Ultrahigh intensity lasers can potentially be used in conjunction with conventional fusion lasers to ignite inertial confinement fusion (ICF) capsules with a total energy of a few tens of kilojoules of laser light, and can possibly lead to high gain with as little as 100 kJ. A scheme is proposed with three phases. First, a capsule is imploded as in the conventional approach to inertial fusion to assemble a high-density fuel configuration. Second, a hole is bored through the capsule corona composed of ablated material, as the critical density is pushed close to the high-density core of the capsule by the ponderomotive force associated with high-intensity laser light. Finally, the fuel is ignited by suprathermal electrons, produced in the high-intensity laser-plasma interactions, which then propagate from critical density to this high-density core. This new scheme also drastically reduces the difficulty of the implosion, and thereby allows lower quality fabrication and less stringent beam quality and symmetry requirements from the implosion driver. The difficulty of the fusion scheme is transferred to the technological difficulty of producing the ultrahigh-intensity laser and of transporting this energy to the fuel.

We use simulations to investigate the interaction of ultra-intense laser pulses with a plasma. With an intensity greater than 10' W/cm, these pulses have a pressure greater than l0 Mbar and drive the plasma relativistically.Hole boring by the light beam is a key feature of the interaction. We find substantial absorption into heated electrons with a characteristic temperature of order the ponderomotive potential. Other eAects include a dependence on the polarization of the incident light, strong magnetic field generation, and a period of intense instability generation in the underdense plasma.PACS numbers: 52.40.Nk, 52.35.Nx, 52.60.+h The ongoing development of ultrabright, short pulse lasers is allowing exploration of new regimes of lasermatter interaction. Experiments are now being carried out [1][2][3] in which plasmas are irradiated by laser light with intensities up to lk"=10' Wpm /cm . Here I is the intensity and A, " the wavelength in microns. At such intensities, electrons oscillating in the field of the light wave are strongly relativistic; i.e. , p, bmoc~I, where p" is the momentum of oscillation, mo the electron mass, and c the velocity of light. This is the first simulation study to address an ultra-intense light wave with a finite spot size normally incident onto an overdense plasmavacuum interface. Some of the key results are strong hole punching and MeV ion generation inward due to the large light pressure, MeV electron generation into the overdense plasma by nonadiabatic heating at the sharp light-plasma interface, and estimates for the absorption of the incident light and characteristic temperature of heated electrons for a range of laser intensities soon to be experimentally accessible.We use a 2D, electromagnetic, relativistic electron, mobile ion, particle-in-cell code (ZOHAR [4]) to study this relatively unexplored, highly nonlinear regime [5].As with previous ID studies at lower intensities [61, these simulations model the interaction of laser light with a collisionless plasma. For the intensities and time scales studied here, inverse bremsstrahlung is weak since the light-plasma interface is quite steep and the effective temperature of the electrons interacting with the light wave is large. We begin with a simulation which focuses on the interaction of high-intensity laser light with a preformed, overdense plasma. Laser light with a Gaussian intensity profile (in the transverse or y dimension) is introduced at the left boundary and propagates (in the x direction) through a region of vacuum onto a slab of overdense plasma. Particles escaping through the right boundary are reemitted with their original temperature.In this example, the system is 36c/cuit long in the x direction and 40c/coo wide, where roti is the light-wave frequency. The plasma is initially 22c/ton long, preceded by 14c/too of vacuum. The electric field of the incident light wave is in the y direction, and the dynamics are followed in the x-y plane (the so-called p-polarized case). In complementary simulations, the electric...

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Several targets are described that in simulations give yields of 1–30 MJ when indirectly driven by 0.9–2 MJ of 0.35 μm laser light. The article describes the targets, the modeling that was used to design them, and the modeling done to set specifications for the laser system in the proposed National Ignition Facility. Capsules with beryllium or polystyrene ablators are enclosed in gold hohlraums. All the designs utilize a cryogenic fuel layer; it is very difficult to achieve ignition at this scale with a noncryogenic capsule. It is necessary to use multiple bands of illumination in the hohlraum to achieve sufficiently uniform x-ray irradiation, and to use a low-Z gas fill in the hohlraum to reduce filling of the hohlraum with gold plasma. Critical issues are hohlraum design and optimization, Rayleigh–Taylor instability modeling, and laser–plasma interactions.

Transport modeling of idealized, cone-guided fast ignition targets indicates the severe challenge posed by fast-electron source divergence. The hybrid particle-in-cell [PIC] code Zuma is run in tandem with the radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel heating, and thermonuclear burn. The fast electron source is based on a 3D explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi two-temperature energy spectrum, and a divergent angle spectrum (average velocityspace polar angle of 52 • ). Transport simulations with the PIC-based divergence do not ignite for > 1 MJ of fast-electron energy, for a modest (70 µm) standoff distance from fast-electron injection to the dense fuel. However, artificially collimating the source gives an ignition energy of 132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields. Uniform fields ∼50 MG are sufficient to recover the artificially collimated ignition energy. Experiments at the Omega laser facility have generated fields of this magnitude by imploding a capsule in seed fields of 50-100 kG. Such imploded fields are however more compressed in the transport region than in the laser absorption region. When fast electrons encounter increasing field strength, magnetic mirroring can reflect a substantial fraction of them and reduce coupling to the fuel. A hollow magnetic pipe, which peaks at a finite radius, is presented as one field configuration which circumvents mirroring.

We report the first direct measurements of total absorption of short laser pulses on solid targets in the ultrarelativistic regime. The data show an enhanced absorption at intensities above 10(20) W/cm(2), reaching 60% for near-normal incidence and 80%-90% for 45 degrees incidence. Two-dimensional particle-in-cell simulations demonstrate that such high absorption is consistent with both interaction with preplasma and hole boring by the intense laser pulse. A large redshift in the second harmonic indicates a surface recession velocity of 0.035c.

Detailed angle and energy resolved measurements of positrons ejected from the back of a gold target that was irradiated with an intense picosecond duration laser pulse reveal that the positrons are ejected in a collimated relativistic jet. The laser-positron energy conversion efficiency is ∼2×10{-4}. The jets have ∼20 degree angular divergence and the energy distributions are quasimonoenergetic with energy of 4 to 20 MeV and a beam temperature of ∼1  MeV. The sheath electric field on the surface of the target is shown to determine the positron energy. The positron angular and energy distribution is controlled by varying the sheath field, through the laser conditions and target geometry.

The minimum energy needed to ignite an inertial confinement fusion capsule is of considerable interest in the optimization of an inertial fusion driver. Recent computational work investigating this minimum energy has found that it depends on the capsule implosion history, in particular, on the capsule drive pressure. This dependence is examined using a series of LASNEX simulations to find ignited capsules which have different values of the implosion velocity, fuel adiabat and drive pressure. It is found that the main effect of varying the drive pressure is to alter the stagnation of the capsule, changing its stagnation adiabat, which, in turn, affects the energy required for ignition. To account for this effect a generalized scaling law has been devised for the ignition energy, Eign ∝ α 1.88±0.05 if v −5.89±0.12 P −0.77±0.03 . This generalized scaling law agrees with the results of previous work in the appropriate limits.