Jill P. Dahlburg

The Nike KrF laser facility: Performance and initial target experiments

Krypton‐fluoride (KrF) lasers are of interest to laser fusion because they have both the large bandwidth capability (≳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.

Direct and large-eddy simulations of three-dimensional compressible Navier-Stokes turbulence

This paper reports results from the numerical implementation and testing of the compressible large‐eddy simulation (LES) model described by Speziale et al. [Phys. Fluids 31, 940 (1988)] and Erlebacher et al. (to appear in J. Fluid Mech.). Relevant quantities from 323 ‘‘coarse grid’’ LES solutions are compared with results generated from 963 direct numerical simulations (DNS) of three‐dimensional compressible turbulence. It is found that the 323 LES results overall agree well with their 963 DNS counterparts. Moreover, the new DNS results confirm several recent conclusions about compressible turbulence that have been based primarily on two‐dimensional simulations.

Numerical simulation of ablative Rayleigh–Taylor instability

Numerical simulations over a wide range of parameters of the linear stability of an ablating laser‐produced plasma show a linear decrease in the growth rate with increasing ablation velocity. Simulations in planar and spherical geometries, with red and blue laser light, at high and low intensities, and at high and low accelerations, all seem to nearly follow a consistent law γ=0.9√kg − 3kva, when va is defined as the mass ablation rate divided by the peak density, in agreement with the eigenvalue analysis of Takabe and co‐workers [Phys. Fluids 26, 2299 (1983); 28, 3676 (1985)].

Observation of Rayleigh-Taylor Growth to Short Wavelengths on Nike

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.

Computational modeling of direct-drive fusion pellets and KrF-driven foil experiments

FAST is a radiation transport hydrodynamics code that simulates laser matter interactions of relevance to direct-drive laser fusion target design. FAST solves the Euler equations of compressible flow using the Flux-Corrected Transport finite volume method. The advection algorithm provides accurate computation of flows from nearly incompressible vortical flows to those that are highly compressible and dominated by strong pressure and density gradients. In this paper we describe the numerical techniques and physics packages. FAST has also been benchmarked with Nike laser facility experiments in which linearly perturbed, low adiabat planar plastic targets are ablatively accelerated to velocities approaching 107 cm/s. Over a range of perturbation wavelengths, the code results agree with the measured Rayleigh–Taylor growth from the linear through the deeply nonlinear regimes. FAST has been applied to the two-dimensional spherical simulation design to provide surface finish and laser bandwidth tolerances for a ...

Saturation of perturbation growth in ablatively driven planar laser targets

Saturation of the mass variation growth during the shock transit time, theoretically predicted for the surface roughness case by Ishizaki and Nishihara [Phys. Rev. Lett. 78, 1920 (1997)] and for the laser imprint case by Taylor et al. [Phys. Rev. Lett. 79, 1861 (1997)], is studied analytically and numerically. The saturation is demonstrated to be essentially the same effect in both cases, caused by the stabilizing action of mass ablation. Scalings of saturation time and saturation level for the two cases are related. For lower-density foam targets, the peak level of mass variation is proportional approximately to ρ01/2 and exactly to ρ0 for the cases of laser imprint and surface roughness, respectively.

Measurements of laser-imprinted perturbations and Rayleigh–Taylor growth with the Nike KrF laser

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 A rms randomly rough surface finish driven by a completely unif...

Richtmyer–Meshkov-like instabilities and early-time perturbation growth in laser targets and Z-pinch loads

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 R...

Three-dimensional multimode simulations of the ablative Rayleigh--Taylor instability

Multimode simulations of the evolution of the laser‐driven, ablative Rayleigh–Taylor instability on planar, plastic targets are performed in three dimensions, with FAST3D–CM. The initial mass density target perturbations are random, with a power law dependence of k−2, a RMS surface finish of 0.1 μm, and perturbation wave numbers ranging from 2π/dmax to √2×(12π/dmax), for dmax=128 μm. At early nonlinear times, the perturbations grow to tile the target with approximately hexagonal bubbles that are of the shortest, initially seeded wavelengths not stabilized by density gradients. This tiling occurs on a time scale that is comparable to the eddy turnover time of the dominant bubble wavelength. When the target thickness is large compared to the dominant, short wavelengths, the bubbles continue to burn into the target and to evolve to progressively longer spatial scales. Predictions from second‐order mode coupling and saturation models are found to be consistent with the simulation results.

The effect of shape in the three-dimensional ablative Rayleigh-Taylor instability. I: Single-mode perturbations

The nonlinear saturation amplitudes attained by Rayleigh–Taylor perturbations growing on ablatively stabilized laser fusion targets are crucial in determining the survival time of those targets. For a given set of baseline simulation parameters, the peak amplitude is found to be a progressive function of cross‐sectional perturbation shape as well as of wave number, with three‐dimensional (3‐D) square modes and two‐dimensional (2‐D) axisymmetric bubbles saturating later, and at higher amplitudes than two‐dimensional planar modes. In late nonlinear times hydrodynamic evolution diverges; the 3‐D square mode bubble continues to widen, while the 2‐D axisymmetric bubble fills in.