Connected Pair of Planar Helices With Straight-Edge Connections for Application in TWTs

A novel planar slow wave structure (SWS) consisting of a connected pair of planar helices with straight-edge connections (PH-SEC) has been proposed in this paper. It is shown with the help of dispersion and coupling impedance characteristics that this structure offers a larger electron-beam tunnel and lower risk of backward-wave oscillation compared with a single PH-SEC. Fabrication using printed-circuit techniques is reported and measured S-parameters over 1-7 GHz are presented to demonstrate the ease of feed-design and wideband characteristics for the new SWS. Using particle-in-cell simulations, hot-test parameters such as the output power, gain, and efficiency are also obtained and reported. It is shown that the connected pair of PH-SECs has a higher gain growth rate than that for the single PH-SEC. The proposed SWS is also compared at frequencies around 60 GHz with a recently reported structure which is derived from meander-line SWS; the comparison shows that the connected pair of PH-SECs offers a significantly higher coupling impedance. With a good potential for fabrication using microfabrication techniques, the connected pair of PH-SECs can be an attractive candidate for application in mm-wave traveling wave tubes.