Low-Frequency Waves and Instabilities on the Positive Column in a Magnetic Field. III. Experiments on the m = 1 Azimuthal Mode

In papers I and II of this series [Phys. Fluids 12, 303 and 316 (1969)], a theory describing low-frequency wave propagation along a weakly ionized positive column in a magnetic field, Bz, was developed and studied numerically. Paper III describes experimental verification of the analysis, based on observation of electron waves propagating along a low-pressure mercury-vapor discharge. The theory predicts that for propagation in the direction of the axial electron drift, an azimuthal mode number m ≥ 1, and rotation in the sense of the electron diamagnetic drift, these helical waves should show a transition with increasing Bz from damping to spatial growth. Measurements of the wave dispersion characteristics as a function of Bz are presented for the first two radial modes of m = 1. They show reasonable quantitative agreement with theory. The behavior of the spectrum of self-excited waves, and the dependence of the steady axial electric field on Bz, are interpreted in terms of the linear perturbation theory. It is suggested that, for a low-pressure discharge, instability sets in first for high-order modes, localized near the outer radius, and spreads to low-order modes as Bz is increased.