DURHAM, N.C. -- A record-setting Russian ultraviolet free-electron laser
(FEL) has begun operating at its new home at Duke University, where investigators
expect to harness it soon for medical research such as improved laser surgical
techniques.
The device also is producing intense beams of gamma rays, which other physicists
hope will help answer major questions in nuclear physics such as understanding
thermonuclear processes inside stars like the sun.
The OK-4 optical klystron FEL, originally developed at the Budker Institute
of Nuclear Physics in Novosibirsk, Russia, was moved to Duke in May 1995
to take advantage of a much more powerful 1.1 billion electron volt electron
"racetrack" at Duke's Free-Electron Laser Laboratory.
After being reassembled and joined to the laboratory's electron ring, the
OK-4 began producing ultraviolet laser light late on Nov. 13. Within two
days, physicists had also induced the system to produce gamma rays by colliding
the laser beam with the same electron beam that feeds it.
Duke's FEL Laboratory is funded by a U.S. Department of Defense program
for advancing laser technology in medicine.
Free-electron lasers are like no others because they produce laser light
by perturbing beams of "free" electrons that have been liberated
from their normal bondage to atoms. Because electrons that provide the energy
of other lasers are held within the structure of their atoms, such lasers
can emit only a limited number of discrete wavelengths of light. FELs, in
contrast, can be "tuned" to a large variety of different wavelengths.
Also, FELs' beam pulse structures and power levels are extremely easy to
manipulate.
FELs produce light by passing such free-electron beams through a series
of magnets. As in all lasers, the energized electrons emit light, which
is then amplified and concentrated into a sharp beam by rapidly bouncing
the light between mirrors within an "optical cavity."
The OK-4's successful lasing at Duke means that it now becomes "the
shortest wavelength and the most powerful ultraviolet wavelength FEL in
the United States so far," said Vladimir Litvinenko, the Duke associate
physics professor who originally designed the OK-4 as a researcher in his
native Russia.
Litvinenko predicted that, within about a month, the Duke FEL lab would
begin trying to produce laser light of even shorter wavelengths than the
OK-4's standing world record for an FEL -- 240 nanometers, or billionths
of a meter
Scientists hope to establish a new FEL record of 193 nanometers, which would
be deep in the ultraviolet. "Our goal in the next half year would be
to cover more or less all of the ultraviolet range, and then go to the extreme
ultraviolet and X-ray ranges," he said.
"I think we can look forward to a series of very impressive demonstrations
and breakthroughs in the development of short wavelength FEL-based light
sources in the immediate years ahead," added John Madey, the Duke FEL
Laboratory's director and the free-electron laser's inventor.
Madey said one almost immediate use of the 193-nanometer OK-4 ultraviolet
light at Duke may be in experimental laser eye surgery.
Conventional excimer lasers currently used in such surgery also operate
at 193 nanometers. The lasers correct nearsightedness and farsightedness
by surgically removing corneal tissue to alter the eye's shape. But doctors
have found that "193 nanometers isn't necessarily the best match, nor
is the pulse structure or peak power generated by excimer lasers ideal for
this procedure," Madey said.
Since the OK-4 is much more flexible than excimer lasers, the Russian device
"will allow exploration of the range of wavelengths immediately above
193 nanometers, and particularly from 190 to 220 nanometers," added
Madey.
Research using the OK-4 also may have far-reaching impact in other fields
of laser surgery, Madey added. The Mark III, the Duke laboratory's infrared
free-electron laser that has been in operation since January 1992, is presently
being used for medical research.
Other scientists at Duke's Triangle Universities Nuclear Laboratory (TUNL)
also are excited about the OK-4's prospects for providing gamma rays of
exceptional intensity. Those gammas are produced through a process called
Compton back scattering.
In some split-second choreography, the Duke team began producing gamma rays
two nights after they first manufactured ultraviolet laser light. The researchers
began by introducing an extra electron bunch into the FEL lab's electron
ring.
Traveling at nearly the speed of light, the extra electron bunch arrived
in the laser's optical cavity at the precise instant that ultraviolet light
created by the previous pulse was bouncing between the mirrors. Ultraviolet
light and electrons thus interacted, resulting in a tremendous boost in
energy which converted ultraviolet light into gamma rays.
Gamma rays are nature's most energetic kind of light, even more powerful
than X-rays. Preliminary results show that the first OK-4 gamma rays measured
energies of about 12 million electron volts.
Such energetic gammas could help answer unresolved questions about "the
most important reaction in nuclear astrophysics," said Werner Tornow,
a Duke physics research professor and TUNL's director.
That helium burning reaction deep within stars like our own sun converts
the isotope carbon-12 into the isotope oxygen-16. Knowing more about the
reaction is critically important for predicting such vital details as the
relative abundance of various elements in the universe. Measuring its details,
however, is very difficult in the laboratory, Tornow added.
A promising alternative is to study the reaction backwards by bombarding
oxygen-16 with gamma rays to produce carbon-12. Up to now, though, scientists
have been unable to study that reverse reaction because gamma ray beams
from other sources have lacked enough intensity, said Henry Weller, another
Duke physics professor and TUNL researcher.
Because gamma rays are generated more efficiently inside its optical cavity,
the OK-4's gamma intensity could be 1,000 times higher than those available
now. That means "instead of getting one count a day, we'll get 15 counts
an hour, a huge improvement," Weller said.
Funded by the U.S. Department of Energy, and located within short walking
distance from the FEL Laboratory, TUNL investigates details about the centers
of atoms by bombarding those with various kinds of high speed subatomic
particles.
If the OK-4's gamma ray energies could be boosted to about 150 million electron
volts, their higher intensities would "open up a whole new field for
TUNL," said Tornow, who has begun a cooperative research program with
Madey's lab.
At the higher energy levels, the OK-4's gamma beams might allow TUNL scientists
to infer the relative masses of the up-and-down quarks within protons and
neutrons -- another major unanswered question in nuclear physics, Weller
added.
Boosting gamma energies that high would require a major upgrade of the FEL
Laboratory's electron ring system. TUNL officials have already approached
the Department of Energy about possible funding for what could then become
a major national gamma ray center.
Duke's short wavelength FEL research project is operated as a collaboration
of the FEL Laboratory's four faculty members, Madey, Litvinenko, Patrick
O'Shea and David Straub.
"One of the single largest research projects of its kind, its success
reflects the dedicated contributions of the students and staff of the FEL
Laboratory as well as key contributions from the Budker Institute of Nuclear
Physics, Stanford University and Argonne National Laboratory," Madey
said.