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Old photographic technique applied
to future energy research

Marshall scientists assist laser fusion experiments

March 24, 1999: One of the oldest photographic techniques is helping scientists study what happens as they push the frontiers of nuclear physics in the quest for a 21st century power source. It might even help take astrophysicists to the edge of the universe without leaving home.

The Astrophysics Branch in the Space Sciences Laboratory at NASA's Marshall Space Flight Center is providing photographic emulsions for use in detectors inside the petawatt laser facility at Lawrence Livermore National Laboratory in Livermore, Calif.

Right: A photomicrograph shows where high-energy electrons blazed through a film emulsion located in a spectrometer outside the target chamber for the petawatt laser tests. The tracks are slightly curved from the emulsion being slightly curved in its holder. Links to 1210x957-pixel, 127K JPG. Credit: NASA/Marshall Space Flight Center and the University of Alabama in Huntsville.

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"We and the University of Alabama in Huntsville (UAH) have done all of the measurements of the energy spectra of the positrons (anti-electrons) and electrons produced in the plasma from those tests," said Dr. Thomas Parnell who recently retired as the chief of the high-energy astrophysics branch. "We also helped with the design of the magnetic spectrometers that those emulsion strips go into."

NASA/Marshall is the last U.S. laboratory that can make large, thick photographic emulsions that record tracks as nuclear particles pass through. NASA/Marshall uses them for cosmic ray research, but emulsions are just as easily applied to nuclear physics - where the technique originated.

"Marshall's role was absolutely mission critical," said Dr. Thomas Cowan of Lawrence Livermore. Cowan is presenting a paper, coauthored with Parnell, Dr. Yoshi Takahashi at UAH, and others, today on " Nuclear Fission and Anti-matter Creation with Ultra-Intense Lasers" at the centennial meeting of the American Physical Society in Atlanta.

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"The emulsion technique which they brought to the work was the key difference in being able to distinguish the high energy electrons and very rare positrons from the flood of other intense radiation coming from the laser-target interaction. Other techniques that use more conventional electronic detectors, were blinded by the radiation flash, and required much more care and development to function reliably."

The emulsions record the tracks of nuclear debris from microscopic gold and uranium targets that the laser vaporizes. If its design can be tailored so the laser operates the way scientists want, advanced descendants of the petawatt laser could be used to trigger tiny thermonuclear fusion reactions. But it also generates conditions that previously were seen only in the most powerful particle accelerators. And they might provide some insight into the cause of gamma ray bursts.

Fusion could be a clean, long-term power source for mankind. It involves joining two lightweight atoms that surrender more energy than it takes to cram them together. But first you have to overcome the natural repulsion between protons, the positively charged particles in the nucleus of an atom, and get them close enough for the strong force to bind two nuclei. For the sun, it's simple. Gravity compresses the core to drive the fusion reaction and generate the heat that keeps us warm.

Left: Schematic of the petawatt laser target chamber shows the laser beam entering from the left and being focused by a glass mirror and a plasma mirror (since a solid mirror would disintegrate under the concentrated flash). The tracking spectrometer measures electron energies from 100 to 2,000 MeV, and the two electron/positron spectrometers measure the relative energies and populations of electrons (e-; 0.2-140 MeV) and positrons (e+; 0.2-40 MeV). Links to 480x418-pixel, 55 K JPG and 963x839-pixel, 112 K JPG. Credit: NASA/Marshall Space Flight Center and Lawrence Livermore National Laboratory.

Human attempts to replicate the pressures with magnetic confinement in vacuum chambers have fallen apart as the plasmas twisted and wiggled out of position. For the past two decades, one of the most promising approaches has been inertial confinement fusion, the idea of zapping a target on all sides with light from several high-energy lasers. The petawatt laser is derived for the 10-laser Nova facility. Its lessons will be incorporated in the National Ignition Facility, under construction at Lawrence Livermore, with more than 100 lasers.

That, too, has proven difficult, but scientists at Lawrence Livermore think they are getting close to the proper technique with the petawatt laser. It uses a special series of mirrors and amplifiers to generate an incredibly brief, powerful pulse of light. The total power delivered to the target is just enough to keep a 100-watt bulb burning for 6 seconds.

But the amplifier system shortened the pulse to just one-half a picosecond (a trillionth of a second), and boosted the power to 1.25 petawatts. That's 1,250,000,000,000,000 watts focused on a gold foil target just 1 mm (1/25th of an inch) across.

That's bright enough that the electrons in the gold atoms hit by the laser do more than just get hot. The are energized enough to produce intense, high-energy gamma rays that knock neutrons from the gold nuclei. Those neutrons cause nuclear fission in the uranium pellet behind the gold target. Freeing electrons from atoms with light is a simple trick, but the high temperatures and electric fields generated by the petawatt flash laser accelerate electrons to as high as 15 million electron-volts (15 MeV), and perhaps as high as 100 MeV.

Some of the gamma rays split into electrons and positrons (anti-electrons), so the tracks in the emulsion provide a measure of gamma ray production, and intensity, in the blast.

"These reactions create lots of electrons, X-rays and gamma rays," Parnell explained. "You need a high spatial resolution detector that accepts everything at once. That points to passive nuclear track detectors. They have the highest resolution and track density of any detectors."

"We don't directly detect the fission products or other nuclear debris with the emulsions," Cowan explained. "These are captured in the target assembly and we detect them after the laser shot with gamma-ray spectrometers. The electrons and positrons are the particles emanating from the target that we directly measure with the emulsions. In terms of understanding the underlying physics of the laser-plasma interaction, all of these are important, but the electron data are the most direct and most influential of all of our measurements."

Right: Artist's conception of the interaction of the Petawatt laser pulse with the gold and uranium target material. The laser forms a plasma plume at the target surface, in which electrons (e-) are produced with very high energies. Some of these electrons make gamma-rays (g) in the target, which in turn can knock neutrons out of the gold nuclei. Those neutrons cause uranium nuclei to fission. Some gamma-rays are converted into matter-antimatter electron-positron pairs (green e+). Links to 640x480-pixel, 62K JPG. Credit: Lawrence Livermore National Laboratory.

The emulsions are strips of photographic film. Each strip is composed of a 500 micron-thick plastic substrate, 1 cm wide and 12 cm long (0.4x4.7 in), coated on both sides with a 50 micron-thick emulsion. The emulsion is about 20 times thicker than what is found on film you use in your camera.

"It's specially made in terms of silver halide content and grain size for nuclear track detection," Parnell said. "But other than that, it's a lot like black & white film, as fine as the best photographic film.

Web Links

Cosmic ray research at NASA/Marshall, including
Peeking at the Innards of the Universe using scintillating optical fibers, May 1997.
Imaging Calorimeter for ACCESS aboard the International Space Station,
Down-to-Earth Fiber Technology Yields Insight into Cosmic Rays, Sept. 2, 1996.

External links

Nuclear Fission and Anti-matter Creation with Ultra-Intense Lasers, a lay language version of the paper to be presented at the American Physical Society on Wednesday.
Petawatt laser fusion project at Lawrence Livermore National Laboratory.

"We make it by pouring the liquid gel on the plastic substrate ourselves. It's a simple mechanical process, and we pour it by hand. Experienced people are just as important as the facility."

Walter Fountain and Mark Christl of NASA and Joy Johnson of the Universities Space Research Association make the emulsions by hand. The 1x12-cm size for the petawatt laser facility is not the largest they can make. They have also produced large trays for placement under metal sheets that intercept cosmic rays and turn them into showers of secondary particles.

For the petawatt laser tests, the film strips are not just hung inside the chamber where the laser target sits, but placed inside three special electron spectrometers.

The two low-energy spectrometers are equipped with strong magnets - 10,000 times more powerful than Earth's magnetic field - that make electrons and positrons curve in opposite directions as they enter the spectrometers at close to the speed of light. These spectrometers let scientists measure the relative population of the two particles.

The third spectrometer also has strong magnets to spread the electrons out by energy, like a prism spreading white light into colors, to measure the where the energy goes in the by-products of a shot.

"Electronic detectors would be saturated by this flux and the density of particles occurring all at once," Parnell explained. But the electrons and positrons leave trails as their passage through the emulsion sensitizes the silver bromide crystals.

After a shot, the emulsions are removed from the spectrometers and returned to NASA/Marshall and UAH for measurement by Fountain, Johnson, and Bei Lei Dong of UAH. The emulsions are developed - at very low temperatures to protect the thick emulsions and ensure even development so the electron tracks appear as tiny dark strips in the emulsion.

"Most of the electron counting is done visually, by eye, with a standard analytical microscope," Parnell explained. "There has been some progress in automating the process, but it's like other kind of image recognition problems, and still requires expert people."

Left: A laser physicist at Lawrence Livermore adjusts a lens inside the laser compression chamber. Credit: Lawrence Livermore National Laboratory.

And a lot of time. Each test shot consumes five emulsion strips, and the lab makes two shots a day.

Over the coming months, the Lawrence Livermore team will use more emulsions, and other detectors, as they refine their understanding of the petawatt laser and how it interacts with the starlike spot of plasma that it creates for an instant.

That starlike quality may take the petawatt facility in a new direction that brings its research closer to work at NASA/Marshall: simulating the sources of gamma-ray bursts coming from sources several billion light years away.

"We hope that there will be an expanding overlap in this research as well," Cowan said. "Prof. Yoshi Takahashi at the University of Alabama in Huntsville and coworkers have proposed that the laser acceleration mechanism, and possibly even the positron production in relativistic laser-plasmas, may provide some experimental insight into the physics of gamma-ray bursts. If there are credible and interesting parallels, these results may help to launch additional interest in laboratory astrophysics."

In other words, astrophysicists may at last get a close-up peek at what happens in the gamma-ray bursts occurring near the edge of the observable universe.

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Author: Dave Dooling
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