  The growth rates of seeded QED cascades in counter propagating lasers are
calculated with first principles 2D/3D QED-PIC simulations. The dependence of
the growth rate on laser polarization and intensity are compared with
analytical models that support the findings of the simulations. The models
provide an insight regarding the qualitative trend of the cascade growth when
the intensity of the laser field is varied. A discussion about the cascade's
threshold is included, based on the analytical and numerical results. These
results show that relativistic pair plasmas and efficient conversion from laser
photons to gamma rays can be observed with the typical intensities planned to
operate on future ultra-intense laser facilities such as ELI or VULCAN.
