In linear potential approach instability of an electron beam in cold plasma for various distribution functions of electrons of a beam on velocities is considered. It is shown that at increase in spread of electrons of a beam on velocities there is a transformation of the mechanism of beam instability in plasma: the hydrodynamic one-particle mode of instability transforms into the hydrodynamic collective mode, or into the one-particle kinetic mode. Analytically and numerically instability growth rates in various modes are defined.
Department of physical electronics, Faculty of Physics, M.V.Lomonosov Moscow State University. Moscow 119991, Russia