Multifrequency generation in free-electron lasers with quasi-optical resonators

Abstract

According to the general concept of the modern theory of self-oscillatory systems with many degrees of freedom, in free-electron lasers (FELs) with multimode resonators and high-quality (low-emittance) electron beams the increase of the electron current results in a sequence of bifurcations. If the excess of the current over self-excitation threshold is moderate, then the electron beam organizes the ‘cold’ resonator modes into strictly periodic wave structures (supermodes). At a large excess over threshold the self-oscillations become stochastic with a quasi-continuous frequency spectrum. In FELs with broad dispersion of the initial electron velocities (high emittance) a large number of modes with random phases can coexist in practice at any excess of the current over threshold; the energy distribution in the mode spectrum can be described by the quasi-linear theory. If the electrodynamic methods ensure suppression of ali the modes except for the operating one, then in the case of low-quality electron beams, high efficiency (up to 50%) can be achieved by stochastic (multiperiodic) wave pumping.