Parametric excitation of ion density fluctuations in the relativistic beam-plasma interaction

When a weak and cold relativistic electron beam interacts with a dense plasma, it generates a nearly monochromatic wave of high intensity which triggers the growth of parametric instabilities. The present work investigates this process by using a hybrid technique in which the fully relativistic motion of the beam particles is treated numerically, while the behavior of the background plasma and the parametrically excited waves is treated analytically through appropriate linear dielectric functions. It is found that the self-consistent excitation of large ion density fluctuations leads to the decoupling of the beam from the background plasma, hence limiting the amount of energy that can be extracted from the beam. By adjusting the various parameters of the problem it is found that up to 23% of the initial beam energy can be transferred to the electrostatic waves. The large ion fluctuations responsible for decoupling the beam correspond to purely growing modes which exhibit secular growth in time.