Excitation of the Talbot-type Supermode in Oversized Electromagnetic System of a Free-electron Laser

In THz electron maser based on the use of a high-current relativistic electron beam radiation, the use of an oversized electrodynamical system is necessary for a number of technical reasons. In this case, it is difficult to provide selective excitation of a definite operating cavity mode, because several modes are in resonance at once. We propose to use excitation of a supermode formed by a fixed set of transverse modes of an oversized waveguide due to the Talbot effect. We present designs of THz-frequency-range free-electron masers fed by a 10 MeV/2kA/200 ns electron beam and based on excitation of a Talbot-type supermodes. The calculated efficiency of these FEMs at the level of 5–10% corresponds to the GW level of the output power.

1. Introduction

There is growing interest in the creation of high power (the GW level) sources operating in the terahertz (THz) and sub-THz frequency ranges. Such sources should be based on the use of relativistic electron beams and oversized microwave systems. However, there are several problems caused by the necessity of oversized electrodynamical systems. The need for this is caused by a few reasons, for example, the problem of microwave breakdown of powerful radiation inside the cavity, the problem of transverse retention of the relativistic high-current beam, ohmic heating of the walls of the cavity, etc. It is difficult to ensure selective excitation of a certain transverse mode in this situation. In this situation, the spectrum of transverse modes is very dense, and it becomes difficult to provide the selective excitation of a single chosen operating transverse mode by the electron beam, as well as to provide selective single-mode feedback in an oversized system [1–5].