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, $B_z$ , 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 $\text{m\hspace{0.17em}≥\hspace{0.17em}1}$ , and rotation in the sense of the electron diamagnetic drift, these helical waves should show a transition with increasing $B_z$ from damping to spatial growth. Measurements of the wave dispersion characteristics as a function of $B_z$ are presented for the first two radial modes of $\text{m\hspace{0.17em}=\hspace{0.17em}1}$ . 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 $B_z$ , 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 $B_z$ is increased.