Limitations on the attainable intensity of high power lasers

It is shown that even a single $e^-e^+$ pair created by a super strong laser field in vacuum would cause development of an avalanche-like QED cascade which rapidly depletes the incoming laser pulse. This confirms the old N. Bohr conjecture that the electric field of the critical QED strength $E_S=m^2c^3/e\hbar$ could never be created.