A novel method by C. Zhou and R. Betti [Bull. Am. Phys. Soc. 50, 140 (2005)] to assemble and ignite thermonuclear fuel is presented. Massive cryogenic shells are first imploded by direct laser light with a low implosion velocity and on a low adiabat leading to fuel assemblies with large areal densities. The assembled fuel is ignited from a central hot spot heated by the collision of a spherically convergent ignitor shock and the return shock. The resulting fuel assembly features a hot-spot pressure greater than the surrounding dense fuel pressure. Such a nonisobaric assembly requires a lower energy threshold for ignition than the conventional isobaric one. The ignitor shock can be launched by a spike in the laser power or by particle beams. The thermonuclear gain can be significantly larger than in conventional isobaric ignition for equal driver energy.

The direct-drive, laser-based approach to inertial confinement fusion (ICF) is reviewed from its inception following the demonstration of the first laser to its implementation on the present generation of high-power lasers. The review focuses on the evolution of scientific understanding gained from target-physics experiments in many areas, identifying problems that were demonstrated and the solutions implemented. The review starts with the basic understanding of laser–plasma interactions that was obtained before the declassification of laser-induced compression in the early 1970s and continues with the compression experiments using infrared lasers in the late 1970s that produced thermonuclear neutrons. The problem of suprathermal electrons and the target preheat that they caused, associated with the infrared laser wavelength, led to lasers being built after 1980 to operate at shorter wavelengths, especially 0.35 μm—the third harmonic of the Nd:glass laser—and 0.248 μm (the KrF gas laser). The main physics areas relevant to direct drive are reviewed. The primary absorption mechanism at short wavelengths is classical inverse bremsstrahlung. Nonuniformities imprinted on the target by laser irradiation have been addressed by the development of a number of beam-smoothing techniques and imprint-mitigation strategies. The effects of hydrodynamic instabilities are mitigated by a combination of imprint reduction and target designs that minimize the instability growth rates. Several coronal plasma physics processes are reviewed. The two-plasmon–decay instability, stimulated Brillouin scattering (together with cross-beam energy transfer), and (possibly) stimulated Raman scattering are identified as potential concerns, placing constraints on the laser intensities used in target designs, while other processes (self-focusing and filamentation, the parametric decay instability, and magnetic fields), once considered important, are now of lesser concern for mainline direct-drive target concepts. Filamentation is largely suppressed by beam smoothing. Thermal transport modeling, important to the interpretation of experiments and to target design, has been found to be nonlocal in nature. Advances in shock timing and equation-of-state measurements relevant to direct-drive ICF are reported. Room-temperature implosions have provided an increased understanding of the importance of stability and uniformity. The evolution of cryogenic implosion capabilities, leading to an extensive series carried out on the 60-beam OMEGA laser [Boehly et al., Opt. Commun. 133, 495 (1997)], is reviewed together with major advances in cryogenic target formation. A polar-drive concept has been developed that will enable direct-drive–ignition experiments to be performed on the National Ignition Facility [Haynam et al., Appl. Opt. 46(16), 3276 (2007)]. The advantages offered by the alternative approaches of fast ignition and shock ignition and the issues associated with these concepts are described. The lessons learned from target-physics and implosion experiments are taken into account in ignition and high-gain target designs for laser wavelengths of 1/3 μm and 1/4 μm. Substantial advances in direct-drive inertial fusion reactor concepts are reviewed. Overall, the progress in scientific understanding over the past five decades has been enormous, to the point that inertial fusion energy using direct drive shows significant promise as a future environmentally attractive energy source.

A distinctive way of quantitatively imaging inertial fusion implosions has resulted in the characterization of two different types of electromagnetic configurations and in the measurement of the temporal evolution of capsule size and areal density. Radiography with a pulsed, monoenergetic, isotropic proton source reveals field structures through deflection of proton trajectories, and areal densities are quantified through the energy lost by protons while traversing the plasma. The two field structures consist of (i) many radial filaments with complex striations and bifurcations, permeating the entire field of view, of magnetic field magnitude 60 tesla and (ii) a coherent, centrally directed electric field of order 10 9 volts per meter, seen in proximity to the capsule surface. Although the mechanism for generating these fields is unclear, their effect on implosion dynamics is potentially consequential.

Polymer chains attached by one end to an impenetrable surface at high coverage exemplify a tethered layer of mesoscopic dimensions. At equilibrium, thermal fluctuations of the segment density profile of the brushlike layer reflect the tethered chain dynamics; the probing of these fluctuations by evanescent-wave dynamic light scattering is reported. By utilizing a set of terminally attached layers with thicknesses (L 0 ) from 45 to 130 nanometers, it was found that there is a preferred wavelength of order L 0 of these fluctuations with a concurrent slowing down of their thermal decay rate. This technique could open the route for the investigation of the largely unexplored area of polymer surface dynamics.When polymer molecules are selectively attached by one end onto a solid surface at relatively high concentrations, the polymer chains avoid one another by extending away from the surface, forming a "brush" (1, 2). The formation of polymer brushes is of technological importance in colloid stabilization and in modifying bulk surfaces and interfaces for improved adhesion, wetting, and wear properties. Such brushes can be created by synthesizing diblock copolymers, in which one shorter block adsorbs strongly to the surface from a solution and the other longer block extends into the solvent. This stretched configuration of tethered chains is used as a model for a wide variety of confined polymer systems (3). Most theoretical and experimental studies (3-10) have addressed the static structure of brushes, such as the density profile above the surface. Although theoretical predictions have been made of their dynamical structure as well (4,5,11,12), direct relevant experimental observation, although much needed (7), is difficult because of the small size of the brush and its low scattering power.We have addressed this problem by performing evanescent-wave dynamic lightscattering (EWDLS) measurements of asymmetric poly(ethyleneoxide-b-styrene) (PEO-PS) copolymers (Fig. 1). Light is reflected from a high-refractive index prism; the polymer is attached at the glass surface at the base of the prism, and the evanescent wave propagates through the polymer brush under conditions of total internal reflection. The evanescent wave is then used as the incident beam in photon correlation spectroscopy (13), which allows fluctuations with wave vector q to be resolved on time scales from 10 Ϫ7 to 10 3 s. From these measurements, we obtained the time-correlation functions C(q, t) of the scatteredlight intensity that reflect concentration fluctuations in the brush. Our results show that long-lived fluctuations have a finite wavelength of order L 0 [O(L 0 )], where L 0 is the equilibrium thickness of the grafted PS layer. This result resembles that for the interal relaxation mode seen in bulk diblock copolymer solutions (14).Detailed material and surface characterization is essential for the interpretation of the results. The PEO-PS model system in toluene was selected on account of the extensive information on adsorbed amount, kinetics of...

The paper presents theoretical analysis and experimental results concerning the major physical issues in the shock ignition approach. These are: generation of a high amplitude shock in the imploding target, laser-plasma interaction physics in the conditions of high laser intensities needed for high amplitude shock excitation, symmetry and stability of the shock propagation, role of fast electrons in the symmetrization of the shock pressure and the fuel preheat. The theoretical models and numerical simulations are compared with the results of specially designed experiments on laser plasma interaction and shock excitation in plane and spherical geometries.

Metal foil targets were irradiated with 1 mum wavelength (lambda) laser pulses of 5 ps duration and focused intensities (I) of up to 4x10;{19} W cm;{-2}, giving values of both Ilambda;{2} and pulse duration comparable to those required for fast ignition inertial fusion. The divergence of the electrons accelerated into the target was determined from spatially resolved measurements of x-ray K_{alpha} emission and from transverse probing of the plasma formed on the back of the foils. Comparison of the divergence with other published data shows that it increases with Ilambda;{2} and is independent of pulse duration. Two-dimensional particle-in-cell simulations reproduce these results, indicating that it is a fundamental property of the laser-plasma interaction.

Dynamics of thin metal foils irradiated by moderate-contrast high-intensity laser beamsThe modeling of petawatt laser-generated hot electrons in mass-limited solid-foil-target interactions at "relativistic" laser intensities is presented using copper targets and parameters motivated by recent experiments at the Rutherford Appleton Laboratory Petawatt and 100-TW facilities ͓Theobald et al., Phys. Plasmas 13, 043102 ͑2006͔͒. Electron refluxing allows a unique determination of the laser-electron conversion efficiency and a test with simulations. Good agreement between experiments and simulations is found for conversion efficiencies of 10%.

Spherical shock-ignition experiments on OMEGA used a novel beam configuration that separates low-intensity compression beams and high-intensity spike beams. Significant improvements in the performance of plastic-shell, D 2 implosions were observed with repointed beams. The analysis of the coupling of the high-intensity spike beam energy into the imploding capsule indicates that absorbed hot-electron energy contributes to the coupling. The backscattering of laser energy was measured to reach up to 36% at single-beam intensities of $8 Â 10 15 W/cm 2. Hard x-ray measurements revealed a relatively low hot-electron temperature of $30 keV independent of intensity and timing. At the highest intensity, stimulated Brillouin scattering occurs near and above the quarter-critical density and the two-plasmon-decay instability is suppressed. V