It is shown that stimulated synchrotron emission and absorption can act simultaneously on a relativistic electron beam with a narrow energy spread to reduce the spread significantly. This beam ‘‘cooling’’ mechanism acts most effectively for beam energies belonging to a sequence of discrete values, one for each synchrotron harmonic. The discrete energy values are absolute, and are not influenced by external parameters such as magnetic field strength. Application of this mechanism to cool beams for free-electron lasers, synchrotron-radiation lasers, and slow-wave cyclotron amplifiers is suggested.

• Received 4 February 1991

DOI:https://doi.org/10.1103/PhysRevLett.66.2312