Mastering microbunching for linac-based light sources

An electron bunch, as seen on the spectrometer screen. By controlling the microbunching instability, this bunch is completely split into about 10 sub–bunches spaced 25 micrometers apart. The ability to control the number of sub-bunches, the distance between them, and their intensity could be beneficial for a number of light sources.

by Boris Podobedov, Sergei Seletskiy, & Xi Yang
http://phys.org/news/2013-08-mastering-microbunching-linac-based-sources.html#jCp
(Phys.org) —Designing accelerators requires years of research and development. Throughout the Lab's history, scientists and engineers at Brookhaven have helped lead the way in designing accelerator technologies for cutting-edge facilities here on site and at institutions around the world.

Our team in the Photon Sciences Directorate and Yuzhen Shen, who is now with the U.S. Patent and Trademark Office, recently tackled a significant problem in accelerator design, a phenomenon called "microbunching instability" that has been identified as one of the most serious challenges to the performance of advanced linear accelerator (linac)-based light sources.

A bit about light sources and microbunching
Light sources, including the National Synchrotron Light Source (NSLS) and the future NSLS-II at Brookhaven, are important tools for producing ultra-bright light that scientists can use to analyze the atomic and molecular structures for advances in different areas of science, ranging from biology and physics to chemistry and geophysics, as well as medicine and materials science. Some light sources are linac-based and others, such as NSLS and NSLS-II, are based on storage rings. In both kinds, the bright light—large quantities of photons—is produced from electron beams that travel in bunches through certain accelerator components, for instance bending magnets or undulators.
Electrons are never distributed completely evenly inside a bunch and any small "noise" in an unstable bunch can cause the electrons to form microstructures as they clump closer or spread further apart. This effect, the microbunching instability, results in chaotic changes of beam density distribution. In extreme cases, especially for more intense bunches that provide higher brightness at light sources, bunches simply split apart. This effect degrades the quality of electron beams and the photon beams they produce.
The microbunching instability is rather common and especially important in the fourth generation light sources, such as the Linear Coherent Light Source at SLAC National Accelerator Laboratory in California. They rely on short-wavelength free electron lasers (FELs) that use short, dense bunches of electrons traveling at nearly the speed of light to make short photon bunches.