R. Jeffery Lawrence

Theoretical analysis of a pulsed-laser-driven hypervelocity flyer launcher☆

Abstract High-power but low-total-energy pulsed lasers can be used to accelerate small-diameter, thin flyers to velocities in excess of several kilometers per second. The geometry under consideration involves placing the flyer on the end of an optical fiber through which the laser pulse is delivered. The blowoff products driving the flyer are thus fully tamped. a model, based on the Gurney theory for explosively driven plates, is derived for predicting the final velocity of these flyers. All but two of the required input parameters are readily available; those two can be extracted from one limited set of experimental measurements. Data on aluminum flyers illustrate that once the input parameters have been determined, the model predicts changes resulting from variations of laser fluence and pulse duration as well as flyer thickness and diameter. Additional data on copper and magnesium indicate that the energy-coupling efficiency can vary by at least 50%, depending on the flyer material.

Enhanced momentum transfer from hypervelocity particle impacts

Abstract When hypervelocity particles impact thick targets, the momentum generated in the target can be substantially greater than that associated with the incident projectiles. Adapting an existing model for vaporization-induced impulse generation, a simple model and various scaling laws are derived for this type of momentum enhancement. The model is compared to limited experimental data and is contrasted with other theoretical predictions. The sensitivity to uncertainties in the various input parameters is determined, and the variation of the momentum coupling efficiency is examined. To emphasize the importance of the enhancement phenomenon, illustrative system-level requirements for delivering specific target loads are calculated.

A simple model for the optimazation of stand-off hypervelocity particle shields

Abstract Impacts at extreme velocities of many tens or even hundreds of kilometers per second are essentially inaccessible to experiment; hence, theory and the resulting scaling laws are the only means to describe the relevant phenomena. One area amenable to this approach involves analyzing the efficacy of stand-off shields for protection against hypervelocity particles. A simple model that allows for optimal design of these shields is developed. Examples and scaling laws that provide estimates for the minimum requirements for such shields are derived from the model.

Plasma-driven Z-pinch X-ray loading and momentum coupling in meteorite and planetary materials

X-ray momentum coupling coefficients, C M , were determined by measuring stress waveforms in planetary materials subjected to impulsive radiation loading from the Sandia National Laboratories Z-machine. Velocity interferometry (VISAR) diagnostics provided equation-of-state data. Targets were iron and stone meteorites, magnesium-rich olivine (dunite) solid and powder (~5–300 μm), and Si, Al, and Fe calibration targets. Samples were ~1-mm thick and, except for Si, backed by LiF single-crystal windows. X-ray spectra combined thermal radiation (blackbody 170–237 eV) and line emissions from pinch materials (Cu, Ni, Al, or stainless steel). Target fluences of 0.4–1.7 kJ/cm 2 at intensities of 43–260GW/cm 2 produced plasma pressures of 2.6–12.4 GPa. The short (~5 ns) drive pulses gave rise to attenuating stress waves in the samples. The attenuating wave impulse is constant, allowing accurate C M measurements from rear-surface motion. C M was 1.9 − 3.1 × 10 −5 s/m for stony meteorites, 2.7 and 0.5 × 10 −5 s/m for solid and powdered dunite, 0.8 − 1.4 × 10 −5 s/m for iron meteorites, and 0.3, 1.8, and 2.7 × 10 −5 s/m respectively for Si, Fe, and Al calibration targets. Results are consistent with geometric scaling from recent laser hohlraum measurements. CTH hydrocode modeling of X-ray coupling to porous silica corroborated experimental measurements and supported extrapolations to other materials. CTH-modeled C M for porous materials was low and consistent with experimental results. Analytic modeling (BBAY) of X-ray radiation-induced momentum coupling to selected materials was also performed, often producing higher C M values than experimental results. Reasons for the higher values include neglect of solid ejecta mechanisms, turbulent mixing of heterogeneous phases, variances in heats of melt/vaporization, sample inhomogeneities, wave interactions at the sample/window boundary, and finite sample/window sizes. The measurements validate application of C M to (inhomogeneous) planetary materials from high-intensity soft X-ray radiation.

THE EQUIVALENCE OF SIMPLE MODELS FOR RADIATION-INDUCED IMPULSE

A number of models that predict the blowoff impulse generated in solid targets by short high-intensity radiation loads are described. It is shown that the impulse is insensitive to the details of the energy deposition and interaction processes. Thus with the proper nondimensionalization and normalization, all the models are shown to be very nearly equivalent.

NUMERICAL AND ANALYTICAL ANALYSIS OF THIN LASER-DRIVEN FLYER PLATES

When a short high-intensity laser pulse is transmitted through a small diameter optical fiber onto a thin metallic foil placed on the fiber end, the inner surface of the foil is explosively vaporized and the bulk of the flyer material is driven off at high velocities. Experiments have been performed, in which the foil velocity has been monitored with time, for a variety of energies, pulse lengths, and fiber diameters. We have compared the data with results of both analytical and computational investigations. The two-dimensional radiation hydrodynamics code, LASNEX, has been used to provide detailed predictions of the acceleration histories, taking into account the reflection, deposition, and re-radiation processes in the driving plasma. Careful comparison with data has increased our understanding of the governing processes. The late-time velocity of the flyer has been studied using a simple model employing global conservation of mass, momentum and energy in conjunction with the well-known Gurney theory for explosively driven flyer plates. Together, these techniques have allowed quite complete understanding of the phenomena, and it has been possible to match most of the features seen in the expanding body of experimental data.

Chapter 9 Memories of Shock Wave Research at Sandia

These individual recollections present a window into the personal experiences of people who participated in the shock wave research program at Sandia. We made a strong effort to contact and encourage as many people as possible to participate. Over 80 people were contacted and about 40 provided recollections of their personal experiences. Each contributor was asked to provide a summary of their role in shock wave research at Sandia, bringing out any interesting events or anecdotes that happened along the way

Chapter 8 Looking to the Future

The synopsis of shock wave science presented in this book describes the pioneering research conducted at Sandia over the past 60 years. The shock wave program was organized and conducted rather differently from that of similar research programs at other institutions. Two separate shock wave research efforts were established in the 1950s, one focused on scientific understanding of shock compression processes and the other on engineering applications.

Chapter 5 The 1980s: Heady Times

The previous two decades of shock wave research at Sandia led to (1) advances in experimental techniques, (2) measurements of dynamic material response for a wide range of materials, (3) state-of-the-art material models, and (4) a family of 1-D and 2-D computer codes that could simulate materials used in weapon components and subsystems with considerable accuracy. However, full three-dimensional (3-D) code capabilities were needed for higher fidelity simulations of weapon components and subsystems.

Chapter 6 The 1990s: Black Monday

The 1990s were turbulent times for shock wave research at Sandia because of the near elimination of experimental shock wave research, including experimental facilities. Three management decisions led to this challenging event. The first decision was implementation of a laboratory-wide restructuring of management in the early 1990s. As a consequence, all Sandia departments, including the second-level (i.e., the original department management level) ones that involved shock wave research managed by George Samara and Jim Asay, were dissolved. The first-level divisions (now renamed departments) that had been supervised by Samara and Asay became individual departments under the direct supervision of two different directors. In addition, Walt Herrmann stepped down as the Director of Engineering Sciences; that was the directorate in which Asay’s shock wave department had resided. The directorate was then eliminated, and the shock wave divisions that Asay had managed were transferred to Ed Barsis, who was the Director of the Computing Research Center. This resulted in a two-level management structure, with each director supervising the direct-reporting managers, as opposed to the previous situation in which three or four second-level managers reported to each director.