RF input for relativistic sectioned amplifiers

In order to input RF radiation into Cherenkov sectioned amplifiers driven by relativistic electron beams mode converters are usually used that convert the TE₁₀ mode of rectangular waveguide to the rotating TE_m1 mode of cylindrical waveguide, which is the operating mode of the first section. The dimensions of such converters are found using simple relations from geometrical optics, which represent the electromagnetic fields with rays propagating at the speed-of-light for modes with large azimuthal indices m. However, even for small m such an approach allows one to find dimensions close to the optimal one, although necessary corrections obtained from cumbersome experiments for conversion coefficients and frequency bandwidths are required. In this present work using computer simulations with the Agilent HFSS [1] code we find the corrections to optimal dimensions giving maximal conversion coefficients over wide frequency bandwidths for such mode converters.