Mark H. Emery

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...

Nonlinear aspects of hydrodynamic instabilities in laser ablation

We report on our investigation of the Rayleigh–Taylor and Kelvin–Helmholtz (KH) instabilities in laser ablatively accelerated targets for a series of single mode perturbations. We find linear growth rates well below classical values and a cutoff in the growth rates for wavelengths less than the foil thickness. However, the striking result is the dominance of nonlinear effects, i.e., the KH rollup, for the short wavelength perturbations. Although the linear growth rate increases as k1/2 up to the cutoff, the KH rollup dominates at large k, drastically reducing the penetration rate of the dense spike below its free fall value in the nonlinear phase and effectively doubling the aspect ratio before foil breakup.

Density and temperature profiles within laser-produced plasmas in the classical-transport regime

Using planar polystyrene targets with embedded Al tracer dots, improved x‐ray spectroscopic measurements of density and temperature profiles in laser‐produced plasmas were obtained. Time‐integrated, spatially resolved spectra of the Al tracer emission were collected at laser intensities where the energy absorption and transport is expected to be classical. The plasma density and temperature profiles were determined by comparing the observed x‐ray line intensities with collisional‐radiative equilibrium calculations. Plasma profiles were obtained for the region blowoff plasma with densities between one‐tenth and twice critical for the 1.05 μ laser excitation. Near agreement is found between the measured density and temperature profiles and 2‐D (cylindrical) hydrodynamic calculations with energy absorption via classical inverse bremsstrahlung and energy transport via classical thermal conduction.

The Rayleigh–Taylor instability in ablatively accelerated targets with 1, 1/2 , and 1/4 μm laser light

The results of a series of detailed numerical simulations of the Rayleigh–Taylor instability in laser ablatively accelerated targets are presented for a fairly wide range of initial conditions. It is shown that the Rayleigh–Taylor growth rate in an ablative environment is a strong function of the laser wavelength. For perturbation wavelengths about three times the in‐flight target thickness, the ratios of the numerical growth rates to the classical growth rates are of the order of 1/1.5, 1/2.5, and 1/3.5 for 1, 1/2 , and 1/4 μm laser light, respectively. The numerical results are in good agreement with the theoretical model presented here based on the ablative convection of vorticity away from the unstable ablation front. These results provide strong evidence for the viability of high‐aspect‐ratio shells in direct‐drive laser fusion.

Vortex shedding due to laser ablation

An investigation of laser‐driven targets shows the generation and subsequent shedding of strong vortex structures from the ablation layer whenever the density and pressure profiles become noncollinear. It is shown here that vortex shedding explains the inhibited linear Rayleigh–Taylor growth, the short‐wavelength cutoff, dynamic stabilization and clarifies the role of the Kelvin–Helmholtz roll‐up.

The Rayleigh-Taylor instability in direct-drive laser fusion

Achieving high energy gain in laser fusion will require the use of moderately-thin pellet shells. But thinner shells are expected to be more susceptible to break-up from the Rayleigh-Taylor instability. Calculations and experimental data thus far indicate that the growth rate of the Rayleigh-Taylor instability is reduced well below the classical value when the surface of the pellet shell is directly ablated by laser light. In addition, the mix of the cold DT shell with the hot ignitor may only slightly degrade the pellet yield. It may therefore be possible to successfully implode these moderately thin shells and produce gains of a few hundred, using a few-megajoule KrF laser driver.

New Measurement Techniques Using Tracers Within Laser-Produced Plasmas

The use of locally embedded tracers within laser-irradiated solid targets has led to a new class of diagnostic methods for laser-produced plasmas. Demonstrated uses of tracers include the first visualizations of hydrodynamic flow of laser-ablated materials and improved spectroscopic measurements of plasma density and temperature profiles; comparisons with a two-dimensional hydrodynamics computer code are shown. Proposed future uses of tracers include the first measurements of fluid velocity profiles and improved determinations of mass ablation rates.

Simulation of the Rayleigh–Taylor instability in colliding, ablatively driven laser foils

This paper reports results from a series of numerical simulations of a pair of independently accelerated rectilinear foils in the presence of the ablative Rayleigh–Taylor (RT) instability on the laser‐side surfaces. The foil thickness and laser intensity are chosen to be in the range relevant to high‐gain inertial fusion pellets, with 80 μm thick plastic (CH) foils accelerated toward each other from a separation distance of 650 μm by a 1/4 μm laser beam with an intensity of 3×1014 W/cm2. At early times the foils are physically well separated from one another, and evolve independently in a way that is fully consistent with the previously studied evolution of ablatively RT unstable planar targets [Gardner et al., Phys. Fluids B 3, 1070 (1991)]. Subsequently, pressure builds up in the region between the foils, causing them to decelerate. This stabilizes the RT growth on the laser sides, while driving the RT instability on the inner sides. For thin foils, laser‐side RT bubbles become rapidly growing inner sur...

Analysis of Stability and Symmetry Implications for ICF

Pellet gains in excess of 100 will probably be necessary for most applications of inertial fusion.1 In order to achieve these high gains a number of critical physics elements must be controlled. These include (1) high coupling efficiency, (2) low fuel preheat, (3) implosion symmetry, (4) implosion stability (ablation pressure) and (5) an ignition scheme. These factors interact with each other providing conflicting requirements. In particular the first two items are directly in conflict with the second two. For example, high coupling efficiency and low fuel preheat requires control of deleterious plasma instabilities. These instabilities generally scale as Iλ2. Therefore they are usually controlled by the use of lower intensities or shorter laser wavelength. But lower laser intensities are generally associated with thinner shell or double shell target designs,2 and these higher aspect ratio designs place severe requirements on laser symmetry and target stability. Smoothing out laser nonuniformities by lateral thermal conduction3 requires that the absorption-to-ablation distance be on the order of the target radius, leading to longer, not shorter wavelengths. This separation distance also produces a lower hydro-dynamic efficiency.

LASER DRIVEN SHOCK INSTABILITIES IN MULTIMATERIAL, LAYERED, SOLID TARGETS*

The ability of recently developed laser smoothing techniques to produce nearly uniform pressures on the target surface is strongly contingent on the degree of thermal smoothing. Thermal smoothing is not effective during the low intensity portion of a laser pulse, and the resultant shock structure mirrors the residual laser asymmetries. We present simulation results of the interaction of uniform/nonuniform laser generated shocks on planar, layered targets.