World's Most Energetic Light Beam Produced At Argonne

The accomplishment demonstrated that such free-electron lasers could provide the true laser-quality X-ray beams needed to open exciting new horizons for research in dozens of scientific fields.

The beam of light produced in the experiment had a wavelength of 385 nanometers, placing it in the ultraviolet region of the spectrum and making it 1,000 times more energetic than the previous record beam from an operational free-electron laser of its kind.

“In the history of synchrotron radiation research, which is only about 45 years old, you can count the true breakthroughs on the fingers of one hand,” said David Moncton, Argonne’s Associate Laboratory Director for the Advanced Photon Source. “This is one of them. Our team received congratulatory emails from dozens of colleagues all over the world who are working on this same problem.”

A report on the research appears today in Science Express, the Web version of Science magazine, and will appear in the print version of the publication in June. The researchers, led by Efim Gluskin and Stephen Milton, are scientists and engineers at Argonne’s Advanced Photon Source.

“Synchrotron radiation” refers to the way high-energy electrons emit light when magnets bend their flight paths. The world’s most powerful X-ray sources, including Argonne’s Advanced Photon Source, use this method to produce their beams.

The next generation of X-ray sources for scientific research will be based on the free-electron laser concept, the latest extension and refinement of synchrotron radiation. Unlike a conventional laser, which requires mirrors, the Argonne free-electron laser uses a powerful electron accelerator in combination with arrays of very long and precise magnets and needs no mirrors for operation.

With further development, free-electron lasers promise to provide laser-like X-ray beams in ultrashort pulses that will enable scientists to study the properties and structures of materials in far greater detail and in far less time than is possible today. Examples include:

“Snapshots” or “movies” of chemical and biological reactions too fast to be observed with today’s sources. Holographic images of proteins and other molecules. The ability to make an image of a single protein molecule with a single X-ray pulse. Scientists would no longer be limited to making images of only proteins that form crystals. The ability to study “warm, dense matter,” a state between one in which all the electrons surrounding a collection of atoms are highly excited and one in which all the electrons and atoms have become so exited that the electrons are stripped from the atoms and the whole collection becomes a hot plasma. Today, X-rays are the most widely used scientific probe for studying the structures and interactions of crystalline materials at the atomic and molecular levels. But many materials do not form crystals, and many reactions take place too quickly to study adequately, even at Argonne’s Advanced Photon Source, which provides the nation’s most powerful X-ray beams for research.

An ideal step forward for X-ray researchers would be to use lasers that produce intense, perfectly focused X-ray beams. Unfortunately, conventional lasers cannot produce beams of light more energetic than ultraviolet light, which is far less energetic than X-rays. This is because conventional lasers rely on mirrors, which become less efficient as they reflect higher-energy light.

Scientists at laboratories around the world have been trying to step around this problem by developing a version of the free-electron laser. This technology relies on a process called “self-amplified spontaneous emission” and does not require mirrors. Instead, a self-amplified spontaneous emission free-electron laser requires a high-quality electron beam and a long array of high-quality magnets, called an “undulator. ”

When the electron beam passes through the undulator, the magnets vibrate the electrons from side to side, causing them to emit light as synchrotron radiation. The higher the electron energy, the higher the resulting light energy.

At high enough electron energies and with a long enough undulator system, a free-electron laser could theoretically produce an X-ray beam with a peak brightness more than one billion times greater than the brightest beams available today, such as those generated by Argonne’s Advanced Photon Source.

Argonne is one of six U.S. research organizations collaborating on developing the free-electron laser technology needed to achieve this national goal. Others in the collaboration include the Stanford Linear Accelerator Center (SLAC), the University of California at Los Angeles, and Brookhaven, Los Alamos and Lawrence Livermore national laboratories. Their collaboration is aimed at demonstrating the feasibility of the proposed Linac Coherent Light Source, a proof-of-principle fourth-generation X-ray light source to be built at SLAC.

The success of the process is gauged by whether the free-electron laser effect has “saturated,” meaning the point at which the maximum power has been given up by the electron beam and converted to coherent synchrotron radiation.

As electron bunches propagate down the undulator, they are bathed in the same light they generate. As they wiggle back and forth through the magnets and interact with the electric field of this light, some gain energy (speed up), and some lose energy (slow down), depending upon their phase relationship with the light and the magnetic fields.

As a result, two mutually reinforcing processes take place: In one process, the electrons begin to form microbunches separated by a distance equal to the wavelength of the light they generate. In the second process, the light waves from the electrons begin to line up more in phase – meaning that the waves’ peaks and valleys overlay each other – reinforcing and amplifying the light’s brilliance and intensity.

Eventually, a favorable runaway instability develops, akin to the feedback squeal of a public address system with its volume turned up too high. The light intensity grows exponentially along the undulator until the process “saturates,” bringing the beam to its highest possible intensity. By the time the light beam emerges, its initial intensity is amplified more than a billion times.

The nation’s first national laboratory, Argonne National Laboratory conducts basic and applied scientific research across a wide spectrum of disciplines, ranging from high-energy physics to climatology and biotechnology. Since 1990, Argonne has worked with more than 600 companies and numerous federal agencies and other organizations to help advance America's scientific leadership and prepare the nation for the future. Argonne is operated by the University of Chicago as part of the U.S. Department of Energy's national laboratory system.