Longest linac makes the world's shortest electron bunches
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November 3, 2003--Using the unique properties of the world's longest linear accelerator (linac), researchers at the DOE Office of Science's Stanford Linear Accelerator Center (SLAC) have made the world's shortest bunches of electrons. The bunches can be converted into bright, short pulses of X-ray light that offers new views of the atomic world. These are the first X-rays made by a linac, and are 1,000 times shorter than those made by storage rings at SLAC and elsewhere to illuminate microscopic materials.
"These ultra-short, very bright X-rays enable experimenters to make direct observations of atomic motion in matter that have never been seen before," said Jerry Hastings, assistant director of the Stanford Synchrotron Radiation Laboratory at SLAC, which Stanford manages for the Department of Energy. Scientists from industry, universities, and labs in the fields of chemistry, biology, and materials science can use these X-rays to take instant pictures of simple chemical reactions in progress in solids and liquids.
The project, called the Sub-Picosecond Pulse Source (SPPS), is an important stepping stone on the way to making even shorter and brighter X-rays later this decade with the world's first free electron laser (called the Linac Coherent Light Source), which also will use the SLAC linac. This spring physicists tested SPPS, which they will turn on again next month to produce X-rays for experiments. It is an international collaboration that includes laboratory and university participants from the United States and abroad.
The SPPS compresses each bunch from 6 millimeters (thousandths of a meter) down to 12 microns (millionths of a meter). Each bunch contains about 21 billion electrons. Traveling the speed of light, the bunch whizzes by a fixed point in 80 femtoseconds (quadrillionths of a second). More electrons packed together equals more charge, which produces brighter X-rays. The compressed bunches reach a peak current of 30 kiloAmperes--about 1,000 times greater than the current that flows through a household fuse.
Manipulating the shape and size of electron bunches has become a science in itself. To compress the bunches, SPPS researchers rely on several tricks that can only be done at SLAC where the electrons pick up speed and power--28 billion electron volts--on their 2-mile journey down the linac. "The big increase in energy from the beginning to the end of the SLAC linac allows us to do the gymnastics of rotating and compressing the bunches to reach such small final dimensions," said staff physicist Patrick Krejcik.
The gymnastics occurs in three stages. First, the bunches are compressed from 6 millimeters down to 1.2 millimeters in a curved section of the machine's injector, just before entering the linac. Electron bunches are usually accelerated through the linac on top of radio frequency (RF) waves, similar to a surfboard riding the crest of an ocean wave. But in this step, the bunches are adjusted so they look like a surfer climbing a wave: The front of the bunch is closer to the top (and thus receives more RF energy) than the back. Going through the curved pipe, the low-energy tail takes the shortest path and catches up to the head, making the bunch shorter.
The second step is similar. Further down the linac, where they have been accelerated to higher energy, the bunches are tipped to ride slightly ahead of the wave crest, so the rear gets accelerated more than the front. Entering a detour with four bends, this time the higher-energy tail takes the shortest path and catches up again, compressing the bunch to 50 microns.
The final step exploits an effect previously considered a nuisance. As the electron bunches travel at the speed of light, they generate an electric wake (similar to the wake a boat makes) called a wakefield. But instead of spreading out and dissipating, the wake made by the head of the bunch bounces off the pipe the electrons travel in and interferes with the tail. This makes another energy gradient between head and tail, resulting in a final compression of the electron bunches to 12 microns.
At this point, the bunches can be wiggled by an undulator magnet in order to emit X-rays for studying various materials. Or they can remain as electrons for use in studying the properties of wakefields.
--by Heather Rock Woods
The article appeared in the October 15, 2003, edition of the Stanford Report, Stanford University, and is reprinted here with permission.
Related Web Links
Sub-Picosecond Pulse Source
Sub-Picosecond Photon Source to Illuminate Chemical Reactions
A Sub-Picosecond Photon Pulse Facility for SLAC
Funding: The Sub-Picosecond Pulse Source project and the Stanford Synchrotron Radiation Laboratory are supported by the DOE Office of Science's Basic Energy Sciences program.
The Stanford Linear Accelerator Center is a U.S. Department of Energy national laboratory whose mission is to design, construct, and operate state-of-the-art electron accelerators and related experimental facilities for use in high-energy physics and synchrotron radiation research.