R. P. Johnson

Laser-induced shock waves in condensed matter: some techniques and applications

Laser-induced shock waves in condensed matter have important applications in dynamic material studies and high pressure physics. We briefly review some techniques in laser-induced shock waves, including direct laser drive, laser-launched flyer plate, quasi-isentropic loading, point and line imaging velocity interferometry, transient X-ray diffraction, spectroscopy and shock recovery, and their applications to study of equation of state, spallation, and phase transitions.

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.

Diagnostics for confined plasma ablation for plate launch and shock generation

Confined plasma ablation is an efficient method to accelerate 1-D metal plates, with the impact of the plate resulting in a well-defined shock being generated in a target material. By using confined plasma to accelerate a plate, some details of the laser parameters are decoupled from the plate impact. Several types of experiments and related diagnostics to evaluate the performance parameters of the laser beam, flyer plate acceleration, and plate conditions are described. Several experiments using the flyer plates to generate shocks in materials to determine pressure-velocity relations, and dynamic spall strength of various metals are presented.

Novel Techniques for Laser‐Irradiation Driven, Dynamic Materials Experiments

Dynamic loading experiments are described using nanosecond scale laser pulses of 2 to 1000 GW/cm2 over a region 5 mm in diameter. The laser irradiance was tailored to generate shocks or quasi‐isentropic compression. The experiments include novel diagnostic techniques with high temporal resolution: transient x‐ray diffraction (TXD) and polarization‐dependent reflectivity (ellipsometry). TXD uses a laser‐produced plasma to form an x‐ray source. These x rays are collimated by a pinhole to form a Bragg scattering source, which allowed powder lines to be detected from polycrystalline samples such as beryllium foils. Ellipsometry has been demonstrated with 50 ps resolution using the reflectance of a pulsed 660 nm laser from silicon and tin samples through lithium fluoride windows. Ellipsometry can indicate phase changes and potentially yields estimates of surface temperature via the dielectric conductivity. The combination of TXD and ellipsometry with VISAR measurements provides precision characterization of dy...

EFFECTS OF PROCESSING TECHNIQUES ON THE SHOCK RESPONSE OF BE

Microstructural effects including material anisotropy, impurities, grain size, and texture alter a materials response to dynamic loading through wave front dispersion and inelastic processes. The spatial variations created by these effects are a challenge for inertial confinement fusion (ICF) as they may seed instabilities, which could reduce thermonuclear yield, if not controlled through material processing. To this end, laser‐driven confined shock experiments have been conducted on Be to characterize its dynamic strength properties and usefulness as an ICF ablator. Disks of Be 3 mm in diameter and 100 to 250 microns thick in the form of single crystal, rolled foil, and equal channel angular extruded were dynamically loaded to 100s kbar while the material behavior was measured with in‐situ diagnostics. Clear two‐wave structures were observed in free surface velocity records, providing a comparison of flow stress and other dynamic properties between Be types.

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.