Two-stream instability in plasmas for arbitrary propagation

The problem of the two-stream instability in warm, collisionless, field-free plasmas is investigated for arbitrary direction of propagation vector, using the moment equations. It is found that the configurations characterized by (i) a beam of electrons with ions immobile, and (ii) all electrons moving through mobile ions, are stable for subsonic interpenetration speeds. For supersonic speeds, however, the configurations are unstable both for electrostatic and electromagnetic perturbations. The former instability is restricted only to a cone of propagation about the streaming direction, while the latter exists for all angles, and has growth rates that are even larger than the electrostatic growth rates in the limit of long-wavelength perturbations.