  In this paper, we are concerned with the structural stability of some steady
subsonic solutions for Euler-Poisson system. A steady subsonic solution with
subsonic background charge is proven to be structurally stable with respect to
small perturbations of the background charge, the incoming flow angles, the
normal electric field and the Bernoulli's function at the inlet and the end
pressure at the exit, provided the background solution has a low Mach number
and a small electric field. Following the idea developed in [19], we give a new
formulation for Euler-Poisson equations, which employ the Bernoulli's law to
reduce the dimension of the velocity field. The new ingredient in our
mathematical analysis is the solvability of a new second order elliptic system
supplemented with oblique derivative conditions at the inlet and Dirichlet
boundary conditions at the exit of the nozzle.
