Laboratory Astrophysics

A photograph of the electrical currents in the Sandia National Laboratory "Z" machine. The filamentary currents are called 'Lichtenstein' figures and represent the morphology that may be expected in either laboratory astrophysics experiments or the structure of the universe itself.

In the laboratory, the currents flow from the energy stored in many capacitors; in the universe, cells of differing types of plasmas play the role of the energy storing capacitors. Thus, from laboratory astrophysics experiments we may infer the structure of the universe.

The year 1996 marked the Centennial Celebration of the founding of Plasma Astrophysics; its origins may be traced to the seminal research of Kristian Birkeland published in 1896 that began his life-long study of laboratory produced cathodic rays and corpuscles (The term `plasma' was not to be coined by I. Langmuir until 1923) and their analogies to astrophysical phenomena.

This work was presented in two papers: "Sur un spectre des rayons catodiques" in Comptes Rendus, 28 September 1896, and a paper in Archives des Sciences Physiques et Naturelles, Geneva, 4th period, vol. I, 1896, that announced his discovery of magneto--cathode rays.

It was in this work that, according to Birkeland (1908): ...I expressed for the first time my belief that the northern lights are formed by corpuscular rays drawn in from space, and coming from the sun. In addition to his solving the mystery of the Aurora with his now-famous terrella experiments; electron beams in vacuum from magnetized copper globe cathodes, Birkeland utilized his data to formulate a theory about a plasma-filled universe populated with {\em systems} of nebula (galaxies).

Birkeland's methodology was simply that first formulated by Francis Bacon: Theory, modeling, and testing. The testing was achieved with experimental setups in the laboratory and spectroscopic measurements above the arctic circle. Birkeland went to great lengths to diagnose and record the results of these investigations, whether they be in the lab or in the arctic, where he made three expeditions to view astrophysical phenomena first hand.

Photographs of the artifical auroras Birkeland produced by bombarding a magnetized copper globe with streams of electrons respresenting the Sun's plasma emission. Well defined aurora curtains and instabilities, not seen until the advent of earth satellites, are identifiable in the photographs.

At first glance, it appears that the globe is spewing out jets of gas. In reality, plasma is streaming into and pinching onto a stress point on the surface of the conducting globe.

Much of Birkeland's work was rediscovered in the 1980s with renewed interest about the role of large scale magnetic fields and currents in explaining astrophysical, galactic, and cosmological scale phenomena, including the origin and structure or galaxies and thenature of interstellar space.

Stellar Physics in the Laboratory

According to our manner of looking at the matter, every star in the universe would be the seat and field of activity of electric forces of a strength that no one could imagine. We have no certain opinion as to how the assumed enormous electric currents with enormous tension are produced, but it is certainly not in accordance with the principles we employ in technics on the earth at the present time. One may well believe, however, that a knowledge in the future of the electrotechnics of the heavens would be of great practical value to our electrical engineers. It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonble therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebul\ae, but in``empty" space.

Kristian Birkeland in the Norwegian Aurora Polaris Expedition 1902-1903, Christiana, Norway, Aschehoug, 1908.

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

The experiments are at the cutting edge of 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 (NIF), a 1.2 billion dollar facility slated for completion in 2003, in which 196 lasers beam will deliver nearly 2 megajoules to a millimeter sized target, reproducing the plasma temperatures within stars. For scale, an oil-tanker is parked in front of the facility.

Artists depiction of some of the laser beams focused on the target region below.

In addition to the high energy densities promised by NIF, plasma astrophysics today is done on another type of facility called a pulsed-power generator.These include Sandia's 500 kilojoule Saturn and the more powerful 2 megajoule Z-machine as well as Los Alamos' Pegasus and extremely high current machine called Procyon. Later, yet higher power machines such as the 20 megajoule X-1 generator, so called after the bright source in the constellation Cygnus A, and Atlas, a large pulsed power generator used to study instabilities in the various states of matter will be added to the arsenal of Z-pinches machines for which an appreciable fraction of machine useage will be devoted to plasma astrophysics.

The official logo of the Institute of Electronics and Electrical Engineers (IEEE) displays the major elements of the "Z-pinch": an electrical current arbitrarily defined to flow in the "Z" coordinate direction and an azimuthal magnetic pinch field self-created by the current. The emblem is surrounded by a rhombus antenna which denotes that the Z-pinch is an excellent emitter of electromagnet radiation. The radiation from a Z-pinch ranges from radio waves to x-rays, and synchrotron radiation over a full range of frequencies to the acceleration of electrons to gamma ray energies.



Whereas the plasma universe is composed of cells of plasma of differing consituencies (the capacitors) measured in hundreds of megaparsecs and the discharge time may be measured in billions of years, pulsed power generators use large banks of capacitors to build and store electrical charges, then simultaneously discharge them in a fraction of a second. However, in spite of the size/time disparity, the energy densities are nearly equal. The data gathered experimentally can be used to test theories of the universe on high performance, high fidelity computers when scaling factors are taken into account. Pulsed power generators are the most prolific producers of microwaves and x-rays on earth. In all cases the source of this intense radiation is the interaction of filamentary plasma. These plasmas are produced by applying megavolts of potential to centimeter long arrays of wires that then rapidly revert to the fundamental plasma state, releasing a full spectrum of electromagnetic energy.

The "Z" machine at Sandia National Laboratories. (Measurements) The energy stored in the capacitor banks are delivered to a miniscule sized load a few centimeters long, whose projection is shown beneath the machine.

The basic geometry of the load: millions of volts are deliverd between an anode and a cathode between which a wire array has been strung. The array bursts into a plasma in a billionth of a second and in the process, hundreds of terawatts of x-rays are produced.

Depending on the geometry of the load, a wide range of spectra is possible. The dashed line shows the Planckian spectrum for a 140 electronvolt plasma, used for inertial confinement and astrophysical studies.

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. X-ray spectroscopy is the study of the absorption and emission of x-rays and can yield significant data on the chemical compositions, temperatures, densities, and magnetization of the plasma universe. The launching of three x-ray observatories starting in 1999 will increase the amount and resolution of x-ray charts of the plasma universe, helping to unfold the extent of regions of energy storage, release, and the electrical currents required in transport.