Dispersion Control and Size Enlargement of Helical Slow-Wave Structure by Double-Positive Metamaterial Assistance

A novel dispersion control technique, using I-shaped double-positive metamaterial (MTM) as the helical slow-wave structure (SWS) support, exhibiting low-loss, broadband response, has been proposed for wideband traveling-wave tubes (TWTs). The proposed SWS provided the desired flat-to-negative wideband dispersion characteristics giving higher interaction impedance than that is obtainable by conventional vane-loaded helical SWS. A coaxial transmission line-based simulation model has been used to retrieve the effective relative permittivity and permeability of the I-shaped MTMs. The electromagnetic analysis in the sheath-helix model was validated against the CST Microwave Studio simulation with respect to the dispersion characteristics within 14%–16% and interaction impedance versus frequency characteristics within < 12%. Moreover, the proposed structure can be made to exhibit positive, flat, or negative dispersion characteristics by controlling the dimensions of the I-shaped metal strips without altering the dimensions of the helical SWS and metal envelope of the structure. Furthermore, with the help of the proposed structure, the diameter of the structure could be increased without adversely affecting the shape of the dispersion characteristics and reducing the value of the interaction impedance. The structure will reduce the fabrication complexity at higher frequencies and make the structure suitable for high-power, high-gain, and high-efficiency wideband TWTs.