Powerful coherent terahertz radiation from short photo-injector electron bunches is required for many applications including mastering terahertz and x-ray frequency ranges. If the effective length of the electron bunch is shorter than the wavelength of the radiated wave, then the coherent spontaneous emission process does not need a special electron bunching and starts immediately. However, the initial coherence of radiation is rapidly broken due to increase in the bunch phase size, in particular, because of the strong Coulomb repulsion of particles inside a dense bunch. Stabilization of the bunch lengths or even compression of dense electron bunches in terahertz electron sources may be provided by various methods described briefly in this work.
A natural problem arising in the case of realization of a THz electron maser with a high-current relativistic electron beam is the use of an oversized cavity when it is difficult to provide selective excitation of a definite transverse mode. Our idea is to give up working on a fixed transverse mode, and to use excitation of a supermode formed by a fixed set of several transverse modes of an oversized waveguide. We propose to use the Talbot effect to create an oversized microwave system that provides a high Q-factor for this supermode. We present a design of a Free-Electron Maser fed by a 10 MeV / 2 kA / 200 ns electron beam and based on excitation of a Talbot-type supermode at a frequency close to 2 THz. The presentation includes results of our multi-frequency multi-mode simulations of the electron-wave interaction during the spatio-temporal process of formation and amplification of the supermode in an oversized microwave system. The calculated efficiency of this FEM at the level of 5-10% corresponds to the GW level of the output power.
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