KINETIC FEATURES OF THE INSTABILITY OF AN ULTRARELATIVISTIC ELECTRON-BEAM IN A DENSE-PLASMA

The instability of a monoenergetic electron beam in a cold magnetized plasma is studied analytically and numerically for relativistic beam e nergies. A study is made of the nonlinear saturation of the growth of the oscillation amplitude caused by the phase bunching of the beam ele ctrons and the dependence of the phase velocity of the wave on the amp litude of its electric field. It is shown that, when the conditions fo r the nonlinear phase matching are satisfied, the velocity of the beam electrons traveling in the decelerating phases decreases with the sam e rate as the wave velocity. As a result, these electrons interact wit h the nonlinear wave longer than in the case of a linear wave and can transfer a larger part of their energy to the wave. For greater beam e nergies, the diffusion of the electrons in velocity space is suppresse d; the evolution of an ultrarelativistic electron beam can be describe d by the linearized hydrodynamic equations; in this case, the instabil ity is stabilized by the plasma nonlinearity.