Stanford Univ. and the U.S. Dept. of Energy are looking to
‘turn conventional wisdom on its head’ with the LINAC Coherent Light Source.
About seven months ago, the Dept. of Energy’s (DOE) Stanford Linear Accelerator Center (SLAC), Menlo Park, Calif., broke ground for the linear accelerator (LINAC) Coherent Light Source (LCLS). The project is an extremely powerful, $400-million laser, designed to photograph molecules and chemical reactions that previously were impossible to see. And, although excavation crews have not completed boring through the sandstone to complete the new tunnel for the LCLS, collaborators of the project have taken a major step into making it a reality.
Forty plus years in the making
The Stanford Linear Accelerator Center, Menlo Park, Calif., houses the longest linear accelerator in the world and may hold the key to unlocking secrects long held captive inside atoms and molecules. Image: Stanford Linear Accelerator Center
There are two basic types of accelerators, circular and linear. These are very widely used; for instance, an ordinary cathode ray tube television set is a simple form of an accelerator. In a linear accelerator, particles are accelerated in a straight line with a target of interest at one end. SLAC's main accelerator, founded in 1962, is 3.2 km long and can accelerate electrons and positrons up to 50 GeV. It is the longest accelerator in the world and is purported to be the world's straightest object.
Initially named Project M, the idea for a 3.2 km LINAC at Stanford was conceived in 1956, proposed in 1957, and authorized by Congress in 1961. The center was completed Feb. 10, 1966, with the mission to design, construct, and operate state-of-the-art electron accelerators and related experimental facilities for use in photon science, particle and astroparticle science, and high-energy physics research.
Even though it has reached middle-age, the SLAC is not only doing what it does best—boosting electrons to near the speed of light—but it is also assisting in new breakthroughs.
First electron beam
This month LCLS physicists and engineers employed the newly-installed electron injector system and successfully created and accelerated a pulse of electrons. The system is installed in a vault adjoining the LINAC and is plugged into the LINAC at the 2 km point along its length. An animation of this process can be viewed at http://today.slac.stanford.edu/a/2007/04-09.htm.
“SLAC’s ability to use the pre-existing LINAC in the LCLS has dramatically reduced the cost—more than $300 million—and technical risks of the project,” says John Galayda, LCLS project director.
However, not everything old is new again. The LCLS requires a special electron beam that must have a very small transverse size and extremely slight divergence. These properties must be achieved at the beginning of the electron accelerator. They can’t be added by “fixing” the electron beam downstream, adds Galayda.
“Now that the injector has been turned on, we can begin the effort to get the electron beam in shape to make the entire LCLS do its job,” says Galayda.
What’s ahead
Much construction work will take place before the electrons can venture beyond the LINAC. As mentioned before, the LCLS will use the final third portion of the accelerator in conjunction with long arrays of special magnets, called undulators. These powerful undulators produce intense pulses of radiation that last less than a billionth of a second. Similar to the flash of a camera, the LCLS will enable scientists to take images of atoms and molecules in motion, revealing the elemental processes of life on an unparalleled level.
The electrons will zip through 130 m of undulators, emitting photons as they go. The photons will interfere with the electrons in each bunch, forcing them to clump together into micro-bunches. Each micro-bunch will then radiate coherent x-ray light with a wavelength of 1.5 Å. In a loop, the x-rays will then stimulate the micro-bunches to release even more photons, flooding the beam pipe with a beam of coherent x-ray light.
LCLS Design Parameters
• X-ray wavelength: 1.5 to 15 Å (on the scale of atoms)
• Ultra-short pulse duration: 1 to 230 fs
• Peak brightness: 0.8 to 0.06 x1033 photons/s/mm2/mrad2/0.1% bandwidth (10 billion times brighter than existing x-ray sources)
• X-rays per pulse: 1.1 to 29 x1012 (one trillion in a needle-thin beam)
• Electron beam energy: 4.5 to 14.3 GeV
• Peak current: 3.4 kA
• Fundamental saturation power at exit: 8 to 17 GW
• Meters of undulator magnets to generate x-rays from electrons: 112
At the end of the undulator hall, the x-ray beam will go to the experimental hutches where it will be used for experiments, while the electrons are sent to a beam dump—a chunk of copper—that soaks up the once-free electrons.
According to Jonathan Dorfan, former director of the SLAC, "The power of this light source to unveil the structure of the molecular world is limitless."
When completed in 2009, the LCLS will be the world's first x-ray free-electron laser producing ultra-fast, ultra-short pulses of x-rays 10 billion times brighter and 1,000 times shorter than any existing source on earth.
Simply put, the LCLS will provide x-rays so brief and precise that it will enable stroboscopic experiments with materials on the nanoscale. And, hoping to turn conventional wisdom on it head, researchers are anticipating its use in applications in fields that do not yet exist.
