A kinetic equation describing the nonlinear evolution of intense
electromagnetic pulses in electron-positron (e-p) plasmas is presented. The
modulational instability is analyzed for a relativistically intense partially
coherent pulse, and it is found that the modulational instability is inhibited
by the spectral pulse broadening. A numerical study for the one-dimensional
kinetic photon equation is presented. Computer simulations reveal a
Fermi-Pasta-Ulam-like recurrence phenomena for localized broadband pulses. The
results should be of importance in understanding the nonlinear propagation of
broadband intense electromagnetic pulses in e-p plasmas in laser-plasma systems
as well as in astrophysical plasma settings.Comment: 16 pages, 5 figures, to appear in Phys. Plasma
