  We study collective processes for an electron beam propagating through a
background plasma using simulations and analytical theory. A new regime where
the instability of a Langmuir wave packet can grow locally much faster than ion
frequency is clearly identified. The key feature of this new regime is an
Electron Modulational Instability that rapidly creates a local Langmuir wave
packet, which in its turn produces local charge separation and strong ion
density perturbations because of the action of the ponderomotive force, such
that the beam-plasma wave interaction stops being resonant. Three evolution
stages of the process and observed periodic burst features are discussed.
Different physical regimes in the plasma and beam parameter space are
demonstrated for the first time.
