Anyone who has seen a strobe light in action knows that short, bright flashes of light can appear to ‘freeze’ the motion of a moving object. It is intuitive that to apparently freeze the movement of ever-faster moving objects, one needs ever-shorter pulses of light. The Sub-Picosecond Photon Source (SPPS) project takes this relationship to the Nth degree in order to produce informative images of movements that occur, even at the atomic level, during chemical reactions.

The SPPS project is made possible by an upgrade to the existing SLAC linear accelerator (linac), along with specialized new instrumentation, to produce and deliver x-rays from bright electron beam pulses. This project provides a fast, relatively inexpensive way to begin experiments with a new generation of very bright, sub-picosecond (less than one millionth of one millionth of a second), hard x-rays.

This illustration shows the general layout of the SPPS (Image courtesy of SPPS)

‘Brightness’ is the measure of the quality of the photon beam. Bright beams have a small cross-section and the x-ray paths are nearly parallel. The ‘pulse length’ is the measure of the time duration of the x-ray beam.

For example, the beam needs to be bright and have a very short pulse length for experiments that begin to probe the motion of atoms relevant to aspects of chemical dynamics. Both features are provided by the SPPS.

Each electron bunch extracted from the linac damping ring is compressed to 1.2 mm in the ring-to-linac beam line. Each electron bunch is further compressed to 50 mm (50 millionths of a meter) in the new 10 meter-long linac bunch compressor chicane that has recently been installed near the
one kilometer point in Sector 10.

The ‘dog-leg’ bend of the Final Focus Test Beam beamline at the end of the linac does the final compression, yielding an electron bunch of 12 mm (which in time equates to less than one tenth of a picosecond). Finally, a magnetic undulator—an array of permanent magnets—bends these bunches back and forth to produce ultra-short pulse, high-brightness x-rays whose time and special properties are derived from the electron beam.

The most unique feature of the SPPS will be its combination of brightness and sub-picosecond pulse length. The peak brightness of SPPS will exceed that of any existing hard x-ray source by several orders of magnitude and its pulse length has been simulated to be about 80 femtoseconds (about a millionth of a billionth of a second).

This combination will allow us to collect diffraction images of the atomic positions of relatively strongly scattering materials as they undergo changes in their atomic arrangements following an impulse from an ultrafast optical laser. These studies will provide the opportunity to gain direct insights into important processes such as structural changes during actual chemical reactions.

The SPPS will also play a role in the accelerator and x-ray optics R&D for the future Linac Coherent Light Source (LCLS) x-ray free electron laser project. For example, researchers will be able to develop and refine the diagnostic tools associated with the production and use of ultra-short electron and x-ray pulses. The researchers will gain valuable experience and expertise that will be needed for the successful early operation of the LCLS when it comes on-line, which is expected to be in 2008.

The Sector 10 chicane was installed last summer in the linac, and should be commissioned this winter. The first SPPS experimental run is scheduled for Spring 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.

For more information on SPPS, contact: Jerry Hastings (SSRL), Ext. 3107, jbh@slac.stanford.edu