THE GROOVE GUIDE AS AN INTERACTION STRUCTURE FOR A MICROWAVE FEL

Microwave FELs need a waveguide to confine the radiation and to control the group velocity of the radiation. If the electron bunches are short (some tens of ps), the control of the group velocity allows the FEL to operate in different regimes (steady-state or superradiant) [1]. The Groove Guide (GG) is a low loss, laterally open structure; it consists of two parallel conducting planes with two grooves trapping the electromagnetic energy. The dimensions of the GG are of the order of some wavelengths, and, differing from the ''standard'' oversized waveguide, the GG supports a single TE(01)-like mode, if suitably designed. Moreover, the electromagnetic energy density, trapped by the grooves, is higher than in the rectangular oversized guide, for the same amount of power flowing, leading to a stronger coupling between the electrons and the radiation. In this paper the advantages of using a GG as a FEL interaction structure are discussed. Experimental measurements and characterization in the Ka band of prototypes of straight section, bend, transition from/to standard WR-28 rectangular waveguide are presented.