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...

An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 10 20 W/cm 2 , high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, we attempt to explain the physical process present that will explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of microns, whereupon they end up being detected in the radiographic and spectrographic detectors.

An intense collimated beam of high-energy protons is emitted normal to the rear surface of thin solid targets irradiated at 1 PW power and peak intensity 3x10(20) W cm(-2). Up to 48 J ( 12%) of the laser energy is transferred to 2x10(13) protons of energy >10 MeV. The energy spectrum exhibits a sharp high-energy cutoff as high as 58 MeV on the axis of the beam which decreases in energy with increasing off axis angle. Proton induced nuclear processes have been observed and used to characterize the beam.

The concept of fast ignition with inertial confinement fusion (ICF) is a way to reduce the energy required for ignition and burn and to maximize the gain produced by a single implosion. Based on recent experimental findings at the PETAWATT laser at Lawrence Livermore National Laboratory, an intense proton beam to achieve fast ignition is proposed. It is produced by direct laser acceleration and focused onto the pellet from the rear side of an irradiated target and can be integrated into a hohlraum for indirect drive ICF.

In our Petawatt laser experiments several hundred joules of 1 µm laser light in 0.5-5.0 ps pulses with intensities up to 3x10 20 Wcm -2 were incident on solid targets producing a strongly relativistic interaction. The energy content, spectra, and angular patterns of the photon, electron, and ion radiations were diagnosed in a number of ways, including several novel (to laser physics) nuclear activation techniques. From the beamed bremsstrahlung we infer that about 40-50% of the laser energy is converted to broadly beamed hot electrons. Their direction centroid varies from shot to shot, but the beam has a consistent width. Extraordinarily luminous ion beams almost precisely normal to the rear of various targets are seen -up to 3x10 13 protons with kT ion ~ several MeV representing ~6% of the laser energy.We observe ion energies up to at least 55 MeV. The ions appear to originate from the rear target surfaces.The edge of the ion beam is very sharp, and collimation increases with ion energy. At the highest energies, a narrow feature appears in the ion spectra, and the apparent size of the emitting spot is smaller than the full back surface area. Any ion emission from the front of the targets is much less than from the rear and is not sharply beamed. The hot electrons generate a Debye sheath with electrostatic fields of order MV per micron which apparently accelerate the ions.

Absorption mechanisms for ultra-intense (I > 10 17 W/cm 2 ) laser pulses incident on solids and overdense plasma slabs are discussed. We focus on the ultrashort pulse regime, i.e., where the laser pulse length is only a few to perhaps thousands of femtoseconds. Starting from well-known results at low intensity and long pulse length, we begin with absorption mechanisms such as inverse Bremstrahlung and classical resonance absorption and survey several additional absorption mechanisms significant for ultrashort, ultra-intense laser light interacting with overdense plasmas. Estimates for the fraction of laser energy absorbed by various mechanisms are given. It is found that the fraction of energy absorbed by the plasma, and the resulting electron temperatures, can depend considerably on the scale length of the plasma at the critical surface. It is also found that two-dimensional (2-D) effects greatly increase the amount of absorption into hot electrons, over the amount predicted using one-dimensional (1-D) theory. The inclusion of kinetic effects, collisionless absorption, and multidimensional effects are crucial to obtaining a complete picture of the interaction. We also review some of the experimental efforts to understand this complex process of absorption.

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