The results of three-dimensional electrodynamic modeling of photonic-crystal
double-gaps resonators designed for use in miniature multi-beams klystrons.
In the new resonators, the solid metal body is replaced by a similarly shaped lattice of
metal rods, allowing to suppress parasitic types of vibrations. It is shown that such oscillatory systems have an improved complex of electrodynamic and mass-dimensional parameters.
The conditions of tuning these systems to a highly efficient two-mode of interaction are investigated
with multi-beam electron flow simultaneously at two frequencies corresponding to the main (antiphase) and
the first higher (in-phase) types of oscillations.