Optimized multibeam configuration for observation of QED cascades

QED cascades in an intense electromagnetic field can occur if the dynamical quantum parameter χ of a seed electron, which in Compton units coincides with the electron proper acceleration, attains the order of unity. We derive a general expression for χ of an initially slow electron in an arbitrary electromagnetic field for a time range t1/ω, where ω 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 1023W/cm2. Such parameters of the beams will be obtained with a new generation of laser facilities, particularly the ELI Beamlines, in the coming years.