Density filamentation nonlinearly driven by the Weibel instability in
  relativistic beam plasmas
  Density filamentation has been observed in many beam-plasma simulations and
experiments. Because current filamentation is a pure transverse mode, charge
density filamentation cannot be produced directly by the current filamentation
process. To explain this phenomenon, several mechanisms are proposed such as
the coupling of the Weibel instability to the two-stream instability, coupling
to the Langmuir wave, differences in thermal velocities between the beam and
return currents, the magnetic pressure gradient force, etc. In this paper, it
is shown that the gradient of the Lorentz factor can, in fact, represent the
nonlinear behavior of a plasma fluid and further that the nonuniform Lorentz
factor distribution can give rise to electrostatic fields and density
filaments. Simulation results together with theoretical analyses are presented.
