The Helically-Wrapped Circular Waveguide.

This paper contains a theoretical investigation into a form of helical waveguide intended for use within a microwave amplifier. The helical waveguide is similar to the commonly-used tape helix in that an electromagnetic wave corkscrews around a central electron beam. The progress of the wave down the longitudinal axis of the structure is thereby slowed, allowing for interaction with space-charge waves on the electron beam and consequent amplification of the electromagnetic wave. The primary advantage of a helical waveguide over a tape helix is the greater heat capacity of the solid structure. Unfortunately, a closed structure like the helical waveguide has a low-frequency cutoff, making it more dispersive than the open-structured tape helix. Previous analyses have shown the helically-wrapped rectangular waveguide to be quite dispersive, limiting its usefulness as a slow-wave structure. This paper considers the possibility of reducing the dispersion of the helical waveguide by using a circular waveguide helix instead of a rectangular waveguide helix. Switching to a circular boundary changes the loading conditions over that of a flat boundary, straightening out the dispersion curves and reducing the low-frequency cutoff of the helical waveguide.