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One of the most promising approaches to reach a high gain in inertial confinement fusion is the fast ignition scheme. In this scheme, a relativistic electron beam is generated; this passes through the imploded plasma and deposits parts of its energy in the core. However, the large angular spread of the relativistic electron beam and the poorly controlled compression of the target affect realization of the fast ignition technique. Here, we demonstrate that indirectly driven (that is, driven by X-rays generated inside a gold hohlraum) implosions with a ‘high-foot’ and a short-coast time of less than 200 ps allow us to tightly compress the shell. Furthermore, we show the ability to optimize the symmetry of the imploding shell by changing the hohlraum length, successfully tuning a suitable tube-shaped shell to compensate for the large angular spread of the relativistic electron beam and to enhance the electron-to-core coupling efficiency via resistive magnetic fields. Benefiting from those experimental techniques, a significant enhancement in neutron yield was achieved in our indirectly driven fast ignition experiments. These results pave the way towards high-coupling fast ignition experiments with indirectly driven targets similar to those at the National Ignition Facility. Experiments realizing the indirect-drive fast ignition scheme for inertial confinement fusion are reported. Enabled by a tightly compressed target, an increase of neutron yield is observed.

The formation and structure of a collisional shock wave in a fully ionized plasma is studied via full particle-in-cell simulations, which allows the complex momentum and energy transfer processes between different charged particles to be treated self-consistently. The kinetic energy of the plasma flow drifting towards a reflecting piston is found to be rapidly converted into thermal motion under the cooperative effects of ion-ion collisions, ion-electron collisions, and electric field charged-particle interactions. The subsequent shock evolution is influenced by the ``precursor'' ion beam before a quasisteady state is reached. The shock wave structure is then analyzed from a two-fluid transport viewpoint, which is found to be affected by ``flux-limiting'' electron transport, the nonthermal ions, and the charge separation electric field.

Plasma effects, such as the multi-component kinetic diffusion and self-generated electromagnetic fields, are recognized as a pivotal key to understanding the physics of interface evolution in inertial confinement fusion and supernova remnants. In this work, a two-dimensional hybrid fluid-PIC code is used to investigate the ion kinetic effects of the single-mode Richtmyer–Meshkov instability (RMI) at the interface between hydrogen plasma and carbon plasma. After an electrostatic shockwave passing through the perturbed interface, the RMI, which reshapes the interface, grows via the vorticity depositing as well as the self-generated magnetic field. After scaling the growth of the interfacial mix region with time, the density transition layer has been found to exceed the disturbance wavelength and lead to a suppression of the instability evolution finally.

Shock-driven hydrodynamic instabilities in a plasma usually lead to interfacial mixing and the generation of electromagnetic fields, which are nonequilibrium processes coupling kinetics with meso- and macroscopic dynamics. The understanding and modeling of these physical processes are very challenging tasks for single-fluid hydrodynamic codes. This work presents a new framework that incorporates both kinetics and hydrodynamics to simulate shock waves and hydrodynamic instabilities in high-density plasmas. In this hybrid code, ions are modeled using the standard particle-in-cell method together with a Monte Carlo description of collisions while electrons are modeled as a massless fluid, with the electron heat flux and fluid–particle energy exchange being considered in the electron pressure equation. In high-density plasmas, Maxwell’s equations are solved using Ohm’s law instead of Ampère’s law. This hybrid algorithm retains ion kinetic effects and their consequences for plasma interpenetration, shock wave propagation, and hydrodynamic instability. Furthermore, we investigate the shock-induced (or gravity-induced) turbulent mixing between a light and a heavy plasma, where hydrodynamic instabilities are initiated by a shock wave (or gravity). This study reveals that self-generated electromagnetic fields play a role in the formation of baroclinic vorticity along the interface and in late-time mixing of the plasmas. Our results confirm the ability of the proposed method to describe shock-driven hydrodynamic instabilities in a plasma, in particular, nonequilibrium processes that involve mixing and electromagnetic fields at the interface.

Kinetic physics has the potential to impact the performance of indirect-drive inertial confinement fusion (ICF) experiments. In near vacuum hohlraums (or vacuum hohlraums), the high-Z plasma expands from the hohlraum wall and collides with the blow-off from the capsule (or the low-density fill-gas). Such collision produces conditions in which kinetic effects may dominate since the ion-ion mean free paths are larger than the size of the interaction region. In the present work, we present the experimental evidence supported by simulations of kinetic effects launched in the interpenetration layer between the laser-driven hohlraum plasma bubbles and the corona plasma of the compressed pellet on the Shenguang-III prototype laser facility. Our study showed kinetic shocks arisen in the hohlraum wall/ablator (or the low-density fill-gas) interpenetration region, which result in efficient acceleration of high energy ions. When these high energy ions were deposited inside the capsule, it resulted in significant low-mode asymmetry of the implosion capsule since there are no high energy ions from the laser entrance holes. These studies provide novel insight into the interactions and dynamics of a vacuum hohlraum and near vacuum hohlraum.

We present the first experimental evidence supported by simulations of kinetic effects launched in the interpenetration layer between the laser-driven hohlraum plasma bubbles and the corona plasma of the compressed pellet at the Shenguang-III prototype laser facility. Solid plastic capsules were coated with carbon-deuterium layers; as the implosion neutron yield is quenched, DD fusion yield from the corona plasma provides a direct measure of the kinetic effects inside the hohlraum. An anomalous large energy spread of the DD neutron signal (∼282 keV) and anomalous scaling of the neutron yield with the thickness of the carbon-deuterium layers cannot be explained by the hydrodynamic mechanisms. Instead, these results can be attributed to kinetic shocks that arise in the hohlraum-wall-ablator interpenetration region, which result in efficient acceleration of the deuterons (∼28.8 J, 0.45% of the total input laser energy). These studies provide novel insight into the interactions and dynamics of a vacuum hohlraum and near-vacuum hohlraum.

The influence of ionization on the formation of plasma channel and the propagation of a relativistic electron beam in a near-critical density hydrocarbon gas are investigated by using two-dimensional (2D) particle-in-cell (PIC) simulations. A magnetic-dipole vortex is formed and self-sustained by its magnetic field pressure inside the plasma channel when the short pulse laser energy is almost depleted. After its formation, the magnetic dipole vortex moves forward with the relativistic electron beam in the plasma channel. In a fully ionized plasma, a high density plasma barrier is usually formed ahead of the plasma channel due to the density profile steepening. Therefore, the deflection of the plasma channel can easily occur during the forward movement of the magnetic dipole vortex. In contrast, the deflection of the plasma channel is suppressed by the ionization effect with hydrocarbon gas. Two main mechanisms to suppress the deflection are found. These mechanisms are the decrease of the plasma density steepening at the front of the plasma channel in a partly-ionized plasma, and the consuming of the electromagnetic field energy due to the ionization itself.

The anomalous mix at the high-Z and low-Z plasma interfaces in an inertial confinement fusion hohlraum is a current topic of interest. The mechanism for such an anomalous mix in the interpenetration layer at the high-Z and low-Z plasma interface and its effects on the laser plasma instabilities have been investigated by particle-in-cell simulations. It is found that a diffusion-driven collisionless shock wave can be generated from an initially sharp high-Z and low-Z plasma interface with total pressure balance and constant temperature in the laser propagation channel. This purely electrostatic shock wave propagates into the high-Z plasma and leads to mix of different species of ions which is significantly faster than a classical mix in the presence of the large electric field. The mix layer width, measured as a separation distance affected by the shock, grows as , where . The effect of the anomalous mix on the linear growth rate of laser plasma instabilities is evaluated.

The effect of an externally applied magnetic field on the ion acceleration by laser-driven collisionless shocks is examined by means of multi-dimensional particle-in-cell simulations. For the interaction of ultra-intense sub-picosecond laser pulses with the near-relativistic critical-density plasma, the longitudinal transport of the laser generated fast electrons are significantly inhibited by the kilo-Tesla (kT) level transverse magnetic field, resulting in a thermal pressure which significantly exceeds the laser radiation pressure in the hot electron accumulation region. As a result, the accumulated plasma expands into the vacuum and leads to acceleration of a supersonic plasma flow in the opposite direction through the rocket effect, which streams into the target and drives a supercritical magnetized collisionless shock. In comparison with the case without the external magnetic field, where an electrostatic collisionless shock can be driven, the energy flux of the shock accelerated quasi-monoenergetic ion beam is considerably increased by an order of magnitude due to the strength enhancement of the magnetized shock.

A new scheme for cone-hollow-assisted fast ignition in inertial fusion is investigated. A hollow is attached to the tip of a conventional gold cone. The transport and heating of the high-current electrons propagating from the cone tip to the compressed fuel core along the hollow is investigated by two-dimensional hybrid simulation. Different hollow geometry sizes are examinized. It is shown that with proper hollow guiding, hot electrons can be collimated between the inner-walls of the hollow by the large interface magnetic fields appearing on the inner surface. When the beam electrons further propagate into the dense region, they are scattered into the gold hollow through collisions with the fuel electrons and ions. The resulting magnetic potential around the hollow then bends beam electrons along the gold hollow to reach the dense core.

In indirect-drive inertial confinement fusion (ICF), the incident laser beam could excite laser plasma instabilities (LPI) such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) and two plasmon decay (TPD) besides gently heat the hohlraum through collisional absorption. These instabilities would largely reduce the X-ray conversion and degrade the drive symmetry of the radiation environment. In addition, when the amplitude of parametric instability increases to a certain level, there would be interplay between different instabilities, which makes LPI complicated and unpredictable. Therefore, LPI has become one of the major challenge in achieving ignition. LPI research during recent few years made great strides in identifying, understanding, and controlling instabilities in the context of laser fusion. This paper reviews the progress in this important field according to laser (L), plasma (P), and instability (I). Prospects for the application of our improved understanding for indirect drive ICF and some exciting research opportunities are also discussed.

The role of ion–ion acoustic instabilities in the formation and dissipation of collisionless electrostatic shock waves driven by counter-streaming supersonic plasma flows has been investigated via two-dimensional particle-in-cell simulations. The nonlinear evolution of unstable waves and ion velocity distributions has been analyzed in detail. It is found that for electrostatic shocks driven by moderate-velocity flows, longitudinal and oblique ion–ion acoustic instabilities can be excited in the downstream and upstream regions, which lead to thermalization of the transmitted and reflected ions, respectively. For high-velocity flows, oblique ion–ion acoustic instabilities can develop in the overlap layer during the shock formation process and impede the shock formation.

Channeling by a train of laser pulses into homogeneous and inhomogeneous plasmas is studied using particle-in-cell simulation. When the pulse duration and the interval between the successive pulses are appropriate, the laser pulse train can channel into the plasma deeper than a single long-pulse laser of similar peak intensity and total energy. The increased penetration distance can be attributed to the repeated actions of the ponderomotive force, the continuous between-pulse channel lengthening by the inertially evacuating ions, and the suppression of laser-driven plasma instabilities by the intermittent laser-energy cut-offs.

A scheme for neutron production is investigated in which an ultra-intense laser is irradiated into a two-layer (deuterium and aurum) spherical shell target through the cone shaped entrance hole. It is found that the energy conversion efficiency from laser to target can reach as high as 71%, and deuterium ions are heated to a maximum energy of several MeV from the inner layer surface. These ions are accelerated towards the center of the cavity and accumulated finally with a high density up to tens of critical density in several picoseconds. Two different mechanisms account for the efficient yield of the neutrons in the cavity: (1) At the early stage, the neutrons are generated by the high energy deuterium ions based on the “beam-target” approach. (2) At the later stage, the neutrons are generated by the thermonuclear fusion when the most of the deuterium ions reach equilibrium in the cavity. It is also found that a large number of deuterium ions accelerated inward can pass through the target center and the outer Au layer and finally stopped in the CD2 layer. This also causes efficient yield of neutrons inside the CD2 layer due to “beam-target” approach. A postprocessor has been designed to evaluate the neutron yield and the neutron spectrum is obtained.

Fast ignition approach to inertial confinement fusion is one of the important goals today, in addition to central hot spot ignition in China. The SG-IIU and PW laser facilities are coupled to investigate the hot spot formation for fast ignition. The SG-III laser facility is almost completed and will be coupled with tens kJ PW lasers for the demonstration of fast ignition. In recent years, for physical studies of fast ignition, we have been focusing on the experimental study of implosion symmetry, M-band radiation preheating and mixing, advanced fast ignition target design, and so on. In addition, the modeling capabilities and code developments enhanced our ability to perform the hydro-simulation of the compression implosion, and the particle-in-cell (PIC) and hybrid-PIC simulation of the generation, transport and deposition of relativistic electron beams. Considerable progress has been achieved in understanding the critical issues of fast ignition.

Three-dimensional particle-in-cell simulation is used to investigate radiation-pressure driven acceleration and compression of small solid-density plasma by intense laser pulses. It is found that multiple impacts by presently available short-pulse lasers on a small hemispheric shell target can create a long-living tiny quasineutral monoenergetic plasma bunch of very high energy density.

The effects of collision on the generation and transportation of fast electrons produced by ultra-intense laser pulse in overdense plasma for densities ranging from below to 400 times critical density are investigated by collisional particle-in-cell code. It is found that a relatively stable state of fast electron energy flux exists in the simulations, where collision contributes to increasing the production of fast electrons. The unexpected increase of production is attributed to the efficient local heating of the thermal electrons, which results in higher thermal pressure and less steepened interface. Therefore, fast electrons can be effectively accelerated through 2ω oscillation from J×B force in the collisional case, while it is suppressed in the collisionless case because of the highly steepened plasma density. The collisional effects on the transportation of fast electrons in the solid target are also discussed.

A 2D analytical model is presented for the generation of plasma wakes (or bubbles) with an ultra-intense laser pulse by taking into account the response of plasma ions. It is shown that the effect of ion motion becomes significant at the laser intensity exceeding 10{sup 21} W/cm{sup 2} and plasma background density below 10{sup 19} cm{sup -3}. In this regime, ion motion tends to suppress the electrostatic field induced by charge separation and makes the electron acceleration less effective. As a result, the assumption of immobile ions overestimates the efficiency of laser wake-field acceleration of electrons. Based on the analytical model, the dynamics of plasma ions in laser-induced wake field is investigated. It is found that only one bubble appears as the plasmas background density exceeds the resonant density and the deposited laser energy is concentrated into the bubble, resulting in the generation of an ion bunch with extremely high energy density.

The effect of pre-plasma on core heating in cone-guiding fast ignition is evaluated by two-dimensional particle-in-cell (PIC) and Fokker–Planck (FP) simulations. If the long-scale pre-plasma exists in the cone, the generated fast electron energy becomes too high for effective core heating. As a result, the energy coupling from laser to core ηL→core is reduced by 80% compared with the case without a pre-plasma. Even for the case without a pre-plasma, ηL→core obtained in the simulation is smaller than that required for 5 keV heating in FIREX-I. In order to enhance ηL→core, we propose a new target design ‘extended double cone with short inner cone wall’ for fast electron guiding to imploded core and show sufficient improvement of heating efficiency using this new cone on the basis of PIC and FP hydro-simulations.

Intense short-petawatt-laser driven relativistic electron beams in a hollow high-Z plasma fiber embedded in low-Z plasmas of different densities are studied. When the plasma is of lower density than the hollow fiber, resistive filamentation of the electron beam is observed. It is found that the electron motion and the magnetic field are highly correlated with tens of terahertz oscillation frequency. Depending on the material property around the hollow fiber and the plasma density, the beam electrons can be focused or defocused as it propagates in the plasma. Relativistic electron transport and target heating are also investigated.

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.

The impact of a laser-accelerated micron-size projectile on a dense plasma target is studied using two-dimensional particle-in-cell simulations. The projectile is first accelerated by an ultraintense laser. It then impinges on the dense plasma target and merges with the latter. Part of the kinetic energy of the laser-accelerated ions in the projectile is deposited in the fused target, and an extremely high concentration of plasma ions with a mean kinetic energy needed for fusion reaction is induced. The interaction is thus useful for laser-driven impact fusion and as a compact neutron source.

Laser plasma interactions in a relativistic regime relevant to the fast ignition in inertial confinement fusion have been investigated. Ultraintense laser propagation in preformed plasmas and hot electron generation are studied. The experiments are performed using a 100 TW 0.6 ps laser and a 20 TW 0.6 ps laser synchronized by a long pulse laser. In the study, a self-focused ultraintense laser beam propagates along its axis into an overdense plasma with peak density 1022/cm3. Channel formation in the plasma is observed. The laser transmission in the overdense plasma depends on the position of its focus and can take place in plasmas with peak densities as high as 5×1022/cm3. The hot electron beams produced by the laser-plasma interaction have a divergence angle of ∼30°, which is smaller than that from laser-solid interactions. For deeper penetration of the laser light into the plasma, the use of multiple short pulse lasers is proposed. The latter scheme is investigated using particle-in-cell simulation. It ...

To increase the acceleration rate for producing highly energetic protons, a scheme using a cone-shaped target with a coated proton layer is proposed and demonstrated by 2D particle-in-cell (PIC) simulation. The simulation results show that because the laser light and electrons are guided along the cone wall, the energy and density of hot electrons are enhanced in the cone target. Thus, the amplitude of the sheath field on the target's rear surface is enhanced, since it is proportional to the hot electrons' temperature and the logarithm of their density. Therefore, protons are accelerated strongly by this sheath field.

An analytical fluid model for JxB heating during the normal incidence by a short ultraintense linearly polarized laser on a solid-density plasma is proposed. The steepening of an originally smooth electron density profile as the electrons are pushed inward by the laser is included self-consistently. It is shown that the JxB heating includes two distinct coupling processes depending on the initial laser and plasma conditions: for a moderate intensity (a 4 gamma(0)n(c), so that no 2 omega resonance will occur, but the longitudinal component of the oscillating ponderomotive field can lead to an absorption mechanism similar to "vacuum heating." (c) 2006 American Institute of Physics.