Hybrid Mode in Traveling-Wave Tube With Partially Plasma-Loaded Helix and Hollow Electron Beam

The linear theory of a partially plasma-loaded helix-type traveling-wave tube in the presence of a hollow electron beam is considered. In this configuration, the formation of hybrid waves and the role of these modes on the coupling of the hollow electron beam/wave are discussed. The plasma effect on the gain and the degree of concentration of the axial fields are investigated in the plasma-loaded region. It is shown that the strong interaction and the gain enhancement occur in the frequency that the hybrid wave is formed. The effects of radius and thickness of the hollow electron beam on the gain are studied as well. Strong gain enhancement is observed when the electron beam comes close to the edge of plasma.