W-band Cherenkov maser based on a periodic surface field structure

Two-dimensional (2D) Periodic Surface Lattices have been useful in producing distributed feedback in a Free-Electron Maser driven by an oversized electron beam of annular geometry [1]. The Periodic Surface Lattices in this case act as frequency selective mirrors allowing the production of narrow band microwave output. Such 2D periodic structures observed using a hollow, copper, cylindrical waveguide with a sinusoidal grating machined into the walls, where the diameter of the waveguide is much larger than λ (D >> λ) have also a potential for application in Cherenkov sources of high power microwave and THz radiation [2]. In this case localised surface fields are excited around the perturbations if the structure is radiated by an external source (for example an electron beam) [3-5]. The eigenfield created within the structure can be described as a superposition of a near cut-off volume field which synchronises the localised surface partial fields. A relativistic electron beam travelling close to the structure interacts with the spatial harmonics of the partial surface field propagating with a phase velocity less than speed of light. In this paper we demonstrate a novel high Q cavity operating at W-band (75-110GHz), where there is coupling between a near cut-off TM_0,6 volume field and an evanescent HE_1,20 surface field, produced within the periodic structure. Results of the numerical modelling of the Cherenkov maser capable of producing high power microwave radiation at W-band frequencies using the PIC code MAGIC will be presented.