Russian Peoples` Friendship University, Moskow, Russia

The particle-in-cell simulation of the electron oscillations in the double layer between the conducting electrodes with the fixed potentials is performed. It is shown that the existence of the plasma-beam system in the high potential side of the layer that is formed by the accelerated and the reflected particles leads to the unstable oscillations. When the low potential region has a sufficient size the formation of the virtual cathode is possible.