ALBERT

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.

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: 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: Experimental results and simulations that study the effects of thin metallic layers with high atomic number (high-Z) on the hydrodynamics of laser accelerated plastic targets are presented. These experiments employ a laser pulse with a low-intensity foot that rises into a high-intensity main pulse. This pulse shape simulates the generic shape needed for high-gain fusion implosions. Imprint of laser nonuniformity during start up of the low intensity foot is a well-known seed for hydrodynamic instability. Large reductions are observed in hydrodynamic instability seeded by laser imprint when certain minimum thickness gold or palladium layers are applied to the laser-illuminated surface of the targets. The experiment indicates that the reduction in imprint is at least as large as that obtained by a 6 times improvement in the laser uniformity. Simulations supported by experiments are presented showing that during the low intensity foot the laser light can be nearly completely absorbed by the high-Z layer. X rays originating from the high-Z layer heat the underlying lower-Z plastic target material and cause large buffering plasma to form between the layer and the accelerated target. This long-scale plasma apparently isolates the target from laser nonuniformity and accounts for the observed large reduction in laser imprint. With onset of the higher intensity main pulse, the high-Z layer expands and the laser light is transmitted. This technique will be useful in reducing laser imprint in pellet implosions and thereby allow the design of more robust targets for high-gain laser fusion. © 2002 American Institute of Physics.

Notes: The classical Richtmyer–Meshkov (RM) instability develops when a planar shock wave interacts with a corrugated interface between two different fluids. A larger family of so-called RM-like hydrodynamic interfacial instabilities is discussed. All of these feature a perturbation growth at an interface, which is driven mainly by vorticity, either initially deposited at the interface or supplied by external sources. The inertial confinement fusion relevant physical conditions that give rise to the RM-like instabilities range from the early-time phase of conventional ablative laser acceleration to collisions of plasma shells (like components of nested-wire-arrays, double-gas-puff Z-pinch loads, supernovae ejecta and interstellar gas). In the laser ablation case, numerous additional factors are involved: the mass flow through the front, thermal conduction in the corona, and an external perturbation drive (laser imprint), which leads to a full stabilization of perturbation growth. In contrast with the classical RM case, mass perturbations can exhibit decaying oscillations rather than a linear growth. It is shown how the early-time perturbation behavior could be controlled by tailoring the density profile of a laser target or a Z-pinch load, to diminish the total mass perturbation seed for the Rayleigh–Taylor instability development. © 2000 American Institute of Physics.

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 technique of near forward laser scattering is used to infer characteristics of intrinsic and controlled density fluctuations in laser-produced plasmas. Intrinsic fluctuations are studied in long-scalelength plasmas where it is found that the fluctuations exhibit scale sizes related to the intensity variation scales in the plasma forming and interaction beams. Stimulated Brillouin forward scattering and filamentation appear to be the primary mechanism through which these fluctuations originate. The beam spray resulting from these fluctuations is important to understand, since it can affect symmetry in an inertial confinement fusion (ICF) experiment. Controlled fluctuations are studied in foam and exploding foil targets. Forward scattered light from foam targets shows evidence that the initial target inhomogeneities remain after the target is laser heated. Forward scattered light from an exploding foil plasma shows that a regular intensity pattern can be used to produce a spatially correlated density fluctuation pattern. These results provide data which are being used to benchmark numerical models of beam spray.

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: 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: Illumination uniformity in laser-pellet interactions is investigated for small kilojoule-range laser systems that are capable of symmetrically illuminating spherical targets. We address a variety of options that might be used to control the illumination uniformity on such targets, including the number of beams, pulse energy, laser wavelength, temporal pulse shape, and beam spatial profile shape. We consider only laser beams that are optically smoothed using methods such as induced spatial incoherence or random phase plates. The parameters that are most important for providing good uniformity are identified. We find that the most important parameters governing the uniformity are the number of beams and the shape of the beam profile. Configurations with a small number of beams (e.g., six) are much more sensitive to the beam profile; using techniques such as splitting the focal spots provides an additional degree of freedom that can significantly increase the illumination uniformity. Additionally, we show that the hydrodynamics of the plasma is important in determining the uniformity, although it is not so easily controlled. Quantitatively, we find nonuniformities (measured by peak-to-valley ablation pressure differences) can be less than 10% for six-beam kilojoule-level 1/4 -μm laser wavelength systems, and of the order of a few percent for comparable 24-beam systems.