Publications

Abstract not provided.

Impact-flash phenomenology has been known for decades, and is now being considered for missile-defense applications, in particular for remote engagement diagnostics. To technically establish this capability, we have conducted a series of experiments at impact velocities of ∼6, ∼11, and ∼25 km/s. Two- and three-stage light-gas guns were used for the lower two velocities, and magnetically-driven flyers on the Sandia Z machine achieved the higher velocity. Spectrally- and temporally-resolved flash output addressed data reproducibility, material identification, and target configuration analysis. Usable data were obtained at visible and infrared wavelengths. Standard atomic spectral databases were used to identify strong lines from all principal materials used in the study. The data were unique to the individual materials over the wide range of velocities and conditions examined. The time-varying nature of the signals offered the potential for correlation of the measurements with various aspects of the target configuration. Integrating the records over wavelength helped to clarify those time variations. © 2006 American Institute of Physics.

Kill assessment continues to be a major problem for the nation's missile defense program. A potential approach for addressing this issue involves spectral and temporal analysis of the short-time impact flash that occurs when a kill vehicle intercepts and engages a target missile. This can provide identification of the materials involved in the impact event, which will, in turn, yield the data necessary for target identification, engagement analysis, and kill assessment. This report describes the first phases of a project under which we are providing laboratory demonstrations of the feasibility and effectiveness of this approach. We are using two major Sandia facilities, the Z-Pinch accelerator, and the two- and three-stage gas guns at the Shock Thermodynamics and Applied Research (STAR) facility. We have looked at the spectral content of impact flash at velocities up to 25 km/s on the Z-Pinch machine to establish the capability for spectroscopy for these types of events, and are looking at similar experiments at velocities from 6 to 11 km/s on the gas guns to demonstrate a similar capability for a variety of research-oriented and applied materials. The present report describes only the work performed on the Z machine.

Abstract not provided.

The Eulerian hydrocode, CTH, has been used to study the interaction of hypervelocity flyer plates with thin targets at velocities from 6 to 11 km/s. These penetrating impacts produce debris clouds that are subsequently allowed to stagnate against downstream witness plates. Velocity histories from this latter plate are used to infer the evolution and propagation of the debris cloud. This analysis, which is a companion to a parallel experimental effort, examined both numerical and physics-based issues. We conclude that numerical resolution and convergence are important in ways we had not anticipated. The calculated release from the extreme states generated by the initial impact shows discrepancies with related experimental observations, and indicates that even for well-known materials (e.g., aluminum), high-temperature failure criteria are not well understood, and that non-equilibrium or rate-dependent equations of state may be influencing the results.

The pulsed-power Z machine, in an isentropic compression experiment (ICE) mode, will allow the dynamic characterization of porous materials - here various ceramic powders, e.g., Al{sub 2}O{sub 3}, WC, ZrO{sub 2} - at roughly half their solid densities. A cylindrical configuration can provide megabar-level loads on an annulus of the sample material. Data will be provided by velocity interferometers that measure free-surface (or possibly interface) particle velocities. Differing sample thicknesses using stepped or conical geometries yield experimental efficiency by allowing multiple data records on single shots. With the p/{alpha} model for porous materials, the one-dimensional Lagrangian hydrocode WONDY provides the needed analyses. Based on static data, both power-law and quadratic crush curves are employed. Within the model constraints, we suggest that the most important parameter for characterizing the material is the crush strength, p{sub s}. With adequate sample thicknesses, the planned velocity measurements differentiate among the various assumptions for p{sub s}.

A systematic computational and experimental study is presented on impact generated debris resulting from record-high impact speeds recently achieved on the Sandia three-stage light-gas gun. In these experiments, a target plate of aluminum is impacted by a titanium-alloy flyer plate at speeds ranging from 6.5 to 11 km/s, producing pressures from 1 Mb to over 2.3 Mb, and temperatures as high as 15000 K (>1 eV). The aluminum plate is totally melted at stresses above 1.6 Mb. Upon release, the thermodynamic release isentropes will interact with the vapor dome. The amount of vapor generated in the debris cloud will depend on many factors such as the thickness of the aluminum plate, super-cooling, vaporization kinetics, the distance, and therefore time, over which the impact-generated debris is allowed to expand. To characterize the debris cloud, the velocity history produced by stagnation of the aluminum expansion products against a witness plate is measured using velocity interferometry. X-ray measurements of the debris cloud are also recorded prior to stagnation against an aluminum witness plate. Both radiographs and witness-plate velocity measurements suggest that the vaporization process is both time-dependent and heterogeneous when the material is released from shocked states around 230 GPa. Experiments suggest that the threshold for vaporization kinetics in aluminum should become significant when expanded from shocked states over 230 GPa. Numerical simulations are conducted to compare the measured x-ray radiographs of the debris cloud and the time-resolved experimental interferometer record with calculational results using the 3-D hydrodynamic wavecode, CTH. Results of these experiments and calculations are discussed in this paper.

Abstract not provided.

Abstract not provided.

Two experiments have been performed to measure the effects of pulsed radiation loads on the front of small tubular structures, using as an energy source the X-ray fluence produced by a Z-pinch at the Sandia National Laboratories Z Facility. The project had two major goals: to establish the feasibility of using the Z machine to study the phenomenology associated with debris generation and propagation down tubular structures with partitions; and to use the resultant experimental data to validate numerical hydrocodes (shock physics codes) so that we have confidence in their use in analyzing these types of situations. Two tubular aluminum structures (5 and 10 cm long and 1 cm inside diameter) were prepared, with aluminum partitions located at the front, halfway down the pipe, and at the rear. Interferometry (VISARs) provided multiple velocity histories for all of the partitions. In both experiments, the first barrier, which was exposed directly to the x-ray fluence, was launched into the pipe at a velocity of ∼2 km/s, accelerating to give a mean velocity of ∼ 2.6 km/s. Loss of plate integrity is inferred from the dispersed launch of the second partition at ∼1 km/s. Wall shocks propagating at 4.5 km/s were inferred. Post-test metallography showed evidence of melting and partial vaporization of the plates, and turbulent mixing with material from the walls. Calculations qualitatively agree with the observed results, but slightly overpredict debris velocity, possibly due to overestimates of total energy fluence. An application for this work is the study of techniques for line-of-sight shock and debris mitigation on high-power pulsed power facilities such as Z and its follow-on machines. © 2001 Elsevier Science Ltd. All rights reserved.

We generally use large-scale hydrocodes to study the dynamic response of targets to influence pulsed radiation loads. However, for many applications where the desired solution does not require a detailed specification of pressure- or velocity-time histories, there are simple analytic approaches that can yield surprisingly accurate results. Examples include determining either the final velocity of a radiation-driven flying plate or the impulse delivered to a structural element. These methods are all based on relatively straightforward use of conservation of mass and momentum, but they typically need one scaling-law parameter. In this context, short pulse means short compared to the characteristic time of the desired response, which allows for the phenomena to be essentially uncoupled. High fluence means that the input energy is great enough to yield vaporization or blowoff of one or more portions of the configuration. We discuss some of these methods, give examples, and suggest limitations and criteria for their use.

Short communication.

We describe a simple engineering model applicable to stand-off “Whipple bumper” shields, which are used to protect space-based assets from impacts by orbital debris particles. The model provides a framework for analyzing: 1) the parameter limits governing the penetration and breakup or decomposition of the hypervelocity debris particle; 2) the behavior of the induced debris cloud, including its velocity and divergence; and 3) the design and optimization of the stand-off shield for a specific threat and level of protection required. The model is normalized to actual stand-off debris shield experiments and multi-dimensional numerical simulations at impact velocities of ~10 km/s. The subsequent analysis of a current space station shield design suggests that: 1) for acceptable levels of protection, stand-off shields can be significantly thinner than previously thought; and 2) with the proper balance between shield thickness and stand-off distance, the total shield mass can be reduced substantially.

A model that predicts the final velocity of high-power, pulsed-laser-driven thin flyers is described. The required input parameters can either be obtained from standard handbooks or simply extracted from one set of data. The model yields a number of features and scaling laws that are well verified by experiment. Specific comparisons of model predictions with experimental results illustrate excellent agreement for variations of laser fluence and pulse width as well as flyer diameter and thickness.

The use of ground-based lasers to launch small payloads but large total masses into low-Earth orbit may prove to be the most innovative and potentially economical approach for accomplishing this important mission. Of the several possible schemes for laser propulsion, two are examined: (1) ablative momentum transfer using pulsed lasers; and (2) heat exchanger thrusters in conjunction with CW lasers. For an entry-level payload of {approximately}50 kg it is found that the former yields payload-to-power ratios < 0.5 kg/MW with a requirement for an average laser power of at least 100 MW, whereas the latter might yield 1 to 3 kg/MW with a laser power of several 10s of MW. One of the promising approaches that could yield a driver for such a system is the reactor-pumped laser FALCON, which scales to these power levels with the potential for long run times.

A number of models that predict the 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 known to be very nearly equivalent. 5 refs., 5 figs., 1 tab.

Stand-off hypervelocity particle shields offer potential weight savings of an order of magnitude or more over conventional homogeneous armors. Based on an earlier complete model for the design and optimization of a stand-of shield system, a more restricted model appropriate for retrofit shields is described. Procedures to minimize the shield mass are provided, and scaling laws for many of the important parameter relationships are illustrated. 6 refs., 5 figs.