  QED cascades in intense electromagnetic field can occur if the dynamical
quantum parameter $\chi$ of a seed electron, which in Compton units coincides
with the electron proper acceleration, attains the order of unity. We derive
general expression for $\chi$ of an initially slow electron in an arbitrary
electromagnetic field for a time range $t\ll 1/\omega$, where $\omega$ is the
field carrier frequency. Using this formula, we consider a special field
configuration of multiple colliding focused laser beams and optimize it to
provide cascade development at laser power below $10$ PW and intensity of the
order of $10^{23}$W/cm$^2$. Such parameters of the beams will be obtained with
a new generation of laser facilities, particularly the ELI Beamlines, in the
coming years.
