FREE-ELECTRON LASERS: Undulators for x-ray FEL pass key milestone

The idea of a free-electron laser (FEL) that produces beams in the x-ray wavelength range has long intrigued researchers, particularly in the biomedical and materials-processing sectors. Three groups are currently developing x-ray FELs, one at Stanford University (Stanford, CA), another near Hamburg, Germany, and the third in Riken, Japan (see www.laserfocusworld.com/articles/282655).

The U.S. project combines Stanford’s Linac Coherent Light Source (LCLS) with the Stanford Linear Accelerator Center (SLAC), the world’s longest and highest energy electron linac. When complete and online, the LCLS will use the last kilometer of the 3 km SLAC to produce 1.5 to 15 Å photons in a 130-m-long undulator with electrons having energies of up to 15 GeV.

As part of the collaborative U.S. effort, scientists at Argonne National Laboratory (Argonne, IL)—the entity charged with designing the undulators for the LCLS and the Advanced Photon Source (APS), another component of the x-ray FEL—recently announced a milestone in the design and construction of the LCLS undulator system: the completion and acceptance of the last of the 40 undulators needed for the LCLS, which means the project is on course for completion in March 2009.

According to Argonne LCLS Project director Geoff Pile, the undulators are the heart of the LCLS FEL, providing a precise magnetic field through which an electron beam will travel. The linac accelerates a linear beam of electrons that pass through undulators, which force the electrons to oscillate back and forth, producing large amounts of x-rays. These x-rays interact with the electrons, forcing them to bunch at x-ray wavelengths. When this bunching occurs, the electrons emit their light coherently, causing a large gain in radiation power. The result will be x-ray beams one billion times brighter than can be produced by any other x-ray source currently available.

Strong forces

Each undulator comprises an array of ultrastrong neodymium iron boron permanent magnets and vanadium permendur magnetic poles (see figure). The magnets and poles are mounted in aluminum structures bolted onto a 3.4 m long straight titanium beam. The beam secures the magnet and pole assemblies, counteracts the very high magnetic forces between the upper and lower magnetic arrays, and is critical in determining the thermal and mechanical stability of the undulator. Precision and stability requirements for the LCLS devices far exceed those for existing undulators at the APS and other light-source facilities, according to Pile.

“There are many more milestones to be reached,” Pile said. “We went through the evaluation and testing of a single undulator without putting a beam through it, then a mock-up with a bunch of ‘dummy’ components. Now we are in the process of proving the production components. Some of the challenges are that we have to keep the beam line components straight within sub-tens of microns and we have to take into consideration thermal changes in the tunnel. A single-degree temperature change can make a considerable difference in the expansion. We’ve also got to be concerned with old-fashioned shrinkage of the floor of the tunnel.”

The first light through the undulators is expected in July or August 2009, with the first experiments utilizing the x-ray FEL expected to come that same year or in 2010, according to Pile.

“A lot of reactions take place at subpicosecond time scales,” said Marion White, senior physicist at Argonne. “If you can take a very sharp snapshot of these processes in a very short amount of time, you can see what is going on. Synchotron systems are limited to 10 to 15 picoseconds, so you need a shorter pulse to freeze motion effectively at subpicosecond levels-for example, to observe chemical processes as they happen. We can determine the beginning and the end of these things but we can’t see what happens in between, such as how each piece of a protein finds its place or what happens during photosynthesis. So we expect a huge number of biological and physics applications with the x-ray FEL.”