Analysis of a broadband quasi-optical mode converter for gyrotrons working in multi modes

A broadband quasi-optical mode converter that could function at three segment-continuous frequencies in three different waveguide modes is investigated. The three modes and frequencies are TE
_11,2
mode at 351-361 GHz, TE
_12,2
mode at 375-385 GHz and TE
_13,2
mode at 398-410 GHz. The mode converter consists of a Vlasov launcher, three reflecting mirrors and an output window. It converts rotating high-order waveguide modes into free-space Gaussian beams. The key to manage broad bandwidths at different modes is to select proper center frequencies corresponding with waveguide modes. Moreover, another crucial part to realize broad bandwidths is a relatively large radius of the Vlasov launcher. From the calculation of a program code called Gyrotron Quasi-optical Mode Converter (GQMC) that is developed, it shows that the mode converter could generate well-shaped Gaussian beam with efficiency higher that 77.3% at different modes and frequencies. This mode converter could be used in step-tunable multi-mode gyrotrons which could achieve continuous broadband radiation in different modes.