The plasma dynamics in the beam double layer between the conducting boundaries with fixed potentials is investigated by means of numerical simulations. It is shown that the electron oscillatios can form the hybrid structure which consists of the potential distribution arising in the genyle regime of the Pierce instability and the layer potential. For the nonlinear oscillations taking account of the ion motion the system returns periodically to the nearly initial state if a size of the high potential region is comparable with the layer width. If the homogeneous regions are narrow, the double layer is stable.