Ion beam ordinary electromagnetic mode instability

The instability of the ordinary mode propagating perpendicular to the magnetic field is discussed for two ion beams counter-streaming along the magnetic field through a plasma of stationary electrons and ions. The instability criterion gives the necessary and sufficient condition for a purely growing mode. Although the instability threshold is high (u/Voi ≳ (2βoiNoi/Noi)-½ if the streaming velocity is 10% larger than the threshold value), large growth rates of the order of the ion cyclotron frequency result.