Cherenkov instability driven by an unmagnetized electron beam in a periodically corrugated waveguide is studied by developing a new version of self-consistent linear theory. The conventional space charge and cyclotron modes of magnetized beam degenerate in this case. The normal electromagnetic modes are pure transverse magnetic (TM) and transverse electric (TE) modes in axisymmetric cases and are hybrid modes having both TM and TE components in nonaxisymmetric cases. Using the developed linear theory, the axisymmetric and nonaxisymmetric Cherenkov instabilities due to the unmagnetized beam in the periodic system are examined numerically. The unmagnetized beam can excite the axisymmetric TM and nonaxisymmetric hybrid modes. A comparison of the numerically obtained results is made with those of the recent C-band Pasotron experiment.
Backward wave oscillators (BWOs) have been studied as a candidate high-power microwave source. To increase the operation frequency, an oversized slow-wave structure (SWS) is used. The operation at reduced voltage is preferable for practical applications. This work is aimed at numerically examining the operation mode of a weakly relativistic oversized BWO. We examine not only the axisymmetric transverse magnetic mode but also the non-axisymmetric hybrid modes of the oversized SWS. Both of them are candidates for the operation modes. These modes are surface waves whose fields are concentrated near the SWS wall. They overlap in frequency and are not separated by stop-bands. For an efficient beam interaction, the injected electron beam needs to be controlled more accurately than in the non-oversized SWS case.
The Cherenkov instability used in slow-wave devices has been well studied in the literature. However, in previous analyses, the beam motion is restricted to the longitudinal direction assuming an infinitely strong magnetic field. For the finite strength magnetic field, the transverse beam perturbation cannot be ignored and leads to the slow cyclotron instability. Recently, a new version of self-consistent field theory considering three-dimensional perturbation has been developed based on a solid beam, in which the effect of the transverse perturbation appears as a surface charge at a fixed boundary. In the case of a thin annular beam, the boundary is modulated and is essentially different from the solid beam case. We propose a self-consistent field theory considering the moving modified boundary surface. The slow cyclotron instability due to the modulation of an infinitesimally thin annular electron beam is presented.
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