Before it’s 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.

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. "It’s 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 compressors—3-sided detours in the linac studded with four magnets that pull the electrons temporarily off course—similar 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 bunch shorter.

The slotted foil is placed at the crest of the bunch compressor’s 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. We’re 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 didn’t 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.