Oversized Modified Bragg Cavities for High-Power Long-Pulse Subterahertz Free-Electron Lasers

We study a new modification of the Bragg structures based on the coupling of traveling and quasicritical waves, as well as the possibility to use it as an electrodynamic system of free-electron masers (FEMs) and lasers (FELs). It is shown that inclusion of quasicritical waves in the feedback loop allows one to improve significantly the selective characteristics of Bragg structures and ensure the stable regime of single-frequency generation at a rather large (as compared to the wavelength) transverse size of the interaction space. Simulation and "cold" electrodynamic tests performed in the millimeter- and submillimeter-wave ranges confirm high efficiency of application of such structures. Operability of the new feedback mechanism has been confirmed in a series of joint experiments performed by the Joint Institute for Nuclear Research (Dubna, Russia) and the Institute of Applied Physics (IAP) of the Russian Academy of Sciences (Nizhny Novgorod, Russia) with FEL/FEM-based generators in frequency bands from K-a to W for transverse cavity sizes of up to five wavelengths. The project of a high-power long-pulse FEL operated in the subterahertz and terahertz frequency ranges, which is developed jointly by the IAP and the Budker Institute of Nuclear Physics, is discussed. The results of modeling an electrodynamic system based on modified Bragg reflectors having diameters of 20 and 40 wavelengths for implementation of FELs in the 300 and 600 GHz ranges, respectively, are presented.