TM11 to HE11 mode converter in overmoded circular corrugated waveguide

The proposed study contains the design and analysis of TM11 to HE11 (hybrid) mode converter at 42 +/- 0.2 GHz which is an integral component of the transmission line that transmits 200 kW power from gyrotron to the plasma in SST-1 Aditya tokamak. HE11 mode is preferred, as the transmission loss in corrugated waveguide is less in comparison with other modes. For generating HE11 mode from TM11 mode, the depth of the annular slots in the mode converter is varied gradually from zero to one-quarter wavelength. The conversion efficiency and radiation pattern of the proposed mode converter depends on the corrugation profile and the length of the mode converter. Both the parameters are optimised numerically to achieve the highest conversion efficiency. Keeping in mind mechanical fabrication of mode converters optimisation has been carried out. It converts 95.6% power of TM11 mode into HE11 (82.2% TE11+13.4% TM11) mode respectively. The 1 dB compression bandwidth of 19.04% (8 GHz) at 42 GHz is obtained. The length of mode converter is 40.83 lambda mm. The reflection coefficient and VSWR are -25 dB and 1.1 respectively. The power in side lobes and isolation from cross polar component is less than -20 dB and -25 dB respectively.