Superposition of Phase-Space Hole Streets in Vlasov Plasmas

We present a study of a collision--less plasma governed by the Vlasov--Poisson system of equations in one space and one velocity dimension; the plasma is subject to initial density perturbations and to both periodic and non periodic space boundary conditions. For sufficiently large values of the perturbation's amplitude and sufficiently small values of the Landau damping rate we observe the development of {\em two} streets of phase--space holes each consisting of two counter--streaming families of holes. The two streets move in the phase space at different speeds and they may consist of a different number of holes. The fast moving street develops faster when periodic boundary conditions are used, but it appears to be more prominent and robust under non periodic boundary conditions. In two instances, the distribution function decays within the slow moving street; it remains unaffected within the fast moving street, but it has much smaller values there.