Improved Suppression of Beam-Tunnel Parasitic Oscillations by Introducing Lossy Diffusive Surfaces

High-power gyrotrons usually suffer from parasitic oscillations, appearing in the beam tunnel region, which degrade the electron beam quality and the interaction efficiency in the cavity. To avoid the development of such oscillations, several design concepts have been proposed and tested in the past, including the stacked beam tunnel concept, the ceramic-loaded smooth-wall beam tunnel, and the fully metallic beam tunnel with broken symmetry. In this work, we focus on the fully metallic concept by examining the possibility of using diffusive surfaces to effectively suppress the parasitic modes. To this end, a beam tunnel with a diffusive surface is designed, having dimensions that are compatible with a typical high-power gyrotron. The performance of the structure is studied by full-wave particle-in-cell (PIC) beam-wave interaction simulations and is compared to that of a smooth metallic beam tunnel of the same dimensions. Both low- and high-Ohmic wall losses are considered in the study. Simulation results demonstrate that the diffusive surface can indeed secure an almost perfect suppression of the parasitic modes, even when the Ohmic wall losses are quite low, i.e., similar to those of copper.