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Diagnosing the state of an unruly plasma.

Diagnosing the state of an unruly plasma

The narrow, high-voltage gap between the positive and negative electrodes in a pulsed-power diode is a hostile environment. Intense electric and magnetic fields, restricted to a small volume, tear matter apart to produce an extremely hot, unruly plasma of ions and electrons. Such extreme conditions make it difficult to monitor what's happening inside the diode--the kind of fundamental information needed for learning how to generate ion beams with the right characteristics to initiate and drive thermonuclear fusion in tiny fuel pellets.

Now researchers are starting to catch glimpses of the hitherto largely unseen world inside such diodes. Using sophisticated spectroscopic techniques developed by Yitzhak Maron of the Weizmann Institute of Science in Rehovot, Israel, they can measure the intensity and extent of the electric fields present, the location and geometry of the plasmas near the electrodes, and the velocities and numbers of particles at various points within the diode gap.

"Maron can be considered a pioneer in the diagnostics of what happens in a very complicated system -- how ion beams are accelerated against a plasma background," says physicist Martin Reiser of the University of Maryland in College Park. Maron described his diagnostic techniques at last week's meeting on atomic processes in plasmas, held in Gaithersburg, Md.

Maron's scheme exploits the effects of strong electric and magnetic fields on light emitted by certain ions that have been excited to particular energy levels. By measuring the precise wavelengths of light emitted by magnesium, aluminum and other ions preent in the diode gap and by unraveling the meaning of these signals, he can glean information about the behavior of the electric fields and plasmas in the gap.

The measurements are difficult to make because so little time is available and the space is so small, Maron says. Pulse-generated plasmas last only 100 nanoseconds, and the diode gap may be less than a centimeter wide. Nevertheless, Maron and his colleagues have managed in several tests to detect plasma and electric-field features as small as 200 microns and events as brief as 5 nanoseconds.

Until Maron's work, researchers had to calculate theoretically what happens inside a pulsed-power diode to predict the characteristics of the ion beam that emerges. "Maron's work has been instrumental in giving detailed measurements of important quantitites inside a diode," says J. Pace VanDevender of the Sandia National Laboratories in alburquerque, N.M.

Maron is now working with Sandia scientists to adapt his diagnostic methods for use in the powerful ion diode for Sandia's Particle Beam Fusion Accelerator II. Earlier this year, this giant accelerator produced an ion bema of record intensity, an important milestone on the road to inertial confinement fusion. In this scheme, beams of energetic ions would irradiate fuel pellets containing the hydrogen isotopes deuterium and tritium. Theoretically, depositing sufficiently large amounts of energy quickly should initiate nuclear fusion in the fuel pellet.

"The understanding that we get from the detailed behavior of the electrons and ions in the diode from the spectroscopic studies will help us do the job," VanDevender says. "The more sophisticated the diagnostics, the faster progress can be made."
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Author:Peterson, I.
Publication:Science News
Date:Oct 14, 1989
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