Roger A. Kopp

Computational study of laser imprint mitigation in foam-buffered inertial confinement fusion targets

Recent experiments have shown that low density foam layers can significantly mitigate the perturbing effects of beam nonuniformities affecting the acceleration of thin shells. This problem is studied parametrically with two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Foam-buffered targets are employed, consisting typically of 250 A of gold, and 50 μm of 50 mg/cm3 C10H8O4 foam attached to a 10 μm foil. In simulation these were characteristically exposed to 1.2 ns, flat-topped green light pulses at 1.4×1014 W/cm2 intensity, bearing 30 μm lateral perturbations of up to 60% variation in intensity. Without the buffer layers the foils were severely disrupted by 1 ns. With buffering only minimal distortion was manifest at 3 ns. The smoothing is shown to derive principally from the high thermal conductivity of the heated foam. The simulation results imply that (1) the foam thickness should exceed the disturbance wavelength; (2) intensities exceeding ...

Foam-buffered spherical implosions at 527 nm

Creation of a low density, high temperature plasma buffer between the absorption and ablation layers of a directly driven inertial confinement fusion implosion capsule has been proposed as a means to reduce “early time” imprint from laser nonuniformities. This thermal smoothing blanket might be created from a low density foam layer wrapped around the deuterium–tritium filled microballoon. Preliminary spherical implosion tests of this concept using a polystyrene foam layer surrounding a glass microballoon were performed at the Nova laser [Rev. Sci. Instrum. 57, 2101 (1986)], using a 527 nm drive wavelength. Comparison of capsule yield and imploded core symmetry showed promising improvements in overall target performance, relative to one-dimensional undegraded hydrodynamic simulations, when the foam-buffer layer was present.

Laser‐Launched Flyer Plates and Direct Laser Shocks for Dynamic Material Property Measurements

The Trident laser at Los Alamos was used to impart known and controlled shocks in various materials by launching flyer plates or by irradiating the sample directly. Materials investigated include copper, gold, NiTi, SS316, and other metals and alloys. Tensile spall strength, elastic‐plastic transition, phase boundaries, and equation of state can be determined with small samples. Using thin samples (0.1 – 1.0 mm thick) as targets, high pressure gradients can be generated with relatively low pressures, resulting in high tensile strain rates (105 to 108 s−1). Free surface and interface velocities are recorded with point‐ and line‐imaging VISARs. The flexible spatial and temporal pulse profiles of Trident, coupled with the use of laser‐launched flyer plates, provides capabilities which complement experiments conducted using gas guns and tensile bars.

Transient x-ray diffraction and its application to materials science and x-ray optics

Time resolved x-ray diffraction and scattering have been applied to the measurement of a wide variety of physical phenomena from chemical reactions to shock wave physics. Interest in this method has heightened in recent years with the advent of versatile, high power, pulsed x-ray sources utilizing laser plasmas, electron beams and other methods. In this article, we will describe some of the fundamentals involved in time resolved x-ray diffraction, review some of the history of its development, and describe some recent progress in the field. In this article we will emphasize the use of laser-plasma as the x-ray source for transient diffraction.

Analysis of Laser‐Driven Shocks in Confined and Unconfined Geometries

Pulsed lasers are convenient generators of shocks in materials. The efficacy of laser shock generation depends on several factors including laser‐target coupling, laser pulse temporal shape and intensity, and the resultant pressure profile generated at the target surface. Target coupling is a dynamic mechanism that changes as the target surface evolves due to heating, ionization and ablation during laser irradiation. Confining a metal target surface using a transparent “tamper” material can increase the impulse transferred to the target, but also can cause decoupling of the laser energy from the target as the heated tamper begins to move the laser absorption region away from the metal surface. We have analyzed this process using the radiation hydrodynamics code Lasnex in an attempt to simulate experiments using planar targets and determine the coupling efficiency versus tamper material properties. Good agreement with experimental measurements of the shock pressure in Al and PMMA were obtain using laser ab...

Laser/matter interaction at intensities of 1012 W/cm2 and below

For single pulsed laser-matter interactions at sufficiently high intensity, the electron density in the ablated vapor is large enough to absorb the laser radiation before it can reach the dense target material. The resulting interaction can be described in terms of energy flows: laser energy is absorbed in the plasma in front of the target and reappears as thermal electron energy and secondary radiation, part of which impinges upon and heats the dense target material at the dense material-vapor interface. This heating in turn drives ablation, thereby providing a selfconsistent mass source for the laser absorption, energy conversion, and transmission. Under typical conditions of laser intensity, pulse width and spot size, the flow patterns can be strongly two-dimensional. We have modified the inertial confinement fusion code LASNEX to simulate gaseous and some dense material aspects for the relatively low intensity, long pulse-length conditions of interest in many laser-related applications. The unique aspect of our treatment consists of an ablation model which defines a dense material-vapor interface and then calculates the mass flow across this interface. The model, at present, treats the dense material as a rigid, two-dimensional simulational mass and heat reservoir, suppressing all hydrodynamical motion in the dense material. The modeling is being developed and refined through simulation of experiments, as well as through the investigation of internal inconsistencies, and some simulation of model problems. The computer simulations and additional post-processors provide a wealth of predictions for possible measurements, including impulse given to the target, pressures at the target interface, electron temperatures and densities, and ion densities in the vapor-plasma plume region, transmission and emission of radiation along chords through the plume, total mass ablation from the target and burn-through of the target material at selected radial locations. We will present an analysis of some relatively well-diagnosed experimental behavior which has been useful in development of our modeling.

Hydrodynamic coupling of a speckled laser beam to an axially flowing plasma

A plasma with a background flow along the direction of propagation of a laser beam is examined. The laser beam is subject to the beam-smoothing properties of a random phase plate. A simple model for the speckled properties of the laser beam is employed to show that the combined effect of the ponderomotive pressure of both the incident beam and stimulated Brillouin scattering can significantly perturb the hydrodynamics of the plasma.

Dynamic materials evaluation by confined plasma ablation and laser-generated shocks

Laser-generated shocks can and have been used to study their effects on single crystal materials during shock compression. While a crystal undergoes shock compression and release, the transient x- ray diffraction (TXD) of the Bragg and Laue signals is indicative of the change in the crystal lattice spacing. The lattice spacing directly relates to the strain in the crystal. From the dynamic lattice data, strain, strain rate, and/or phase change in a material may be determined. Confined ablation plasmas can efficiently launch a flyer plate for direct impact on a target material imparting a well-characterized shock input and generate kilobar to megabar pressure pulses over a wide range of pulse duration (= 20 ns). The laser-launched flyer plates are analogous to those launched by gas guns, but the smaller size provides an experimental method not easily accessible by larger gas gun experiments. With lasers, diagnostic equipment can be easily synchronized to study dynamic material parameters, i.e., single crystal shock dynamics, interfacial bond strengths of thin coatings, grain-interfaces, texture, and high strain rates (106 - 109 sec-1).

Foam-buffered laser-matter interactions

Recent experiments indicate that low-density foam buffer layers can significantly mitigate the perturbing effects of beam non-uniformities in direct drive laser-matter interactions. Results of a computational study with a 2D ALE code are reported here. Typical targets consisted of 50 {micro}m of 50 mg/cm{sup 3} C{sub 10}H{sub 8}O{sub 4} foam attached to a 10 {micro}m foil and covered with 250 {angstrom} of gold. These targets were exposed to {approximately} 1.2 ns, flat topped, green light pulses at {approximately} 1.4 {times} 10{sup 14} W/cm{sup 2} intensity, bearing 30 {micro}m lateral perturbations. Without the buffer layers the foils were severely disrupted after 1 ns of laser illumination. Buffering could provide stability for more than 2 ns of full shell acceleration. This study shows that the high thermal conductivity of the foam results in flattened shocks in the foam plasma, communicating a smoothed laser drive to the accelerated shells. Preheat from the gold hastens conversion of solid foam to the smoothing heated plasma.

High-speed optical and x-ray methods for evaluating laser-generated shock waves in materials and the corresponding dynamic material response

Optical diagnostic techniques including interferometry, electronic streak photography, and transient x-ray diffraction are used to study the dynamic material response to shock loading by direct laser irradiation and impact by laser- launched plates. The Los Alamos Trident laser is one of several lasers that have been used to generate shocks of 10 Kbar to several Mbar in single crystal and polycrystalline materials. Incorporating optical velocity interferometry (line-VISAR and point-VISAR) with transient x-ray diffraction can provide a complete understanding of the dynamic material response to shock compression and release. Laser-launched flyer plates provide an ideal method to generate one- dimensional shocks in materials. The quality of the one- dimensionality of the launch and acceleration of plates is evaluated by line-imaging VISAR. The line-imaging VISAR images the fringes along a line across the diameter of the plate. Each fringe maxima and minima provide acceleration and velocity information at the specific point on the plate. By varying the fringe constant, number of fringes and fringe spacing on the plate, detailed experimental data can be obtained. For our experiments, most plates are 3-mm diameter and accelerated to 0.2 - > 6 km/sec.