Laboratory experiments on ion waves associated with parallel electric fields

Abstract

We have performed laboratory experiments to study the property of waves along a quasi‐static parallel electric field in a plasma column. The parallel electric field is setup by allowing two different sources of plasmas to interpenetrate each other. Since all ions accelerated by the electric field will gain the same energy across the potential drop, the lighter ion species will acquire higher velocities. The wave modes associated with a single ion species (Ar+) and two ion species (He+ and Ar+) are quite different. In the plasma with only one species, the fast mode of the ion‐electron acoustic waves is observed in our experiment. This mode is always damped, and the results are analyzed by the theory. When the plasma consists of two ion species, the linear theory of ion acoustic waves indicates that the velocity difference between two ion species of different masses can excite ion‐ion acoustic waves via the two‐stream instability. Experimentally, unstable waves generated by the costreaming He+ and Ar+ ions are found to exist. By actively launching waves near the unstable frequencies at the plasmas, we observe that the waves grow at a certain range of distance. The results are quantitatively explained with the kinetic dispersion relations for the ion acoustic waves.