Skip to main content

Latest Free-Electron Laser Will Reach X-Ray Range

Duke's Russian-built "Blue Devil" OK-5 is slated to emit light pulses as short as "femtoseconds," a state in which "new science starts"

 

The components of a next-generation laser that could allow three-dimensional holographic images of objects as small as molecules and events as rapid as chemical reactions are now being tested in preparation for installation at the Duke Free-Electron Laser Laboratory (FELL).

Such fine three-dimensional images have never before been achieved, said physicists constructing the laser, and could give researchers an unprecedented view of the infinitesimal gyrations molecules undergo as they react.

Called the "Blue Devil" OK-5, the new Russian-made laser is part of a 12-year-old research partnership between Duke and a Siberian physics laboratory, the Budker Institute of Physics.

Once the OK-5 is up and running, tentatively next summer, it will produce laser light that is the most powerful achieved in the laboratory and at wavelengths as short as the outer limits of the X-ray range.

When operated in the "Super-Pulse" mode, the OK-5 will emit bursts of light as brief as 50 to 100 femtoseconds. A femtosecond is one thousands of a trillionth of a second --a period so short that light, moving at 186,000 miles a second, has time to journey only one tenth of one thousandth of an inch.

The results will eventually be startling new research, including the ability to produce X-ray holograms of molecular-sized structures, and to seemingly "freeze frame" blindingly fast chemical reactions in mid-flight, said Vladimir Litvinenko, a Duke associate physics professor who is FELL's associate director for light sources.

"We will be able to laze with more power, and at wavelengths we previously could not reach," Litvinenko said. "And, fundamentally, a hundred femtoseconds is a range where new science starts."

The OK-5 is the laboratory's latest free-electron laser (FEL). FELs are unique in that they produce laser light by perturbing beams of electrons freed of their normal bondage to atoms. This mechanism allows such lasers to be tuned to emit laser beams at a variety of different wavelengths, a flexibility that makes them extremely useful in research.

The two previous devices at FELL include the 60 foot long Mark III, housed within a tunnel, where a 40 million volt stream of electrons passes through a device called a "wiggler" to generate infrared laser light. A second machine, the OK-4, uses a more advanced "optical klystron," with three wigglers, to generate ultraviolet light. It also relies on a more-powerful energy source: a 300-foot-in-circumference electron storage ring combined with a 270 million volt linear accelerator.

The Russian-born Litvinenko trained at the Budker Institute in Novosibirsk, Siberia, where he designed and built the OK-4, the world's first ultraviolet FEL. The OK-4 was brought to Duke in 1995 to take advantage of the FELL's electron storage ring. The machine was transferred under a memorandum of understanding between Duke and the Budker Institute which included joint U.S.-Russian research and funding from the Office of Naval Research.

Last summer, James Siedow, Duke's vice provost for research, signed a new agreement with Budker director Alexander Skrinsky to continuing pursuing research of mutual interests in the quest to develop futuristic light sources of various wavelengths.

The new pact was signed about a month after the OK-5 was moved to Duke from Novosibirsk, where the new FEL was constructed. The Russian institute is currently building an electron booster ring that will more than quadruple the injection energy into the storage ring, to 1.2 million electron volts, after installation in 2004.

That upgrade was funded partially by a $3.2 million grant from the U.S. Department of Energy through the Triangle Universities Nuclear Laboratory (TUNL), a joint project of Duke, the University of North Carolina at Chapel Hill, and North Carolina State University.

In a laboratory connected to the FELL physicists at the Triangle University Nuclear Laboratory are taking advantage of another feature of both the OK-4 and OK-5 lasers -- their ability to produce intense, pencil-sized beams of gamma rays, the most energetic form of light.

The physical properties of these gamma ray beams make them especially useful for basic studies of nuclear systems. For example, the gamma rays are generated at energy ranges especially useful for probing nuclei of atoms, said Calvin Howell, a Duke physics professor at TUNL.

Another advantage of the light from the OK-4 and OK-5 lasers is that it is polarized, meaning that the light's electric field vibrates in specific, well-defined directions. Because of the OK-5's advanced design, its users will have the option of choosing either "linear" polarizations oriented along a plane or "circular" polarizations that rotate. Such control of polarization for gamma rays will allow them to use gamma beams to probe the way constituents of atomic nuclei are themselves polarized.

It was Litvinenko's calculations in 1992, that convinced him that free-electron lasers of the OK-4 and OK-5 design could produce both laser light and non-lazing gamma rays -- which are generated when laser light particles collide with speeding electrons within the lasers.

The higher permitted power will allow the OK-5 to produce laser light at small enough wavelengths to reach the "soft" X-ray range -- meaning at the boundary between X-rays and ultraviolet light. Because its wavelengths are so short, a soft X-ray laser beam would be able to resolve details the size of very large molecules, capturing their images for study as 3-dimensional holograms.

Litvinenko's goal is wavelengths as short as 4 billionths of a meter, a range that can penetrate the water in cells to allow holograms of molecules in living tissue, including DNA and proteins.

And because the OK-5 design allows laser pulses as short as femtoseconds, the beams could also act like a strobe camera to seemingly "stop" jiggling atoms in the midst of movement. That capability would allow new kinds of studies of chemical reactions, said Litvinenko.