Lasers Light Up the Future of Synchrotron Light Sources

Argonne’s Advanced Photon Source (APS) is a third-generation synchrotron light source that currently produces the nation’s most brilliant X-ray beams for studies in materials science, biological and environmental research. However, looking toward the future, the APS is also a leader in developing the technology for the next leap in X-ray brilliance, a new “fourth generation” of powerful synchrotron radiation light sources capable of producing hard X-rays—those with the highest energy and most penetrating power—with a peak brilliance billions of times greater than currently available anywhere in the world. Fourth-generation synchrotron radiation X-ray sources will be based on the free-electron laser concept. The technology promises to provide ultrashort laser-like X-ray pulses enabling scientists to study the properties and structure of materials in far greater detail and in far less time than possible today. For example, researchers could make “snapshots” or “movies” of chemical and biological reactions too fast to be observed with today’s sources. Such a new source would also make possible the creation of holographic images of molecules such as proteins.

“When the trajectory of a charged particle, such as an electron, is bent by a magnetic field it emits light, called ‘synchrotron radiation,’” explained Argonne physicist Stephen Milton. “This is the mechanism used at the APS, where powerful magnetic arrays, called undulators, shake bunches of high-energy electrons, which in turn emit intense beams of X-rays. Ordinarily, individual electrons within a bunch emit this light incoherently—like soldiers marching out of step. However, if one can force the electrons within the bunch to emit coherently, tremendous gains in brilliance can be achieved. We have a way of doing this; it is called the free-electron laser process.”

A free-electron laser carries synchrotron radiation to new levels of intensity, but until recently, because of mirror technology and electron beam quality issues, they were limited to longer wavelengths. At the APS, researchers are exploiting a process called “self-amplified, spontaneous emission” (SASE). Unlike other free-electron laser methods that use mirrors or seed lasers, SASE requires none and so is scaleable to X-ray wavelengths.

In a fourth-generation X-ray source, high-energy bunches of electrons traveling at nearly the speed of light would be sent through a long undulator magnet. As they propagate down the undulator, the electron bunches are bathed in the light they are generating. As they wiggle transversely through the undulator magnets, they also interact with the electric field of this light, some gaining energy (speeding up), some losing (slowing down), depending upon their phase relationship with the light and the magnetic fields. The net result is that a microbunching begins within the bunch with a length scale equal to the wavelength of the light being generated.

As microbunching starts, 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. This in turn bathes the electron bunch in even more light and 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 instead grows exponentially along the undulator until the process saturates. By the time the light beam emerges, its intensity is in effect amplified more than one hundred billion times. This “exponential growth” is the essence of a free-electron laser operated in SASE mode.

Milton and his colleagues at Argonne were the first in the world to observe self-amplified, spontaneous emission of light at a wavelength of 530 nanometers—green—and to date are the only laboratory to have measured the signature exponential growth of the SASE light at such short wavelengths. These experiments took place at Argonne’s Low-Energy Undulator Test Line Facility, where nine undulators, totaling 21.5 meters in magnetic length, are installed in series.

“Last December, when we first measured exponential growth, the beam power doubled roughly every 1.2 meters,” Milton said. “Without exponential growth, the optical power would have grown only fourfold compared to a single undulator section, but instead we measured a 40-fold growth of the signal. Recent data indicates even better performance.”

Argonne is one of six U.S. research organizations collaborating on developing the technology for a fourth-generation synchrotron radiation X-ray light source. 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.

For more information please contact Catherine Foster at 630-252-5580

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