Public Release: 

Duke's Russian Laser Making Ultraviolet Light And Gamma Rays

Duke University

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.

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