Effect of Plasma on Spectra of the Cherenkov Beam Instability in a Dielectric Waveguide

We report a study of the spectrum of the Cherenkov beam instability in a plasma dielectric waveguide. In the absence of plasma, the instability results from the induced Cherenkov radiation of many transverse slow electromagnetic modes of the waveguide. In the presence of plasma, the long-wavelength part of the spectrum is due to the excitation of a potential Langmuir wave, aperiodic self-modulation of the beam, and Cherenkov radiation of a surface plasma wave; the high-frequency part of the spectrum weakly depends on the presence of plasma in the waveguide.