By Heather Rock Woods
Using all two miles of the linear accelerator (linac), as
well as loops and bends in the beam, and a usually troublesome effect
called a wakefield, SLAC has made the world’s shortest bunches of
electrons: 12 microns (millionths of a meter) long and 80 femtoseconds
(one quadrillionth of a second) fast.
During its first run in May, the Sub-Picosecond Pulse
Source (SPPS) made high current, ultra short bunches of electrons and
turned them into very bright, ultra short pulses of x-ray light. These
first x-rays made by a linear accelerator are 1,000 times shorter than
those made by storage rings like SPEAR, enabling direct observations of
atomic motion in matter that have never been seen before.
Physicists have always packed billions of electrons into
bunches in order to acquire enough meaningful data. Now, manipulating the
shape and size of the bunches has become like a science in itself.
SPPS relies on several tricks to compress the bunches,
which contain 21 billion electrons, in order to reach a peak current of 30
kiloAmperes. That’s about 1,000 times greater than the current found in a
household fuse. "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 SPPS
accelerator physicist Patrick Krejcik (AD).
The gymnastics occur in three stages, starting as the
bunches leave the damping rings near the beginning of the linac. There, a
bunch travels around the curve of the ring-to-linac (RTL) beamline and
gets compressed from 6 mm down to 1.2 mm.
To compress electron bunches, 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.)
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. Bunches can be adjusted to ride on the slope of
the wave, where they receive less energy as the slope descends. In the RTL,
the bunch looks like a surfer climbing a wave: the front of the bunch has
more energy (i.e., is closer to the top) than the back. Going through the
curved path of the bending magnets, the low-energy tail takes the shortest
path and catches up to the head, making the bunch shorter.
The second step in bunch compression takes place at Sector
10, one third of the way down the linac, where the electrons have been
accelerated to nine billion electron volts of energy. Here the bunches are
tipped to ride slightly ahead of the wave crest, so the rear gets
accelerated more than the front. Entering a chicane with four bends, the
higher-energy tail is able to take the shortest path and catch up again,
compressing the bunch to 50 microns. Paul Emma (ARDA) calculated that this
was just the right place to bend the beam. Lynn Bentson (AD) oversaw
installation of the chicane, and Cherrill Spencer (NLC) designed the bend
magnets in a way that would not introduce any optical aberrations into the
The final step in compressing the bunch is something that
could only be done at SLAC, picking up energy along the remaining 1.3
miles of the linac and using 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. In
free space, the wake would spread out perpendicular to the travel path of
the electrons, but in the beam pipe, the wake made by the head of the
bunch bounces off the pipe and interferes with the tail.
Thus the tail has less energy than the head when a bunch
reaches the end of the linac. Fortuitously, the bunch can be routed
through the Final Focus Test Beam (FFTB), where the beam line jogs right
then left. This geometry forces the higher-energy front to take a longer
path, and the rear catches up again. Here, the bunch has rotated upright
again and is now 12 microns long. At this length, the bunch of 21 billion
electrons whizzes by a fixed point in 80 femtoseconds. After the
compression, the bunches are wiggled by an undulator magnet to generate
the x-rays. Eric Bong (AD) installed the undulator, on loan from Argonne
"We need a way to measure the bunch length, so part two of
the project is inventing new technologies to measure on the sub-picosecond
timescale," Krejcik said.
The group resuscitated a specialized accelerator cavity
first used here in the 1960’s that kicks the beam vertically (see TIP
September 2000, "Rediscovering Deflecting Structures at SLAC") and
inserted it into the beam line. When turned on by a klystron, this
transverse deflecting cavity samples a bunch by sweeping it vertically
across a screen where the vertical length gives a projection of bunch
length when it is 50 microns. The SPPS collaboration is developing
electro-optic sampling techniques, borrowed from the world of fast laser
technology, to measure the bunches in the FFTB.
SPPS will operate over the next two fiscal years, taking
data in anticipation of the Linac Coherent Light Source (LCLS) that will
make even brighter x-rays. The ultra-short bunches will also be delivered
to the E-164 experiment during its run in the next fiscal year.
UPDATE: October 1, 2003:
SSRL is coordinating and managing SPPS as a consortium involving
laboratory and university participants. Foreign partners, especially
Sweden's Uppsala University and Germany's DESY, are also making
significant contributions. —Heather Woods