Influence of Collisions on the Instability of Cold and Warm Electron Beams in a Plasma

The effect of Coulomb collisions on the growth of the instability of monoenergetic beams and beams of finite initial velocity spread in a plasma has been investigated experimentally. The results are compared with existing theories and with computations based on a beam with Lorentzian distribution function. It is shown that the instability is suppressed by collisions if the beam has a sufficiently large velocity spread in agreement with the prediction of Ascoli and Singhaus and that their stability criterion is valid over a larger parameter range than had previously been assumed. Evidence is presented for a self-quenching of the instability of an initially monoenergetic beam due to the combined effect of collisions and the smearing out of the beam distribution function by the unstable waves.