By Anna Gosline
Before itís even built, SLAC physicists are making it better. Computer
simulations have shown that by using a cleverly placed piece of slotted
foil, the Linac Coherent Light Source (LCLS) will be able to produce
brilliant x-ray pulses that are a staggering 1 femtosecond (quadrillionth
of a second) in duration.
How long is a femtosecond? In one second, an
electron traveling near the speed of light can almost reach the moon.
In one femtosecond, it can just pass through a sheet of plastic wrap.
(Image provided by Diana Rogers)
This is the latest achievement in a long line of research completed in
anticipation of the LCLS facility, due to start construction in 2006. The
one femtosecond x-ray pulse length will allow physicists to see the
fleeting movement of matter at subatomic scales. "Itís the holy grail of
light sources," says SLAC researcher Paul Emma (ARDA), who first imagined
the foiling plot.
Without the foil insert, the x-ray pulse design standard is 230
femtoseconds; fast enough to record the making and breaking of chemical
bonds and atomic scale processes of liquid flow, melting and freezing. But
there are two reasons to push for even shorter pulses.
Like the shutter speed on a camera, pulse length dictates the speed of
movement that can be observed by a light source. "The shorter the pulse
length the better the resolution. You cannot observe phenomena that occur
in the 1 femtosecond time scale with a 200 femtosecond pulse," said Max
Cornacchia (ASD), who coordinated the thin foil research for LCLS.
Long pulses of high energy radiation also have a tendency to destroy
the molecules they are trying to illuminate. Ultra-short pulses will allow
researchers to use bright light to view complex molecules before they begin to break down.
The key to short x-ray pulses is compressing the electron bunches that
create them. In the LCLS, bunches will be shortened with bunch
compressorsó3-sided detours in the linac studded with four magnets that
pull the electrons temporarily off courseósimilar to a traffic circle. The
slotted foil will take advantage of the bunch orientation within the
compressor to weed out 99 percent of the electrons and produce an
effective bunch only 1 femtosecond long.
As electron bunches proceed down the linac, they are pumped with 14.3
billion electron volts of energy on the rollercoaster of radio frequency
(RF) waves. On their final dip towards the bunch compressor, the tail of
the electron bunch has more energy than the head. Like a race car on the
inside track, the higher energy electrons at the end of the pack take a
shorter route around the bend and catch up to the leaders, making the
The slotted foil is placed at the crest of the bunch compressorís bend,
where the electrons are spread out perpendicular to their trajectory. A
mere 100 million electrons in the center of the bunch successfully pass
through the 250 micron (one millionth of a meter) slit in the foil; the
other six billion electrons penetrate the foil and are subsequently too
hot and scattered to produce x-ray radiation further down the line. It is
this selective scattering that yields a 1 femtosecond slice of cool
electrons, which then create the ultra-short x-ray pulse.
"Using a foil to scatter electrons is nothing new. Weíre just using the
scattering in a different way," said Emma.
The LCLS is a delicate and sensitive machine, and researchers were
concerned the proposed electron scatter could ruin the light source
completely. Physicists didnít even know if a 1 femtosecond pulse was
possible until Zhirong Huang (ARDA) demonstrated it with computer
calculations. Using the foil can also introduce a wakefield, a nuisance
effect created by electrons as they travel through the foil. Karl Bane
(ARDA), Gennady Stupakov (ARDA) and Holger Schlarb (DESY) studied this
potential problem and revealed that it would not interfere with x-ray
production. Finally, Dieter Walz (EFD) showed that the foil itself could
withstand continual electron bombardment.
Using the foil also comes at a certain cost. Paring down the effective
electron bunch means that less x-rays are produced. Though the decreased
intensity will not be a limiting factor for any planned experiment,
researchers working on LCLS design are continually searching for ways to
shorten pulse length while maintaining the full intensity.
With the continued ingenuity of SLAC researchers and support from DOE,
LCLS will be an internationally unparalleled light source, giving
scientists a look at the magic of matter at previously unimaginable