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Lasers & Sources

Bright lights, big accelerator

Eye on Technology - x-ray sources

From oemagazine July 2001
30 July 2001, SPIE Newsroom. DOI: 10.1117/2.5200107.0003

By generating a high-energy electron beam using the Advanced Photon Source (APS) linear accelerator (linac) and passing this beam through a series of specialized magnets called undulators, researchers at Argonne National Laboratories (Argonne, IL) have achieved saturation of self-amplified spontaneous emission (SASE) at ultraviolet wavelengths. Eventually, they hope to use the process to generate tunable, coherent x-ray pulses that are 109 times brighter and 103 shorter than the 100 ps pulses produced by major synchrotrons.

When an electron is accelerated back and forth by an undulator, it emits an electromagnetic (EM) wave at a well-defined wavelength. The bunches of electrons in an electron beam are randomly distributed, however, so the radiation emitted is incoherent. With SASE, the initial incoherent light is powerful enough to generate feedback between the electron beam and light, driving the electrons to emit coherent light. The power transferred from electron beam to photon beam increases exponentially until the process saturates.

Currently, the Argonne team has achieved saturation at 385 nm. The emission wavelength for SASE is inversely proportional to the square of the electron beam energy, however, so given a sufficiently high-energy electron beam, it is theoretically possible to generate coherent x-rays. "Our linac is limited to 650 MeV, so the shortest wavelength we can achieve is roughly 60 nm," says project leader Stephen Milton. "To get to x-rays using this process, you need about 15 GeV." Achieving those values requires a linac about 2 km long, similar to the one at the Stanford Linear Accelerator Center (SLAC; Stanford, CA).

The system has been referred to as an SASE free-electron laser (FEL), but the name is something of a misnomer. "It's debatable whether you'd want to call it a laser," says Milton, noting that although the beam is coherent transversely, it is not completely coherent longitudinally. On the other hand, Milton says, "It's extremely bright and coherent enough to do many of the experiments that a lot of people want to do."

Experiments like those of Michael Pellin, group leader in the Material Science division at Argonne. "We're interested in the FEL for tunability below 200 nm. There is really no tunable source with this kind of brightness," says Pellin. His group does trace analysis of small samples using photoionization techniques. "The SASE allows you to photoionize in a way that is not fragmenting so you see the intact molecules you knock off."

At the TESLA facility (Hamburg, Germany), researchers at DESY have generated output at 80 nm using the SASE technique. "We have not achieved saturation yet, but our data correspond with the theoretical expectations," says Ute Wilhelmsen of DESY. Groups in Italy, Japan, and elsewhere in the United States are also at work on the technique.