  When the inflaton couples to photons and amplifies electric fields, charged
particles produced via the Schwinger effect can dominate the universe after
inflation, which is dubbed as the Schwinger preheating. Using the hydrodynamic
approach for the Boltzmann equation, we numerically study two cases, the
Starobinsky inflation model with the kinetic coupling and the anisotropic
inflation model. The Schwinger preheating is not observed in the latter model
but occurs for a sufficiently large inflaton-photon coupling in the first
model. We analytically address its condition and derive a general attractor
solution of the electric fields. The occurrence of the Schwinger preheating in
the first model is determined by whether the electric fields enter the
attractor solution during inflation or not.
