The Compact Light Source builds on the US investment in large synchrotrons, but with a new idea that allows the source to be very compact. Existing synchrotron light sources at US facilities employ multi-GeV electron beams stored in large rings of magnets to generate intense, bright 1 Â wavelength radiation. The Compact Light Source uses a marriage of an electron beam and laser beam to accomplish the same effect. To achieve 1 Â wavelength radiation with a 2 cm-period undulator, the energy of the beam must be about 5 GeV; to achieve this energy the synchrotron is about 1000 ft in diameter. On the other hand, a laser beam colliding with an opposing electron beam has the same effect as an undulator. The electric and magnetic fields cause the electron to wiggle and induce a radiation spectrum similar to a long undulator magnet. This radiation is typically referred to as Compton Scattering or Inverse Compton Scattering. If a laser beam with a wavelength of one micron is used instead of an undulator, the electron beam energy necessary for 1 Â radiation is only about 25 MeV. This reduces the scale of the device by a factor of about 200; a 25 MeV electron storage ring is so small that it can easily fit within the footprint of a large desk (click here for size comparison).

A conceptual model of the CLS is shown in Fig. 1. The ring is injected with a short linear accelerator that accelerates the electron beam to the full energy desired in the ring. For an energy of 25 MeV, the accelerator is similar to those found in medical linacs. The CLS electron source produces only a single electron bunch using a Radio Frequency (RF) gun with a laser photocathode. A focused UV laser pulse striking a cathode emits electrons via the photoelectric effect in a very controlled process. The injector periodically (60-100 Hz) refreshes the electron bunch in the storage ring to maintain high beam quality.

The storage ring is designed to allow the bunch to circulate in a stable fashion for about one million turns. The beam is kept tightly bunched by an RF cavity. On one side of the ring in a straight section the electron beam is transversely focused to a small spot. This straight section also serves as one path of an optical cavity for the laser pulse. A CW mode-locked laser resonantly drives the optical cavity to build-up a high-power laser pulse. The cavity length is adjusted so that the photon pulse round-trip travel time is equal to the revolution time of the electron bunch in the ring. By suitable timing, the electron bunch and the laser pulse collide each turn at the interaction point producing a burst of X-rays.

The laser pulse/electron bunch collision produces an X-ray spectrum equivalent to a 20,000 period undulator magnet. The X-rays are directed in a narrow cone in the direction of the electron beam. They can be focused using conventional X-ray optics down to a size of about 60 microns. Fine-tuning of the X-ray energy (for scans near absorption edges) is achieved with a monochromator adjustment as is currently done in synchrotron beamlines. A large change in X-ray energy necessary for targeting different elements is achieved by directly tuning the electron beam energy. The configuration as described above operates with a similar photon flux up to X-ray energies of many tens of kilovolts. Since a significant fraction of the X-rays are already within the energy bandwidth useful for most X-ray applications, the total X-ray power is a fraction of a watt, instead of the kilowatts of power produced at synchrotrons. This naturally narrow bandwidth simplifies the required X-ray optics since component heating and additional radiation shielding are avoided.

Figure 1. Conceptual model of the CLS.