John H. Gardner

Direct-Drive Laser Fusion; Status and Prospects

Techniques have been developed to improve the uniformity of the laser focal profile, to reduce the ablative Rayleigh–Taylor instability, and to suppress the various laser–plasma instabilities. There are now three direct-drive ignition target designs that utilize these techniques. An evaluation of these designs is still ongoing. Some of them may achieve the gains above 100 that are necessary for a fusion reactor. Two laser systems have been proposed that may meet all of the requirements for a fusion reactor.

Steady-state planar ablative flow

Steady‐state planar ablative flow in a laser produced plasma is studied. The calculations relate all steady‐state fluid quantities to only three parameters, the material, absorbed irradiance, and laser wavelength. The fluid is divided into three regions; the subcritical expanding plasma, the steady‐state ablation front, and the accelerated slab. Boundary conditions at the interfaces of these regions are given. If the absorbed irradiance is nonuniform, the nonuniformity in ablation pressure is calculated. Results are compared with experiment and fluid simulation for both uniform and nonuniform illumination.

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.

Chemical-acoustic interactions in combustion systems

Abstract : A review is presented of chemical-acoustic coupling in terms of its role as a basic interaction which can alter the behavior of combustion systems. Effects resulting from this interaction include sound amplification, changes in sound speed and frequency, sound-induced changes in reaction rates, and acoustic stimulation of chemical oscillation and instabilities. Such effects are important in a variety of problems including combustion instability in jet and rocket engines, the structure of propagating detonations, and turbulence in chemically reacting flows. Background material is presented starting with the early work Lord Raleigh and continuing into a discussion of the relevant properties of sound waves. This leads to a discussion of the influence of energy release on sound waves and the influence of sound waves on chemical reactions. The conclusion is a discussion of chemical-acoustic coupling in combustion environments.

Characteristics of ablation plasma from planar, laser-driven targets

The momentum, energy, and velocity characteristics of plasma ablating from planar targets irradiated by long Nd‐laser pulses (4 ns,<1014 W/cm2) are measured and the dependence of ablation parameters upon absorbed irradiance is determined. Large laser spots are used in these experiments so that the results are not sensitive to boundary effects.

Evidence in the second‐harmonic emission for self‐focusing of a laser pulse in a plasma

Short‐pulse (300 psec), high‐intensity (1014−1015 W/cm2) Nd‐laser light was propagated into variable scale length plasmas (Ln≡n/∇n=200–400 μm at 0.1 critical density) preformed by long‐pulse (4 nsec), low‐intensity (≂6×1012 W/cm2) irradiation of planar targets. For high short‐pulse intensities (≥5×1014 W/cm2), time‐integrated images show filament‐shaped regions of second‐harmonic (2ω0) emission from the low density (0.01≤ne/nc≤0.2) region of the ablation plasma. Two‐dimensional computer calculations of the hyrodynamics and laser beam propagation indicate that these filaments are consistent with ponderomotive self‐focusing of the short pulse. A theoretical model that explains the 2ω0 generation mechanism within low‐density filaments is also presented.

Ablative acceleration of planar targets to high velocities

Laser irradiated targets are ablatively accelerated to velocities near those required for fusion pellet implosions while remaining relatively cool and uniform. The target velocities and velocity profiles are measured using a double-foil method, which is described in detail. Also, the ablation plasma flow from the target surface is spatially resolved, and the scalings with absorbed irradiance of the ablation pressure, ablation velocity, and mass ablation rate are determined. Results are compared with hydrodynamic code calculations.

Hydrodynamic target response to an induced spatial incoherence‐smoothed laser beam

One of the critical elements for high‐gain target designs is the high degree of symmetry that must be maintained in the implosion process. The induced spatial incoherence (ISI) concept has some promise for reducing ablation pressure nonuniformities to ≊1%. The ISI method produces a spatial irradiance profile that undergoes large random fluctuations on picosecond time scales but is smooth on long time scales. The ability of the ISI method to produce a nearly uniform ablation pressure is contingent on both temporal smoothing and thermal diffusion. In the start‐up phase of a shaped reactorlike laser pulse, the target is directly illuminated by the laser light and thermal diffusion is not effective at smoothing residual nonuniformities in the laser beam. During this period in the laser pulse, the target response is dominated by the initial shock generated by the laser pulse and the results indicate that this first shock can be the determining factor in the success or failure of the implosion process. The resu...

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.