Synchrotron light and the Lyncean Compact Light Source
Conventional synchrotron light sources store multi-GeV electron beams in large rings of magnets to generate intense, bright 1 Å wavelength radiation. In a third generation synchrotron, the radiation is produced as the electron beam passes through a special magnet called an undulator magnet. An undulator magnet with a 2 cm-period requires an electron beam with an energy of about 5 GeV and a storage ring that is about 300 m in diameter.
The Compact Light Source uses a "laser undulator" to accomplish the same result. A laser beam colliding with an opposing electron beam has the same effect as an electron beam passing through an undulator magnet. The electric and magnetic fields of the laser beam cause the electron to wiggle and induce a radiation spectrum similar to that from a long undulator magnet. This radiation is often referred to as Inverse Compton Scattering. If a laser beam with a wavelength of one micron is used, the electron beam energy necessary for 1 Å radiation is only about 25 MeV. This reduces the scale of the device by a factor of about 200 and results in a storage ring with a footprint comparable to that of a large office desk.
A model of the Lyncean CLS is shown in Figure 1. An electron pulse is generated using an RF photocathode. This electron pulse gains energy by passing through accelerator sections, similar in size to the accelerators used in medical linacs. Once a chosen energy is reached, the pulse is injected into the storage ring. The storage ring is designed to allow a bunch to circulate for about one million turns. Periodically, a new pulse is injected to refresh the stored beam in order to maintain a steady, high quality X-ray output beam.
Figure 1. Conceptual model of the Lyncean CLS
On one side of the storage ring, the electron beam is transversely focused to a small spot. This side of the ring also serves as one path of an optical cavity for a laser pulse. A laser resonantly drives the optical cavity to build-up a high-power laser pulse. By using an enhancement cavity, for example, a 10W laser can build up about 100kW of circulating power. The cavity length is adjusted so that round-trip travel time of the photon pulse in the optical cavity is equal to the revolution time of the electron bunch in the storage ring. The electron bunch and the laser pulse collide in the interaction region producing a burst of X-rays which are directed in a narrow cone in the same direction as the electron beam.
The native X-ray spectrum of the laser pulse/electron bunch collision is equivalent to that of a 20,000 period undulator magnet. The X-rays can be focused using conventional X-ray optics down to a size of about 45 microns. Fine-tuning of the X-ray energy (for scans near absorption edges) is achieved with a monochromator adjustment just as with a synchrotron beamline. A large change in X-ray energy, necessary for targeting different elements, is achieved by tuning the electron beam energy. The configuration described above operates with a similar photon flux up to X-ray energies of many tens of kilovolts. The naturally narrow bandwidth of the X-ray spectrum means the total produced X-ray power is already within the energy bandwidth useful for most X-ray applications. And since this power is only a fraction of a Watt, the relatively low power minimizes unwanted background scatter and simplifies X-ray optic design.