Spontaneously excited longitudinal backward waves in dusty plasmas

Abstract

A spontaneously excited longitudinal backward dust wave was experimentally examined. Dust particles were confined in a vertical glass cylinder. The spontaneously excited wave propagated downward. A dispersion relation was obtained by measuring the wavenumber and phase velocity. The dispersion relation showed that the wave consisted of two different modes, including a backward wave. The Trivelpiece–Gould (TG) mode was invoked as a dispersion relation to explain the backward wave, although the cyclotron angular frequency, ωc, appearing in the TG mode was replaced with ωa ∼ ωpd, where ωpd is the dust angular frequency. The experimental results were compared with the calculated dispersion relation, including the TG mode. The group velocity of the spontaneously excited backward wave is larger than the value predicted with the TG mode. It is necessary to explain the backward wave to modify the TG mode model or to construct a new model.