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Friday, 27 February 2004

LCLS - Faster with Foil

Paul Emma, K. Bane, M. Cornacchia, Z. Huang, H. Schlarb, G. Stupakov, and D. Walz

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 extremely short, a few femtoseconds (a femtosecond is a quadrillionth of a second), in duration. This pulse length, which is a factor of more than 200 times shorter than the LCLS baseline design, will dramatically increase LCLS' x-ray time resolution, giving scientists the ability to study the movement of matter at atomic scales and accessing the structural changes occurring in the making or breaking of chemical bonds. The key to short x-rays 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 created by 4 magnets that pull the electrons temporarily off course. The slotted foil will take advantage of the bunch orientation within the compressor to perturb 99% of the electrons and produce an effective bunch as short as a few femtoseconds in duration.

As electron bunches proceed down the linac, for LCLS they will be accelerated with 14.3 billion electron volts of energy by radio frequency (RF) waves. Using a special mode of acceleration, the tail of the electron bunch can be given a higher energy than the head (technically referred to as a chirped beam). When the higher-energy electrons at the end of the bunch take a shorter route around the bends through the compressor magnets, they catch up to the front, in effect making the bunch shorter. The slotted foil will be placed at the crest of the bunch compressor's bend, where the electrons are spread out perpendicular to their trajectory (and there is a correlation between space and time). A mere 100 million electrons in the center of the bunch will successfully pass through the 250 micron (one millionth of a meter) slit in the foil unaltered; the other 6 billion electrons will penetrate the foil and are subsequently scattered in such a manner that they are not amplified by the laser amplification process. It is this selective amplification that yields a few femtosecond slice of electrons, which then create the ultra-short x-ray pulse when they are passed through the long LCLS undulator.