Researchers develop 'landmark' X-ray lasers
After more than a decade and a half of effort, scientists appear to have finally created X-ray lasers in the laboratory. Two groups of researchers - one here at the Lawrence Livermore National Laboratory, the other from Princeton University's Plasma Physics Laboratory - are reporting major progress toward achieving this hitherto illusive goal at an American Physical Society meeting in Boston this week.
X-ray laser technology forms part of President Reagan's ''star wars'' missile defense program.
While an X-ray laser works in basically the same fashion as the now familiar optical laser, X-ray lasing has proven much harder to create and control than scientists had expected. Since 1967, when Michel Duguay of Bell Labs and P. M. Rentzepis published the first significant paper outlining theoretical methods for achieving an X-ray laser, a number of scientists from around the world have claimed this achievement. But their work was later refuted.
Speaking of the Livermore work, however, Charles H. Townes of the University of California at Berkeley says, ''This is a landmark, all right.''
Recipient of the Nobel Prize in 1964 for his work on the maser, the microwave predecessor to the laser, Dr. Townes was called in by researchers at the national laboratory to evaluate their effort before it was presented. ''(Livermore) certainly seems to have an X-ray laser. I understand the Princeton group has something as well, but I am not familiar with their work,'' he adds.
The Livermore researchers took a brute-force approach to the problem. They employed the lab's giant optical laser, Nova, to create the conditions necessary for X-ray lasing, explains Dennis Matthews, head of the 40-scientist research team.
Nova's green-light beam was focused down to a narrow line. The beam was also split into two parts and aligned so that they strike two sides of a target, which consists of a thin plastic film on a postage-stamp-size frame.
On one side of the film is an even thinner layer of an exotic metal, either selenium or yttrium. When the fiery, 3-trillion-watt optical beam strikes the metal film, it is vaporized into an electrically charged gas called a plasma. In less than a millionth of a second, the plasma is heated to 10 million degrees Celsius.
At this point, about 250 trillionths of a second after the beam hit, the metallic atoms are in a state conducive to lasing. They have been stripped of all but an average of 10 electrons and these are in a condition called ''inverted population.'' Somewhat analogous to a temperature inversion in the atmosphere, an atom's electrons are in outer orbits, or high energy states, while the inner orbits are empty. This is an inherently unstable condition, one which atoms do not hold for long.
It is also a condition where atoms can be stimulated to emit X-rays of a characteristic frequency. If an X-ray photon of the right sort impinges on an atom in such a state, it emits another X-ray of the same wavelength. So, when these X-rays travel the length of the plasma column, they start a chain reaction that creates the intense laser beam.
In the Livermore experiments, they got a brief X-ray beam that flashed at right angles to the optical laser. Although it only contained 300 to 400 watts of power, it represents an amplification of 700 times more power at the characteristic wavelength than would be emitted by a nonlasing source - more than enough to technically qualify it as a laser, says Dr. Matthews. The ''wall-plug efficiency'' of the process was a billionth of a percent, although they have ideas that could improve this by a factor of as much as 10,000, to a thousandth of a percent overall efficiency.
Princeton researchers, headed by Dr. Szymon Suckewer, took a more efficient route. Using a much smaller, carbon dioxide laser, they create a carbon plasma and raise its temperature to the point at which all the electrons are stripped from the carbon atoms. The plasma, confined by an intense doughnut-shaped magnetic field, is allowed to cool rapidly. At this point, electrons recombine with the nuclei preferentially in the outer orbitals, thus creating the essential population inversion. The atoms are stimulated, electrons cascade from the outer to the inner orbitals, releasing X-rays in the process. Dr. Suckewer reports a 100-fold amplification in X-ray production.
Both the Livermore and Princeton experiments involve the production of ''soft X-rays.'' Visible light has a wavelength of 4,000 to 7,000 Angstroms. (An Angstrom is four billionths of an inch.) The X-rays produced at Livermore are 206 and 209 Angstroms with selenium and 155 Angstroms with yttrium. Using carbon , Princeton researchers produced emissions at 182 Angstroms. The X-rays used for medical examinations are termed ''hard'' because their wavelength is much shorter, about 0.5 Angstroms, and they contain more energy.
The researchers see a number of potential scientific and commercial applications for X-ray lasers, although Dr. Townes cautions that ''there is a lot of work left before they become usable.''
X-ray lasers are well suited for making more detailed analysis of chemical reactions and improved measurements of crystals and microscopic surfaces. They might also be used to produce holographs - three dimensional X-ray photographs - of biological processes like DNA self-replicating.
Semiconductor companies are interested because it might enable them to cram even more complex integrated circuits onto tiny silicon chips. And X-ray lasers might improve CAT scanners and similar medical X-ray devices.