Radiative cooling of relativistic electron beams

Radiative cooling is a natural and effective method of phase-space cooling for stored electron beams. In electron storage rings the average effects of synchrotron radiation from the bending magnets cause the beam emittances in all three degrees of freedom to damp towards equilibria, determined by the fluctuating nature of quantum emissions. In this paper, we show that the radiation damping in a focusing system is fundamentally different from that in a bending system. Quantum excitation to the transverse dimensions is absent in a straight, continuous focusing channel, and is exponentially suppressed in a focusing-dominated ring. Thus, the transverse normalized emittances in such systems can in principle be damped to the Compton wavelength of the electron, limited only by the Heisenberg Uncertainty Principle. In addition, we investigate methods of rapid damping such as radiative laser cooling. We propose a laser-electron storage ring (LESR) where the electron beam in a compact storage ring repetitively interacts with an intense laser pulse stored in an optical resonator. The laser-electron interaction gives rise to fast cooling of electron beams and can be used to overcome the space-charge effects encountered in a medium-energy circular machine. Applications to the designs of ultra-low-emittance damping rings and compact X-ray sources are also explored.