Nonquasineutral theory of ion-acoustic resonances in bounded nonuniform plasmas and comparison with experiments

A theory is developed of ion‐acoustic resonances in inhomogeneous plasmas of the positive column type using a fluid plasma description. Poisson's equation is properly taken into account, instead of applying the usual assumption of wave quasineutrality. The order of the differential equation describing the wave is thereby increased by two and the singularity that previously occurred as a result of the presence of a sheath region is thereby removed. It is shown that ion‐acoustic resonances are confined within the plasma core, the effective boundary not being the wall, but the region in the plasma where the macroscopic ion flow velocity attains the Bohm velocity ${\mathrm{(KT}}_e{\mathrm{/m}}_i{)}^{\text{(1/2)}}$ . The theoretical predictions are substantiated by a number of experiments taken from the literature and supplemented by a series of new experiments. The theory sheds new light on the problem of wave reflection from a negatively polarized wall and the damping of ion‐acoustic waves propagating in inhomogeneous plasmas.