Electro-mechanical modes in plasma waveguides

In addition to increasing the cut-off frequencies of TM-modes in a waveguide, the introduction of a plasma column into the waveguide also introduces new modes of propagation. The properties of these modes, including the effect of an axial d.c. magnetic field but neglecting ion motion, have been studied by solving the field equations considering the electron plasma as a dielectric. The new modes generally have phase velocities much less than the velocity of light; one type exists down to zero frequency and another type is a backward wave. Neither the metallic conductor nor the axial magnetic field is essential to the existence of slow modes. Angular-dependent modes can exhibit Faraday rotation of polarization. A qualitative explanation of these modes is given in terms of an equivalent electrical circuit for the transmission line. Many of the properties of these modes have been verified experimentally by measuring phase velocity of waves along a mercury-arc discharge in an axial magnetic field. These modes are closely related to spacecharge waves in electron beams, and several interesting microwave applications are suggested.