Competition between Buneman and Langmuir Instabilities

The electron-ion beam instabilities are studied by one-dimensional electrostatic particle-in-cell simulation.The simulation results show that both the low-frequency Buneman mode and high-frequency Langmuir wave(LW)are excited in the nonlinear phase.The power of Buneman instability is stronger than that of the LW.The Buneman instability is firstly excited.Then the backward LW appears,which is probably excited by the particles trapped in the wave potential and moving opposite to the original beam direction.After some time,the forward LW can be found,which has a larger maximum frequency than that of the backward LW.With the decrease of the electron drift velocity,the instabilities become weaker;the LW appears to have almost equal intensities and becomes symmetric for forward and backward propagation directions.The LW can also heat the electron,so the relative drift speed cannot far exceed the electron thermal speed,which is not helpful to the development of Buneman instability.