  A hybrid kinetic-fluid model is used to study ionization waves (striations)
in a low-current plasma column of DC discharges in noble gases. Coupled
solutions of a kinetic equation for electrons, a drift-diffusion equation of
ions, and a Poisson equation for the electric field are obtained to clarify the
nature of plasma stratification in the positive column and near-electrode
effects. A simplified two-level excitation-ionization model is used for the
conditions when the nonlinear effects due to stepwise ionization, gas heating,
and Coulomb interactions among electrons are negligible. It is confirmed that
the nonlocal effects are responsible for the formation of moving striations in
DC discharges at low plasma densities. The calculated properties of
self-excited waves of S, P, and R types in Neon and S type in Argon agree with
available experimental data. The reason for Helium plasma stability to
stratification is clarified. It is shown that sustaining stratified plasma is
more efficient than striation-free plasma when the ionization rate is a
nonlinear function of the electric field. However, the nonlinear dependence of
the ionization rate on the electric field is not required for plasma
stratification. Striations of S, P, and R types in Neon exist with minimal or
no ionization enhancement. Effects of the column length on the wave properties
have been demonstrated in our simulations.
