LCLS, a free-electron x-ray laser, will use the last kilometer of the three kilometer SLAC linear accelerator; the world's longest & highest energy electron linac.

Short, intense bunches of electrons are injected into the linac. As the linac accelerates the bunches they pass through bunch compressors which pack them into even shorter bunches. The electrons are then excited (agitated) as they pass through an undulator magnet. Once accelerated & compressed these electron bunches pass through a long undulator magnet, where they emit radiation (x-rays) as they oscillate in the alternating magnetic field. This magnet trick is basically how undulators in the 50-odd synchrotron light sources around the world operate.

One key difference between current synchrotron sources and LCLS is that the x-rays will be emitted coherently and at the same wavelength—the essential properties of a laser. Coherent means that all the x-ray photons are in phase with each other and going in the same direction, like skiers making simultaneous, in-sync S-turns down a mountain slope. [Conventional lasers excite electrons that are bound to atoms within the lasing cavity. LCLS is a “free electron” laser because the electrons are independent from atoms: they fly down the linac unchaperoned.]

LCLS photons will be emitted between 1.5 and 15 Å corresponding to 8keV to 800eV, much shorter wavelength and much higher energy than visible light. In particular the short wavelength end, 1.5 Å, is ideal for studies on the atomic scale, where dimensions are of this order. There are 10 billion Ångströms in one meter.

To make the x-ray beam tremendously bright, LCLS will use a new technique called Self-Amplified Spontaneous Emission. SASE takes advantage of the interaction of an internal electron bunch with the spontaneously emitted x-rays travelling along with this bunch as it traverses the periodic magnetic field of the undulator. This interaction results in a microbunching of the electron beam which greatly amplifies the number of emitted x-rays. This means one bunch of 6 billion electrons can generate a pulse with one trillion coherent x-rays. More x-rays results in sharper pictures of smaller things.

This powerful combination—laser-like x-ray beams with extreme brightness (a trillion x-rays in a needle-thin beam), with short wavelength (on the scale of atoms) and short pulse duration (1 to 230 femtoseconds)—makes LCLS a revolutionary machine.