Planar slow-wave structure with parasitic modes control

Summary form only given: While planar helix structures have been previously investigated [1], the problem posed by mode competition in large aspect ratio structures has not been addressed. The paper focuses on this issue, with the goal to design a structure that can accommodate a large aspect ratio sheet beam and provide high gain, broad bandwidth while being immune to backward wave instability. Previously, we reported analysis and simulation of a sheath-like structure that can accommodate a sheet beam generated at the Naval Research Lab [2]. In this paper, we investigate a modified version which includes a mechanism for selective damping of parasitic waves. The structure proposed in [2] is considered the central region where the beam will propagate through. Connected to this region on both sides are tunnels with smaller spacing and conducting sheets with pitch different from central region. The dielectric supports on the sides are also made to be more lossy than one in the central region. The whole structure is then enclosed in conducting rectangular waveguide. The differences in sheath´s spacing and conducting sheets´ pitches will exclude the fields of the operating mode from the side regions. The beam and operating mode occupy the central region, where coupling between them occurs. The competing parasitic modes have large group velocities in the transverse direction and tend to be localized near a sheath, thus will freely extend to the side regions. This will act to reduce the Pierce impedance for the competing modes relative to operating mode. In addition, the lossy dielectric on the side regions will serve to increase the attenuation of the competing modes relative to the operating mode. With loss tangent of the lossy dielectric equal to 0.75, voltage and current of the beam source assumed to be 30.6kV and 4A, and based on the analysis of [3] and [4], the maximum stable length is computed to be 3.8cm with corresponding primary gain equals to 20dB. Investigat- on on bandwidth shows a high and steady gain from 35GHz to 50GHz.