Coherent radiation in an optical klystron

In the past few years, following Madey's success at Stanford, much attention has been paid to free-electron lasers. In these devices coherent light is produced by the three-body interaction between an undulator (periodic transverse magnetic field), an ultrarelativistic electron beam, and an external light wave. Practically this interaction results in an electron bunch where the electrons are no longer uniformly distributed along the propagation axis but tend to aggregate into microbunches separated by the external light wavelength lambda/sub L/. Then all the electrons work with the same phase with respect to the external light which is amplified (or absorbed) as it passes through the undulator. When placed into an optical cavity this device constitutes a free-electron laser. However, the electron microbunching produces another noticeable effect. The radiation normally emitted by the electrons inside the undulator becomes coherently enhanced at wavelengths corresponding to the bunch-to-bunch separation: lambda/sub L//3,lambda/sub L//5,....(Only upper harmonics are emitted along the propagation axis.) When one has only limited length in which to put an undulator, and this is the case on most storage rings, these two effects can be greatly enhanced by using the optical klystron invented at Novosibirsk instead of an undulator. The optical klystron is an undulator where the central periods are replaced by a special magnetic section (dispersive section) whose role is to enhance the bunching of the beam before entering the last section (radiator). There the energy exchange with the external light and the coherent emission take place. The coherent emission of the various harmonics can be considered as a frequency multiplication of the input light although the energy is taken from the electrons kinetic energy