Optical Klystrons and High-Gain Harmonic Generation

Optical klystrons have been in use for decades, and the first use was in an ultraviolet free-electron laser oscillator. An optical klystron (OK) is fundamentally composed of a modulator wiggler that imposes a velocity modulation on the electrons followed by a magnetic dispersive section that enhances the modulation prior to injection into a radiator wiggler that takes the interaction with the modulation-enhanced electrons to saturation. The magnetic dispersive element is, typically, a three- or four-dipole chicane. We have discussed the use of strong chicanes to compress the electron bunch and weak chicanes to control the phase shift between the electrons and the optical field in the gaps between wigglers. The chicanes used in optical klystrons are intermediate between these other two applications. The radiator can be tuned to the fundamental or a harmonic of the modulator, in which case the interaction is referred to as high-gain harmonic generation (HGHG). In this chapter, we discuss the physical concept underlying the optical klystron and present a comparison between an optical klystron and a conventional wiggler. We also discuss multistage optical klystrons and high-gain harmonic generation (HGHG) in both a single, second harmonic configuration and a multistage harmonic cascade.