  The speed at which ions enter a sheath is a fundamental property of a plasma
that also provides a useful boundary condition in modeling. A recent theory
proposed that this can be significantly influenced by an instability-enhanced
friction force arising from two-stream instabilities in the presheath when
multiple ion species are present. Although experiments appeared to confirm this
theory, recent particle simulations have brought it into question. We reconcile
this controversy using direct numerical solutions of the dispersion relation,
which show that there is a dependence on the electron-ion temperature ratio
that was not considered previously. In addition, particle-in-cell simulations
are used to show that ion-ion two-stream instabilities can arise near the
sheath edge and generate an enhanced ion-ion friction force. Only by accounting
for the instability-enhanced friction force can theory predict the simulated
ion speeds at the sheath edge.
