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Notes: The role played by radiation in the radiation-preheated direct-drive laser fusion target design is discussed. The soft x-rays emitted during the foot of the laser pulse—at a few 1012 W/cm2—preheat the low-opacity foam ablator which helps to control the Rayleigh–Taylor instability. The foam opacity is, however, thick enough to stop that radiation, keeping the fuel on a low adiabat. Radiation effects are also important in the blow-off corona of the target because they establish a long scale-length plasma. This may help to shield the ablation region from the nonuniformities in the laser absorption. © 2000 American Institute of Physics.

Notes: The uniform and smooth focal profile of the Nike KrF laser [S. Obenschain et al., Phys. Plasmas 3, 2098 (1996)] was used to ablatively accelerate 40 μm thick polystyrene planar targets with pulse shaping to minimize shock heating of the compressed material. The foils had imposed small-amplitude sinusoidal wave perturbations of 60, 30, 20, and 12.5 μm wavelength. The shortest wavelength is near the ablative stabilization cutoff for Rayleigh–Taylor growth. Modification of the saturated wave structure due to random laser imprint was observed. Excellent agreement was found between the two-dimensional simulations and experimental data for most cases where the laser imprint was not dominant. © 1999 American Institute of Physics.

Notes: The Nike laser (∼2–3 kJ, ∼1014 W/cm2) has been used to ablatively accelerate planar liquid deuterium targets. These experiments are designed to test some aspects of a high gain direct drive target design. The target consists of a low-density foam that is filled with liquid deuterium and covered with a thin polyimide membrane. The measured target trajectory agrees well with one-dimensional (1D) simulations. The growth of the areal mass modulations were measured with a new, 1.26 keV x-ray backlighter. The modulations appear later and grow to a smaller amplitude when the foot of the laser pulse is made spatially smoother. A thin layer of gold on the front of the target reduces the modulations. The results are compared with 2D modeling.

Notes: A new laser fusion target concept is presented with a predicted energy gain of 127 using a 1.3 MJ KrF laser. This energy gain is sufficiently high for an economically attractive fusion reactor. X rays from high- and low-Z materials are used in combination with a low-opacity ablator to spatially tune the isentrope, thereby providing both high fuel compression and a reduction of the ablative Rayleigh–Taylor instability. © 2000 American Institute of Physics.

Notes: Nike is a 56 beam Krypton Fluoride (KrF) laser system using Induced Spatial Incoherence (ISI) beam smoothing with a measured focal nonuniformity 〈ΔI/I〉 of 1% rms in a single beam [S. Obenschain et al., Phys. Plasmas 3, 1996 (2098)]. When 37 of these beams are overlapped on the target, we estimate that the beam nonuniformity is reduced by 37, to (ΔI/I)≅0.15% (excluding short-wavelength beam-to-beam interference). The extraordinary uniformity of the laser drive, along with a newly developed x-ray framing diagnostic, has provided a unique facility for the accurate measurements of Rayleigh–Taylor amplified laser-imprinted mass perturbations under conditions relevant to direct-drive laser fusion. Data from targets with smooth surfaces as well as those with impressed sine wave perturbations agree with our two-dimensional (2-D) radiation hydrodynamics code that includes the time-dependent ISI beam modulations. A 2-D simulation of a target with a 100 Å rms randomly rough surface finish driven by a completely uniform beam gives final perturbation amplitudes similar to the experimental data for the smoothest laser profile. These results are promising for direct-drive laser fusion.

Notes: Krypton-fluoride (KrF) lasers are of interest to laser fusion because they have both the large bandwidth capability ((approximately-greater-than)THz) desired for rapid beam smoothing and the short laser wavelength (1/4 μm) needed for good laser–target coupling. Nike is a recently completed 56-beam KrF laser and target facility at the Naval Research Laboratory. Because of its bandwidth of 1 THz FWHM (full width at half-maximum), Nike produces more uniform focal distributions than any other high-energy ultraviolet laser. Nike was designed to study the hydrodynamic instability of ablatively accelerated planar targets. First results show that Nike has spatially uniform ablation pressures (Δp/p〈2%). Targets have been accelerated for distances sufficient to study hydrodynamic instability while maintaining good planarity. In this review we present the performance of the Nike laser in producing uniform illumination, and its performance in correspondingly uniform acceleration of targets. © 1996 American Institute of Physics.

Notes: This paper describes theoretical and experimental investigations of induced spatial incoherence (ISI), a technique for achieving the smooth and controllable target beam profiles required for direct-drive laser fusion. In conventional ISI, a broadband laser beam (coherence time tc=1/Δν(very-much-less-than)tpulse) is sliced into an array of mutually incoherent beamlets by echelon structures that impose successive time delay increments Δt〉tc. A focusing lens then overlaps those beamlets onto the target, which is usually located at the far field. Here, we evaluate the ideal target beam profiles for practical ISI focusing configurations, and examine the perturbing effects of transient interference, laser aberration, and plasma filamentation. Analytic and numerical calculations show that nonuniformities due to interference among the beamlets are smoothed by both thermal diffusion and temporal averaging. Under laser-plasma conditions of interest to inertial confinement fusion (ICF), average ablation pressure nonuniformities ∼1% should be readily attainable. We also investigate a partial ISI scheme, which allows widely spaced beamlets to remain mutually coherent; the resulting high spatial frequency interference structure can be effectively smoothed by thermal diffusion alone. A perturbation analysis shows that the average target profile 〈I(x)〉 remains relatively insensitive to laser beam aberration when the scale length of that aberration is larger than the initial beamlet width. This aberration will tend to broaden and smooth 〈I(x)〉, rather than introduce any small-scale structure. The broadening is largely controllable because it depends only upon spatial averagesof the aberrated quantities over the entire laser aperture; the uncontrollable perturbations can be reduced to ∼1% in practical cases. Filamentation in the underdense plasma has been studied numerically using a 2D propagation/hydro code selfoct, which includes both ponderomotive and thermal effects. For 0.25-μm light, this code predicts that ISI should suppress filamentation in plasmas of interest to ICF. We review recent planar target experiments carried out at the Naval Research Laboratory using 1.054- and 0.527-μm light, which show that the combination of ISI and shorter wavelength substantially reduces all evidence of plasma instabilities. Finally, we review a promising alternative technique for achieving ISI in KrF lasers without using echelons.