Low-Frequency Waves and Instabilities on the Positive Column in a Magnetic Field. II. Study of Asymmetric Modes

An analysis was presented previously [H. N. Ewald, F. W. Crawford, and S. A. Self, Phys. Fluids 12, 305 (1969)] for the normal modes of low-frequency wave propagation on the positive column in an axial magnetic field, and dispersion characteristics were discussed for axisymmetric (m = 0) ion-acoustic and electron wave modes. These studies are complemented with an examination of asymmetric modes (m ≠ 0). First, the stability of a low-pressure positive column without axial drift has been investigated. It is found that with increasing magnetic field, instability first occurs in high-order modes. Next, the theory has been applied to the positive column over a wide pressure range. The computed results are compared with previous approximate analyses of the current-convective instability. Finally, the specific prediction of the theory that asymmetric electron wave modes, rotating in the sense of the electron diamagnetic drift (m ≥ 1), should grow convectively for sufficiently large magnetic field has been checked experimentally. Measurements on the propagation of the m = 1 wave on a low-pressure mercury-vapor positive column are presented which support this prediction.