A promising new tool in shock wave physics is the generation of shock waves in test materials through the impact of small, laser-accelerated discs ("flyers"). In order to achieve the necessary one-dimensional condition of uniaxial strain in the shockloaded material, it is vital that flyers maintain a nearly planar geometry during the acceleration and impact processes. The geometry of the flyer is significantly influenced by the spatial intensity profile of the driving laser beam. With the goal of achieving a nearly uniform drive intensity for this application, we have evaluated a diffractive, microlens-array beam shaper for use with a high-energy, Nd:Glass laser driver. Based on the near-field spatial profile of this multimode laser, a 30-mmdiameter array containing multiple hexagonal diffractive lenslets was designed and fabricated. In combination with a primary integrator lens of 76.2-mm focal length, this optical element was intended to produce a uniform intensity distribution over a 2-mm-diameter spot at the focal plane of the primary lens. Beam profiling studies were performed to determine the performance of this optical assembly. At the focal plane of the primary lens, the beam shaping optics generated a reasonably uniform profile over a large portion of the focused beam area. However, a small amount of undiffracted light resulted in a high-intensity, on-axis spike. A beam profile approaching the desired "top hat" geometry could be obtained by moving the flyer launch plane a few mm inside or outside of the focal plane. The planarity of flyers generated using this optical assembly was evaluated using a line-imaging, optically recording velocity interferometer system (ORVIS). Results of these measurements demonstrate the deleterious effect of the on-axis spike on flyer planarity. Acceptable conditions for useful flyer impact experiments can be obtained by operating at a position that provides a near-top-hat profile.
This report summarizes a multiyear effort to establish a new capability for determining dynamic material properties. By utilizing a significant reduction in experimental length and time scales, this new capability addresses both the high per-experiment costs of current methods and the inability of these methods to characterize materials having very small dimensions. Possible applications include bulk-processed materials with minimal dimensions, very scarce or hazardous materials, and materials that can only be made with microscale dimensions. Based on earlier work to develop laser-based techniques for detonating explosives, the current study examined the laser acceleration, or photonic driving, of small metal discs ("flyers") that can generate controlled, planar shock waves in test materials upon impact. Sub-nanosecond interferometric diagnostics were developed previously to examine the motion and impact of laser-driven flyers. To address a broad range of materials and stress states, photonic driving levels must be scaled up considerably from the levels used in earlier studies. Higher driving 3 levels, however, increase concerns over laser-induced damage in optics and excessive heating of laser-accelerated materials. Sufficiently high levels require custom beam-shaping optics to ensure planar acceleration of flyers.The present study involved the development and evaluation of photonic driving systems at two driving levels, numerical simulations of flyer acceleration and impact using the CTH hydrodynamics code, design and fabrication of launch assemblies, improvements in diagnostic instrumentation, and validation experiments on both bulk and thin-film materials having wellestablished shock properties. The primary conclusion is that photonic driving techniques are viable additions to the methods currently used to obtain dynamic material properties.Improvements in launch conditions and diagnostics can certainly be made, but the main challenge to future applications will be the successful design and fabrication of test assemblies for materials of interest.
Motivated by interest in optical firing systems for initiating explosives, laser-induced damage thresholds have been investigated in step-index, multimode fibers having pure fused silica cores. A compact, multimode Nd/YAG laser operated at a pulsewidth of 16 ns was used for the experiments. The focusing geometry for introducing the beam into the fiber was chosen to avoid damage along the core/cladding interface as observed in previous studies. Five lots of twenty fibers each were tested, with polishing steps varied between successive lots to produce improved fmishes on the fiber end surfaces. Each fiber was subjected to a sequence of progressively increasing energy densities up to a value more than 80 J/cm2. Initial damage was monitored by observing scattered HeNe laser light from the fiber faces using magnified video cameras.The majority of the fibers damaged initially at the rear fiber face, once a "laser conditioning" process at the front fiber face was completed. In this process, a visible plasma was generated at the front face for one or more laser shots. Rather than produce progressive damage at the front surface, this process apparently improved the surface finish in nearly all cases, resulting in improved resistance to damage. Other modes of damage along the fiber length were observed either at locations of handling stresses or at the location of highest static tensile stress corresponding to the fiber's minimum bend radius. A fraction of the fibers in each lot never damaged, and this fraction increased as the successive lots were tested. The fibers that did not damage in the final lot were subjected to an additional series of laser shots at the maximum input conditions, and the majority still did not damage. These results, together with some insights into the dominant damage processes suggested by previous studies, are encouraging in terms of realizing high damage thresholds in this type of optical fiber. However, several important areas for further study are indicated.
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