Scale-invariant Helical Magnetic Fields from Inflation

We discuss a model which can generate scale-invariant helical magnetic fields on large scales ($\lesssim 1$Mpc) in the primordial universe. It is also shown that the electric conductivity becomes significant and terminates magnetogenesis even before reheating is completed. By solving the electromagnetic dynamics taking conductivity into account, we find that magnetic fields with amplitude $B\simeq {10}^{-15}\mathrm{G}$ at present can be generated without encountering a backreaction or strong coupling problem.