Design and simulation studies of a metal PBG cavity for millimetre wave gyrotrons

Abstract

The design and simulation studies of a metal Photonic Band Gap (PBG) cavity operating in TE 32 mode for 140 GHz gyrotron are presented. The radius of cylindrical PBG cavity at 140Hz has been calculated as 2.73 mm. This is realized by making a defect in a regular periodic PBG lattice that is approximately equal to radius of the conventional waveguide for TE 32 mode at 140 GHz frequency. The Eigen mode analysis for the removal of both seven and nineteen rods has been studied. The operating for the PBG cavity is located in the global band gap diagram which is obtained through the electromagnetic mode analysis of regular PBG structure. In the present work seven rods are removed to have the desired radius of the cavity in TE 32 mode at 140 GHz. The periodicity and radius of the rod are calculated as 2.03mm and 0.79mm respectively. The designed metal PBG cavity is modelled and simulated using “CST Microwave Studio”. The reflection and propagation characteristics of the PBG cavity are studied at 140 GHz using the time domain solver. The reflection (S 11 ) and transmission (S 21 ) coefficients are obtained as −22 dB and −0.86 dB respectively at 140 GHz. The mode confinement inside the PBG cavity has been studied using the Eigen mode solver. We observed that the designed TE 32 mode has confined at 139.58 GHz that ensures the mode purity at millimetre wave operation. Further, the quality factor of cavity operating in TE 32 mode has been calculated as 8890. The quality factor of other nearby competing modes also has been observed. The nearby modes including TE 23 , and TE 13 are well generated inside the defect at 174.16 GHz and 149.68 GHz respectively. The quality factors of PBG cavity is compared with the quality factor of simple cylindrical cavity for the desired and competing modes. It was observed that the quality factor of TE 32 mode in PBG cavity is greater than other competing modes as compared to simple cylindrical cavity. The metal PBG structures offer good mode purity; hence the mode competition problem in gyrotron is eliminated.