It is shown that the interpenetration of two ionized streams is arrested, as a rule, not because of individual collisions between particles belonging to opposite streams, but because the whole system of charged particles is unstable. The smallest wavelength of an unstable oscillation is λmin where $\lambda_{min} = \surd \left(\frac {\pi m_0 U^2}{2N \epsilon^2}\right) \left(1 - \frac {U^2}{c^2} \right) ^{-\frac {3}{4}}$ Here ± U are the velocities of the undisturbed streams, and N is the density of electrons in each.

A further calculation for the non-relativistic case deals with the amplification of the plasma oscillations present in two colliding streams. It is shown that these grow rapidly and that $\tau_{crit} \surd (m_0 U^2|\pi N \epsilon^2$ is the distance of interpenetration achieved before the counterstreaming of the electrons is brought to a halt. The value of τcrit depends only insensitively on the ratio of the internal plasma energy densities Tpl to the kinetic energy densities Tkin in the streams. For example, τcrit = 9·0 when Tpl:Tkin = 1:10, and τcrit = 19·0 when Tpl : Tkin = 1:105.