Stellar Physics in the Laboratory

Today, strangely enough, the field of laboratory astrophysics is being paced by large experiments intended to delve in the physics of nuclear weapons. The reason is that both energetic space-plasma phenomena and nuclear weapons belong to a class of science called high-energy-density physics. New experimental techniques, improved simulation codes, and experimental diagnostics developed for the US Department of Energy's program to keep the nation's arsenal of nuclear weapons safe, secure, and reliable in the absence of underground testing are indirectly benefiting our understanding of the universe.

The experiments are at the cutting edge to technology and physics and involve both intense laser beams and pulsed-power facilities. The most powerful laser project under construction is the National Ignition Facility, a $1.2 billion facility slated for completion in 2003, in which the beam from 196 lasers will deliver nearly 2 megajoules of power to a millimeter-sized target, reproducing the temperatures within stars.

Plasma astrophysics is currently being done with machines called pulsed-power generators, which currently operate in the 0.5-2 megajoules range and are located at Sandia National Laboratories and Los Alamos National Laboratory. Under construction is Atlas, a large pulsed power generator for studying instabilities in the various states of matter. Also mentioned is X-1, a 20-megajoule behemoth , so called after the bright source in the constellation Cygnus A. An appreciable fraction of machine usage for these large machines will be devoted to plasma astrophysics.

Pulsed-power generators are the most prolific sources of microwaves and X rays on Earth. They produce radiation through the interaction of filamentary plasmas created when megavolts of electrical energy discharge through centimeter-long arrays of wires. The energy pulses are so powerful that the wires are rapidly vaporized and ionized into

plasma filaments that release a full spectrum of electromagnetic energy.

In addition, the current coursing through the plasma of ionized wire induces the Z-pinch effect, in which a powerful current in a plasma in pinched into a filament by the magnetic field produced by the current. In the laboratory, the currents flow from energy stored in many capacitors, while in the universe, cells of differing types of plasmas are thought to play the role of the energy-storing capacitors. Thus, from laboratory astrophysics experiments we may infer the structure of the universe.

The X-ray spectrum of the universe (from 1 to 140 angstroms) allows an in-depth study and comparison of the spectra produced by laboratory Z pinches. In particular, X-ray spectroscopy, the study of the absorption and emission of X rays, can yield significant data on the chemical compositions, temperatures,, densities, and magnetization for the plasma universe. The launching of three X ray observatories started in 1999 will increase the number and resolution of X-ray charts of the plasma universe, helping to map regions of energy storage, areas of release, and the electrical currents required in transport.

—A. P. and G. C. S.

Filamentary electrical currents produced deep inside the Sandia National Laboratories "Z" machine shown here represent the morphology that may be expected in either laboratory astrophysics experiments or the structure of the universe itself.

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