer target irradiated with a 130 fs laser pulse. The polarization degree is determined as a function of overcoat thickness of the target. It is found that the emission from a layer closer to the target surface is polarized parallel to the surface direction whereas emission from a deeper region in perpendicular to the surface. The experimental result is analyzed using a two-dimensional (2D) Maxwellian distribution function for hot electrons. Beam-like distribution was found in the depth of plasma.

Fast electrons are effectively generated from solid targets of cone-geometry by irradiating intense laser pulses, which is applied to fast ignition scheme. For realizing optimal core heating by those electrons, understanding the characteristics of electrons emitted from cone targets is crucial. In this paper, in order to understand the generation and transport processes of hot electrons inside the cone target, two-dimensional (2D) particle-in-cell (PIC) simulations were carried out. It is shown that hot electrons form current layers which are guided by self-generated surface magnetic field, which results in effective energy transfer from laser pulse to hot electrons. When the hot electrons propagate through the steep density gradient at the cone tip, electrostatic field is induced via Weibel instability. As a result, hot electrons are confined inside and emitted gradually from the target, as an electron beam of long duration. Energy spectrum and temporal profile of hot electrons are also evaluated at the rear side of the target, where the profile of rear side plasma is taken from the fluid code and the result is sent to Fokker-Planck code.

Experimental study on energy transport in ultra-high intensity laser plasma was made. X-ray emission from a triple-layer target irradiated at 10 19 W cm -2 was observed with x-ray spectrographs, monochromatic imagers and an x-ray polarimeter to provide a temperature profile in the depth of the target, lateral extension of the heated region and the velocity distribution function of hot electrons. For PW plasma, a very shallow region (∼0.5 μm from the target surface) was heated up to 650eV but the temperature of deeper region (up to 5 μm) was around 100eV. These depths are much shorter than those expected from the classical penetration of the hot electrons. The localized energy deposition is also found for the plasma generated at 10 17 W cm -2 , and the degree of polarization of the helium-like Cl-Heα line (1s 2 1 S 0 -1s2p 1 P 1 ) from the surface region is polarized parallel to the surface direction whereas that from a deeper region is in perpendicular to it. The experimental result is analysed using a two-dimensional Maxwellian distribution function for hot electrons. Beam-like distribution was found in the depth of plasma.

Reviewed are the present status and future prospects of the laser fusion research at the ILE Osaka. The Gekko XII and Peta Watt laser system have been operated for investigating the implosion hydrodynamics, fast ignition, and the relativistic laser plasma interactions and so on. In particular, the fast ignition experiments with cone shell target have been in progress as the UK and US-Japan collaboration programs. In the experiments, the imploded high density plasmas are heated by irradiating 500 J level peta-watt laser pulse. The thermal neutron yield is found to increase by three orders of magnitude by injecting the peta-watt laser into the cone shell target. The Rayleigh-Taylor instability experiment results are also reviewed is this paper.

Kα line emission from partially ionized material was studied to diagnose energy transport in ultrahigh-intensity laser produced plasmas. Dependence of the emission intensity on initial target conductivity was investigated by using a plastic target (C2H3Cl coated with CH) and a metallic target (Al coated with Mg) irradiated with a 1-TW Ti:sapphire laser pulse of 132 fs at 2×10 17 W/cm 2 . Depth of the heated region was deduced from Cl or Al He-α line intensities given as a function of overcoat thickness. No significant difference was observed between the two types of targets. Density–depth product of the hot region was typically 0.1 mg/cm 2 for both targets and the temperature of hot electrons measured by an electron spectrometer was 50 keV . The experimental results were analyzed with an integration calculation with a one-dimensional (1D) relativistic Fokker–Planck code, a 1D radiation hydrodynamics (rad-hydro) simulation, and an atomic–kinetic model developed for ionization-shifted Cl Kα lines. The observed transport features are discussed.

Population kinetics and spectral synthesis codes have been developed to analyze Kα emission from partially ionized chlorine in hydrocarbon plasmas irradiated with high-intensity ultra-short laser pulses. The population kinetics processes are calculated using a bi-Maxwellian electron temperature distribution composed of fast and cold electrons. The fast electrons dominantly contribute to ionize the K-shell bound electrons. i.e., inner-shell ionization, while the cold electrons produce ionization from the outer-shell electrons. The resultant Kα emission provides a distinct spectral signature for each charge state. Also included in the calculation are the opacity effects in the framework of an escape probability method. It is shown that the Kα emission is saturated at a plasma thickness of more than a few microns. Further, we find that unresolved satellite lines overlap significantly with the corresponding parent Kα lines of the next charge state, and make a large contribution to composite spectrum. Finally, time-dependent properties of the Kα emission are also discussed briefly.

The study of fast electron spectrum optimization by suppression of preformed plasma in fast ignition targets is presented in this work. Integrated fast-electron spectra for electron energies below 3 MeV—the energy range responsible for core heating—are compared for different preformed plasma conditions. The pulse contrast (the ratio of peak-to-pedestal laser intensities) is compared for 108, 109 and 1011 conditions at constant laser energy (~500 J), pulse duration (2 ps), spot size (30% encircled energy on 50 µm diameter) and laser intensity (around 1 × 1019 W cm−2). The best electron spectrum optimization, consisting of maximized electron number for energies below 3 MeV was obtained with 14 µm thick cone targets. The energy coupling efficiency from heating laser to core plasma, assuming typical core plasma parameters, was estimated to be 2%, although 0.37% was obtained with previous conditions with poor pulse contrast and a 7 µm thick cone target.

A calculation method for fusion product momentum deposition in dense plasma spheres has been developed and applied to laser imploded DT targets. The net radial momentum deposition from αparticles accelerates the expansion of the plasma sphere and thus reduces the rate of thermonuclear reactions. However, this effect is not so significant as previously expected because the rate of momentum deposition by the α particles is much less than the local pressure gradient around the burn front. The momentum deposition from neutrons is negligibly small because of its long mean free path.

Within the framework of a one dimensional study, the accuracy is examined of two kinds of flux limited diffusion model: one is the artificially flux limited model developed by Corman and the other is based on the Levermore-Pomraning theory, for evaluating the alpha particle heating rate in laser imploded DT targets. It is found that the former model cannot correctly evaluate the ignition condition and the gain since it significantly underestimates the heating rate in the region where alpha particles are born. Contrary to this, in the latter model, which can estimate well the profile of the alpha particle heating rate, not only the time integrated burn properties but also the fuel evolution in space and time are in good agreement with results obtained on the basis of accurate transport calculations.

2D PIC simulations for fast ignition with a planar target and an ultrahigh intensity laser have been performed to investigate effects of laser temporal profile on fast electron characteristics. Six different laser profiles, which are combination of two pulse shapes, namely Flattop and Gaussian shapes, and three different maximum intensities 1 × 1020 , 5 × 1019 , and 2.5 × 1019 W/cm2 , are studied under the condition of fixed laser energy. Time-integrated energy spectra, coupling efficiencies from the heating laser to fast electrons, and divergence angle of fast electrons are shown and discussed.

Kα line emission from Mo was experimentally and theoretically studied using clean, ultrahigh-intensity femtosecond laser pulses. The absolute yields of Kα x-rays at 17 keV from Mo were measured as a function of the laser pulse contrast ratio and irradiation intensity. Significantly enhanced Kα yields were obtained by employing high contrast ratio at optimum irradiance. Conversion efficiencies of 4.28 × 10 −5 /sr, the highest values obtained to date, was demonstrated with contrast ratios in the range of 10 −10 to 10 −11 .

One of the most advanced fast ignition programmes is the fast ignition realization experiment (FIREX). The goal of its first phase is to demonstrate ignition temperature of 5 keV, followed by the second phase to demonstrate ignition-and-burn. The second series experiment of FIREX-I, from late 2010 to early 2011, has demonstrated a high (>10%) coupling efficiency from laser to thermal energy of the compressed core, suggesting that the ignition temperature can be achieved at laser energy below 10 kJ. Further improvement of the coupling efficiency is expected by introducing laser-driven magnetic fields. © 2013 IAEA, Vienna.

In the electron-driven fast-ignition approach to inertial confinement fusion, petawatt laser pulses are required to generate MeV electrons that deposit several tens of kilojoules in the compressed core of an imploded DT shell. We review recent progress in the understanding of intense laser plasma interactions (LPI) relevant to fast ignition. Increases in computational and modeling capabilities, as well as algorithmic developments have led to enhancement in our ability to perform multi-dimensional particle-in-cell (PIC) simulations of LPI at relevant scales. We discuss the physics of the interaction in terms of laser absorption fraction, the laser-generated electron spectra, divergence, and their temporal evolution. Scaling with irradiation conditions such as laser intensity are considered, as well as the dependence on plasma parameters. Different numerical modeling approaches and configurations are addressed, providing an overview of the modeling capabilities and limitations. In addition, we discuss the comparison of simulation results with experimental observables. In particular, we address the question of surrogacy of today's experiments for the full-scale fast ignition problem., Comment: 48 pages, 13 figures, submitted to Nuclear Fusion

Laboratory generation of strong magnetic fields opens new frontiers in plasma and beam physics, astro- and solar-physics, materials science, and atomic and molecular physics. Although kilotesla magnetic fields have already been produced by magnetic flux compression using an imploding metal tube or plasma shell, accessibility at multiple points and better controlled shapes of the field are desirable. Here we have generated kilotesla magnetic fields using a capacitor-coil target, in which two nickel disks are connected by a U-turn coil. A magnetic flux density of 1.5 kT was measured using the Faraday effect 650 μm away from the coil, when the capacitor was driven by two beams from the GEKKO-XII laser (at 1 kJ (total), 1.3 ns, 0.53 or 1 μm, and 5 × 10(16) W/cm(2)).