Search

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.

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.

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.

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%.

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.

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.

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.

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

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

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.

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.

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.

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.

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

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.

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.

We have developed a simulation code to treat behaviors of wall ablation by X-rays, α particles and charged particles from fusion burning targets. The absorption of energy of α particles and charged particles by ablated wall plasmas was estimated using the stopping power obtained by dielectric functions of plasmas and Bethe formulas. We show that ablated wall material (Pb) moves as clump. We clarify that the screening effects of charged particles expanding from fusion burning by ablated wall material (Pb) decrease the ablation depth of liquid Pb compared with that of no screening effects.

Overall fluid dynamics of the implosion, laser-plasma interaction and super hot electron generation, and super hot electron energy deposition within the core are key issues in the Fast Ignition scheme. As both time and space scales of these phenomena are much different, it is impossible to simulate all phenomena with one code, and we must simulate each phenomenon with individual codes and integrate them. We will run these codes individually, that are collaborating each other with data transfer via the computer network. Since communication among these codes is very straightforward, we design a lightweight protocol, Distributed Computing Collaboration Protocol (DCCP), to transfer data. We have planned to simulate the Fast Ignition with self consistent fields in a full range of parameters using DCCP and just started challenging Fast Ignition Integrated Interconnecting code project.

For enhancing the core heating efficiency in electron-driven fast ignition, we proposed the fast electron beam guiding using externally applied longitudinal magnetic fields. Based on the PIC simulations for the FIREX-class experiments, we demonstrated the sufficient beam guiding performance in the collisional dense plasma by kT-class external magnetic fields for the case with moderate mirror ratio (~, 11pages, 8figures

The x-ray laser-matter interaction for a low-$Z$ material, carbon, is studied with a particle-in-cell code that solves the photoionization and x-ray transport self-consistently. Photoionization is the dominant absorption mechanism and nonthermal photoelectrons are produced with energy near the x-ray photon energy. The photoelectrons ionize the target rapidly via collisional impact ionization and field ionization, producing a hot plasma column behind the laser pulse. The radial size of the heated region becomes larger than the laser spot size due to the kinetic nature of the photoelectrons. The plasma can have a temperature of more than 10 000 K ($g1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$), an energy density greater than ${10}^{4}$ $\mathrm{J}/{\mathrm{cm}}^{3}$, an ion-ion Coulomb coupling parameter $\ensuremath{\Gamma}\ensuremath{\ge}1$, and electron degeneracy $\ensuremath{\Theta}\ensuremath{\ge}1$, i.e., strongly coupled warm dense matter. By increasing the laser intensity, the plasma temperature rises nonlinearly from tens of eV to hundreds of eV, bringing it into the high energy density matter regime. The heating depth and temperature are also controllable by changing the photon energy of the incident laser light.

On the basis of coupled transport-hydrodynamic calculations, the neutronic effects in reactor-size volume ignition targets are investigated. It is shown that contrary to the case of central spark ignition, neutrons effectively heat the plasma in the ignition phase and reduce the threshold temperature for ignition. In practically-interesting regions where the areal density of compressed targets < 15 g/cm 2 , this positive effect exceeds the negative one, i.e. a shortening of the confinement time due to excessive heating in the burn phase. Moreover, the suprathermal fusion reactions induced by neutron recoils appreciably contribute to the energy production. Thus, in the volume ignition scheme, the inclusion of neutronic processes results in lowering the ignition temperature and increasing the maximum fuel gain.

Including a density jump between a cone tip and imploded plasmas, we have performed preliminary FI 3 simulations for fast ignition with cone-guided targets, and found that the density gap with 2n was effective at heating the core. But this effect is not successful under the realistic parameters for recent fast ignition experiments. Even so, it was found that the core could be heated up to 0.44 keV, but we could not get core temperature observed in the experiments, 0.8 keV. From those results, it should be emphasizes that integrated simulations under realistic conditions are essential for the fast ignition research.

On the basis of coupled neutronic/hydrodynamic calculations, we examine the neutron heating effects on the ignition and burn propagation in laser-imploded D-T pellets. The fusionproduced neutrons deposit their energy all over the pellet region since the mean-free-path of the neutron is long. The fraction of neutron energies deposited to the central spark region during the ignition phase is too small to reduce the threshold energy of the laser for ignition. In the burn phase, the neutron heating decreases the maximum compression ratio and accelerates the plasma expansion. The inclusion of neutron heating hence decreases the pellet gain from the value in the case without neutron heating. Calculations neglecting the transport of neutron recoils overestimate the neutron heating rate in the reactor-grade pellets.

In the implosion phase of the fast ignition scheme, most critical issues are breakup of the cone tip and the formation of high -R core plasma to improve its heating efficiency. For the integrated fast ignition experiment at ILE Osaka University, robust and reliable implosion must be redesign. In this paper, feasible target design under the constraint condition of existing GXII and LFEX facilities is studied using two-dimensional radiation hydrodynamic simulations, and an optimum target design based on low velocity implosion is proposed. The advantages of low velocity implosion are low adiabat, robust against Rayleigh- Taylor instability, which are verified. Also longer life time of compressed core plasma which is preferable for fast ignition is confirmed in this study.

We propose a new type of target for the fast ignition of inertial confinement fusion. Pre-formed plasma inside a cone target can significantly reduce the energy coupling efficiency from the ultra-high intense short-pulse laser to the imploded core plasma. Also, in order to protect the tip of the cone and reduce generation of pre-formed plasma, we propose pointed shaped cone target. In our estimation, the shock traveling time can be delayed 20-30ps by lower-Z material with larger areal density compared to the conventional gold flat tip. Also, the jet flow can sweep the blow-off plasma from the tip of the cone, and the implosion performance is not drastically affected by the existence of pointed tip. In addition, the self- generated magnetic field is generated along the boundary of cone tip and surrounding CD or DT plasma. This magnetic field can confine fast electrons and focus to the implosion core plasma. Resultant heating efficiency is improved by 30% compared to that with conventional gold flat tip.

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)).

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.