Longest Linac Makes the World's Shortest
Electron Bunches
Researchers at the U.S. Department
of Energy's Stanford Linear Accelerator Center used
the world's longest linear accelerator to develop the
world's shortest bunches of electrons. Conversion of
these bunches into bright, short pulses of X-ray light
enables researchers to directly observe atomic motion
never seen before in solids and liquidsallowing
for instantaneous snapshots of simple chemical reactions
in progress.
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The Klystron Gallery—the
two-mile long linear accelerator (linac) at the
Stanford Linear Accelerator Center—is one
of the longest facilities buildings on Earth. It
was built in the mid-1960s about 31 miles south
of San Francisco and stretches through the rolling,
oak-studded hills behind the Stanford University
campus to the base of the Santa Cruz mountains.
Since this powerful scientific instrument began
operating, SLAC has been generating intense, high-energy
beams of electrons and photons for research on the
structure of matter. Physicists using its facilities
have received three Nobel prizes for the discovery
of the quarks and the tau lepton, both recognized
today as fundamental building blocks of matter. |
November 3, 2003Using
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 kiloAmperesabout
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 power28 billion electron voltson
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.
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To compress electron bunches,
the SPPS accelerates them below the crest of RF
energy waves (shown top). That way, one end of the
bunch has more energy than the other. When the bunch
goes through the chicane in Sector 10, the lower-energy
head of the bunch takes the longer path (shown middle)
and the tail catches up (shown bottom), effectively
rotating the bunch to be shorter. (Graphic by Patrick
Krejcik.) |
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.
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