Role of electron temperature at extremely low density in negative positive ion plasma

Abstract

It is pointed out that the role of electrons in the dynamics of pair ion and negative positive ion plasmas cannot be neglected even at extremely low density of electrons, i.e., ne0n+0≪mem+ (where nj0 is the background density of jth species and mj is the mass of the particles of the j-species while j = e, +, −) because electron thermal velocity is almost always larger than thermal velocities of ions, i.e., vT± ≪ vTe. An analysis of electrostatic waves in unmagnetized negative positive ion electron (NPIE) and pair ion electron (PIE) plasmas is presented for the case ωpe ≪ ωp+ (where ωpj=(4πnj0e2mj)1/2 is the plasma oscillation frequency corresponding to j-species). The electron dynamics contribute to electrostatic perturbations at ion plasma oscillation time scale at longer wavelengths for λDe2k2<1 where λDe=(Te4πne0)1/2 is the electron Debye length. On the other hand, the electron plasma wave turns into thermal wave when the conditions ωpe ≪ ωp± and 1≪λDe2k2 hold simultaneously and ion acoustic wave approaches the sum of ion plasma oscillation frequencies of positive and negative ions. The only electrostatic normal mode of such a plasma is the ion plasma wave corresponding to longer wavelengths, which also includes the contribution of electrons. The electron thermal wave is separated from plasma oscillations and electron time scale disappears with respect to electrostatic perturbations. Similar situation also occurs in plasmas having negatively charged dust particles. To elaborate these points, the analytical results are applied to the two experiments with NPIE and PIE plasmas.