A Microfabricated Planar Helix Slow-Wave Structure to Avoid Dielectric Charging in TWTs

Modifications to a slow-wave structure (SWS) consisting of a planar helix with straight-edge connections are proposed so as to avoid dielectric charging problem when it is used in a traveling-wave tube (TWT). The planar helix SWS is suitable for microfabrication using materials such as silicon (Si) and silicon dioxide (SiO₂). First, through a simple dielectric slab model, it is shown that the phenomenon of dielectric charging can be accurately simulated using Computer Simulation Technology Particle Studio. Next, a more realistic double-dielectric model is considered to obtain guidelines to avoid dielectric charging. Based on these guidelines, two simple modifications are suggested for the planar helix SWS when it is microfabricated using Si wafers and a layer of SiO₂. The modifications consist of partial removal of the SiO₂ layer and a careful enhancement of the conductivity of the Si layer. The simulation results are presented for a Ka-band planar helix SWS to demonstrate a very significant reduction in dielectric charging while maintaining a low insertion loss when these modifications are incorporated. The proposed technique is compatible with microfabrication and is expected to be useful in microfabricated millimeter-wave TWTs.