Roles of Ion Motion and of Return Currents in Transverse Instability of Colliding Plasmas

Proper account of the ion dynamics is shown to lead to a significant change in the extent of the unstable range of transverse modes. However, no very high growth rates can be produced by the ions. For relativistic electron beams in a cold plasma, the role of return currents induced in the plasma and of plasma collisions are analyzed. The return currents do not influence the transverse instability except for the extreme relativistic region γnb/np>1 where the plasma electrons “take over” in maintaining the growth rate high. Collisions are unimportant unless their frequency, ν≥ωp2 (plasma) /ωp (beam), where transverse oscillations cease to be electrically neutralized, so that pinching becomes impossible and the growth rate of the instability goes to zero as 1/ν.