  Ion sound instabilities driven by the ion flow in a system of a finite length
are considered by analytical and numerical methods. The ion sound waves are
modified by the presence of stationary ion flow resulting in negative and
positive energy modes. The instability develops due to coupling of negative and
positive energy modes mediated by reflections from the boundary. It is shown
that the wave dispersion due to deviation from quasineutrality is crucial for
the stability. In finite length system, the dispersion is characterized by the
length of the system measured in units of the Debye length. The instability is
studied analytically and the results are compared with direct, initial value
numerical simulations.
