A Study of Electron Dynamics in a Split-Cathode Coaxially Within an Anode

The split cathode is a novel type of cathode structure applied in a cylindrical high voltage gap built from a coaxial vacuum transmission line that feeds it. A hollow electron beam is emitted from it. The cathode is attached to a conducting circular plate, the reflector, by a central metal rod. The cathode, rod, and reflector are at almost the same potential. An applied axial magnetic field magnetizes the electron beam which accelerates toward the reflector where it slows down and returns toward the cathode. For strong magnetic fields, while charge is trapped inside the cathode-anode-reflector space and has nowhere to go, the energy of the electrons reduces to what is known as a squeezed state. For lower magnetic fields, due to beam expansion in the radial direction which can result in anode current and overflow beyond the cathode or the reflector, squeezing is only partial. We show that quasiperiodic rotating transverse electron orbits appear in this two-stream flow, evidence for the diocotron instability. The dependence of the appearance of these low period shapes on various parameters and the possible connection of this behavior to the dynamics of the electromagnetic coupling in a magnetron type slow-wave structure is investigated.