ACOUSTIC DOUBLE-LAYERS IN MULTISPECIES PLASMA

Particle simulation in a one-dimensional bounded system is used to examine the formation of acoustic double layers in the presence of two ion species. Double-layer formation depends critically on the details of the distribution functions of the supporting ion populations, and their relative drifts with respect to the electrons. This paper examines the effect of having two ion components, an H+ and an O+ beam, on double-layer evolution from ion acoustic turbulence driven by an electron drift relative to the H+ beam of almost-equal-to 0.5u(e), where u(e) is the electron thermal speed. The ratio of ion drifts is taken to be consistent with acceleration by a quasi-static auroral potential drop (i.e., nu(H)/nu(O) = square-root M(O)/M(H) = 4.0). It is found that acoustic double layers form in either ion species on the time scale tau almost-equal-to 100omega(ps)-1, where omega(ps) is the ion plasma frequency for species ''s'' and s = H or O, and for drifts relative to the electrons lower than that required for double layer formation in simulations of single ion component plasma. The effect of including a background (H+) component instead of an O+ beam is examined elsewhere [1].