Energy recovery linacs as synchrotron light sources

Pushing to higher light-source performance will eventually require using linacs rather than storage rings, much like colliding-beam storage rings are now giving way to linear colliders. r 2003 Elsevier Science B.V. All rights reserved. PACS: 07.85.Qe; 07.85. m Synchrotron radiation sources have proven to be immensely important tools throughout the sciences and engineering. In consequence, synchrotron radiation usage continues to grow, assuring the need for additional synchrotron radiation machines well into the foreseeable future. Storage rings, the basis for all major existing hard X-ray synchrotron facilities, have inherent limitations that constrain the brightness, pulse length, and time structure of the X-ray beams. Since large synchrotron X-ray facilities are expensive and take many years to bring to fruition, it is important to consider if there are alternatives to storage rings that offer advantages. The Energy Recovery Linac (ERL) is a proposed alternative synchrotron radiation source that will extend synchrotron radiation science into new realms not possible with storage rings while still being able to serve most existing storage ring applications. The usage of synchrotron radiation has become more specialized and sophisticated. Although no single source will meet every need, the trend is towards higher brilliance and intensity, and shorter bunches. These trends are driven by a desire to study smaller and smaller samples that require more and more brilliance, because tiny samples tend to have both low absolute scattering power and require micro-X-ray beams. An increasing number of experiments also require transversely coherent X-ray beams. It can be shown that transverse coherence increases with the brilliance of the source. Another trend is towards examination of time-resolved phenomena in shorter and shorter time windows. The time resolution of many experiments is ultimately limited by the duration of the X-ray pulse. If the time envelope of the X-ray pulse is sufficiently short and the source is sufficiently brilliant, it can be shown that an increasing number of X-rays will also be longitudinally coherent, that is, in the same quantum mode. So the optical characteristics generally desired from synchrotron radiation sources are brilliance and short pulses. These desired characteristics immediately translate into requirements on the electron bunches. The average brilliance scales inversely to the bunch transverse emittances and linearly with the average *Corresponding author. Tel.: +1-607-255-3441; fax: +1-