Space Charge Waves in Cylindrical Plasma Columns

TLDR

Space‑charge waves can propagate in finite‑cross‑section plasmas even without drift or thermal motion, offering a diagnostic tool that operates at frequencies below the plasma frequency. The authors model the plasma as a dielectric, solve the field equations, and examine the influence of a steady axial magnetic field while neglecting ion motion and electron temperature. They find low‑frequency and backward waves, verify their properties experimentally in a mercury arc column, and use them to measure electron density, confirming results from other methods.

Abstract

When a plasma is of finite transverse cross section, space-charge waves may propagate even in the absence of a drift motion or thermal velocities of the plasma. Some of the properties of these space charge waves have been investigated by regarding the plasma as a dielectric and solving the resulting field equations. The effect of a steady axial magnetic field is considered, but motion of heavy ions and electron temperature effects are neglected. Waves are found to exist at frequencies low compared with the plasma frequency as well as waves with oppositely directed phase and group velocities (backward waves). Many of the features of these waves have been verified experimentally by measuring phase velocity and attenuation of waves along the positive column of a low pressure mercury arc in an axial magnetic field. Measurements of electron density have been made using these waves and the results are compared with those obtained by other methods. An interesting feature of these measurements, of value in plasma diagnostics, is that they can be made with frequencies which are small compared with the plasma frequency.