  When studying the interaction of matter with extreme fields using
multipetawatt lasers, there are two limiting cases maximizing either the
electric field or the magnetic field. Here, the main attention is paid to the
optimal configuration of laser beams in the form of an m-dipole wave, which
maximizes the magnetic field, and the corresponding production of pair plasma
via a QED cascade using 10-PW class lasers. We show that the threshold of
vacuum breakdown with respect to avalanche-like pair generation is about 10 PW.
Using 3D PIC modeling in the specified fields, we go deeper into the physics of
vacuum breakdown, i.e. we examined in detail the individual trajectories of
particles produced in inhomogeneous electric and magnetic fields, the
space-time distributions of pair densities on the avalanche stage, and the
energy distributions of charged particles and gamma photons. The forming plasma
structures represent concentric rings around the central magnetic axis, which
can result in significant change of laser-plasma interaction in comparison with
the case of an e-dipole wave.
