Filamentation instability of ion acoustic waves driven by a relativistic electron beam

Filamentation has been observed in several beam-plasma interaction experiments. The well-known Weibel or filamentation instability has been suggested as a mechanism to account for the experimental observations. However, unless the plasma is very collisional, the two-stream branch of the beam-plasma instability will have a larger growth rate than the Weibel branch. The high-frequency two-stream instability heats the plasma electrons while leaving the ions relatively unperturbed. When this happens, the electron temperature will rapidly exceed the ion temperature and ion acoustic waves can be excited in the system. In addition to the usual return current driven ion acoustic or Buneman instabilities, the authors find that the beam can excite ion acoustic waves directly. Because the speed of relativistic beams greatly exceeds the sound speed in the plasma, the instability exists only very obliquely to the beam direction. Thus, this instability would manifest itself by filamentation of the beam and plasma. The fully electromagnetic dispersion relation for relativistic beams has been analyzed for this instability. Details of the linear dispersion analysis and particle-in-cell simulation results are presented