Beams & Hydrodynamcis

The Beams and Hydrodynamics team is dedicated to conceiving, designing, and fielding state of the art physics experiments oriented towards increasing our understanding of weapons physics. Our team is composed of four highly skilled technicians, five professional staff, and two contractors who collaborate with organizations throughout the Laboratory on a variety of experiments such as: advanced radiography experiments at the Integrated Test Stand, laser based experiments, and hydrodynamic experiments at Trident, x-ray imaging at Pegasus, advanced imaging detector development, anomalous energy loss and advanced accelerator experiments at the High Brightness Sub-picosecond Accelerator, and other basic physics experiments.

Nevada Test Site

P-22 is deeply involved in protecting and archiving the volatile test data it took during more than three decades of underground nuclear testing at the Nevada Test Site (NTS). Our goal is to bring the group's data to a stable and readily accessible state. These data will be used to benchmark all future calculational tools. The archiving activities constitute a significant effort in P-22 and involve individuals responsible for the original execution of underground nuclear tests as well as trainees. Many of the numerical algorithms developed for analyzing the information from underground tests have been ported to modern computer platforms as part of our effort to preserve this valuable and unique data.

In addition, P-22 continues to participate in experiments performed underground at NTS, both to maintain our readiness to support a resumption of nuclear testing should the need arise, and to study the physics of weapons performance and materials (Fig. 3). These experiments increase our understanding of weapons science by allowing improvements in code calculations and in estimates of the severity of problems and changes occurring in the nuclear stockpile as it ages.

At present, we are supporting the Los Alamos Dynamic Experimentation (DX) Division on experiments to measure the properties of material ejected from shocked plutonium. These experimental efforts are discussed in detail in a research highlight in Chapter 2. By performing these experiments underground at NTS, the plutonium is handled and contained in a manner similar to that used for underground nuclear tests, maintaining the readiness training necessary to support the potential for future nuclear tests.

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High-Energy-Density Physics at Pegasus

The Pegasus Pulsed-Power Facility which fired its last shot in 1999, provided a unique capability for delivering strong, converging, shock-driven or adiabatically driven compressions with excellent diagnostics. Pegasus allowed physicists to gather important data on material behavior at high-energy-densities, which are necessary for weapons physics and basic science.

Our studies of the hydrodynamic flow of materials under extreme conditions are crucial to developing and testing weapons models. Our experiments focussed on instabilities at the interface between two materials of different densities.

Our studies of the properties of materials under extreme conditions included topics such as material failure through spall and ejecta, plastic deformations, strain and strain-rate effects, and interfacial friction. One significant series, carried out in collaboration with Livermore, focussed on spallation of shocked aluminum targets and the growth of instabilities.

We explored the electronic properties of materials in the presence of strong magnetic fields, and we collaborated with Russian scientists to study liner stability. In preparation for the future Atlas facility, we also conducted experiments on mechanical joints that can carry high current-densities.

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Instabilities in Taylor-Sedov Blast Waves

The stability of Taylor-Sedov blast waves in low-density gases was investigated. Theoretical results have shown that the stability of propagation in uniform gases depends on the adiabatic index of the gas. This was verified with a LASNEX simulation. Both stable and unstable propagation was observed in experiments at the Trident laser facility. The experimental verification of the adiabatic index criterion for stability is not yet completed.

Additional information (PDF-390Kb).

Bremsstrahlung Target Plasma Expansion Experiments

A critical issue for high resolution radiography is the integrity of the bremsstrahlung converter during the electron beam pulse and, for multiple pulse radiography, the spatial extent of the plasma plume for subsequent pulses. Energy loss calculations indicate that the bremsstrahlung converter on DARHT will be heated to ten's of eV by the electron beam. Expansion velocities of several cm/microsecond are possible which may lead to various instabilities in the propagation of the electron beam thereby resulting in degradation of the spot size. For multiple pulse radiography, in order to avoid transiting the plasma plume, subsequent pulses will have to be moved transversely on the converter. How far they must be moved is as yet unknown.

Initial experiments are underway on the Integrated Test Stand to measure expansion velocities and compare these measurements to 2-D hydrodynamic calculations. A streak camera and optical back lighter are used to determine the temporal evolution of axial expansion velocities. At present, experimentally measured expansion velocities are a factor of 3 higher than calculations would indicate. Efforts to resolve this discrepancy are underway and are critical to ensure confidence in calculations and predictions for conditions which will be reached at DARHT.

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