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Investigation of Instabilities in a Folded-Waveguide Sheet-Beam TWT

Particle-in-cell (PIC) simulation was used for analyzing the instabilities due to zero-drive oscillations in a 220-GHz folded-waveguide (FW) traveling-wave tube. The equivalent circuit analysis based on the transmission line approach was used for the parametric design of the FW structures (FWSs) with circular and rectangular (horizontal and vertical) beam holes. Experimental validations with reference to the dispersion and interaction impedance characteristics were carried out on a scaled model of the slow wave structure in the Ku band. The oscillations observed in various configurations of the FWS were subsequently analyzed using the Brillouin diagram. The FWS with horizontal-rectangular beam hole was found to have a better immunity to instability compared to the other two configurations.