Cherenkov radiation of electromagnetic waves by electron beams in the absence of an external magnetic field
In conventional sources of coherent Cherenkov electromagnetic radiation, the electrons move linearly, guided by external magnetic fields. In the absence of such fields, the electrons can move radially, being affected by the beam self-fields as well as by the radial component of the electric field of the wave. This radial motion can, first, improve the coupling of electrons to the field of a slow wave localized near the wall of a slow-wave structure, and second, cause an energy exchange between the electrons and the wave due to an additional transverse interaction. This interaction, in particular, can lead to an experimentally observed excitation of nonsymmetric transverse electric waves in Cherenkov devices. In plasma-filled sources, the beam self-fields can be compensated for by ions, leading to a known ion focusing of the beams. In such regimes, the beam can be surrounded by an ion layer creating a potential well for electrons which can be displaced from stationary trajectories by transverse fields of the wave. The operation of such sources when the presence of ions and the radial electric field of the wave play competing focusing and defocusing roles, and electron interception by the walls restricts the output power level, is analyzed in stationary and nonstationary regimes.
Nusinovich
GS
Institute for Plasma Research, University of Maryland, College Park, College Park, Maryland 20742, USA.
Bliokh
YP