Physics of two-beam free-electron lasers

The paper is concerned with the physical background of the operation of two-beam free-electron lasers (FELs). Both quasilinear and quadratic mechanisms of the superheterodyne amplification of electromagnetic waves are considered. It is asserted that, at the low- and medium-power levels, two-beam FELs will become the main amplifying devices in commercial systems operating in the submillimeter and IR wavelength ranges.