Excitation of eigenwaves of a layered superconductor of cylindrical configuration by an electron beam

The instability of a nonrelativistic thin tubular electron beam propagating in vacuum above a cylindrical layered superconductor is theoretically investigated. The crystallographic c-axis of the superconductor is aligned with the cylinder axis. An analysis of the eigenwave spectra reveals that for non-zero azimuthal mode indices, the eigenwaves are hybrid, resulting from a superposition of ordinary and extraordinary Josephson plasma waves (JPWs). A new type of wave, termed a hybrid pseudosurface Josephson plasma wave, is predicted, which is shown to be the only mode that can exist in a specific frequency range above the Josephson plasma frequency. It is found that at the Josephson plasma frequency, a transformation of the spectrum occurs, where hybrid surface JPWs convert into the corresponding pseudosurface modes. The polarization of the eigenmodes is investigated, showing a transition from linear and circular polarization for surface modes near the vacuum light line to a generally elliptical polarization for pseudosurface modes at larger longitudinal wave numbers. An analysis of the instability growth rates, driven by both Vavilov–Cherenkov and anomalous Doppler effects, indicates that the Vavilov–Cherenkov mechanism yields the largest growth rates. These findings suggest that employing layered superconductors as slow-wave structures represents a novel approach for the development of THz radiation sources, offering advantages such as lower ohmic losses, dynamic frequency tuning, and a simpler geometry.