  The backreaction effect and plasma oscillation in pair production for rapidly
oscillating electric fields are investigated by solving quantum Vlasov
equation. Contrary to previously thought, it is found that the backreaction
effect can be neglected in the pair production for a rapidly oscillating but
weak electric field, particularly, for a subcritical external electric field
with frequency chirp. In some cases the oscillation period of created
electron-positron plasma can be described by a simple formula constructed based
on the Langmiur oscillation frequency, but it is impossible in general case
because the plasma oscillation period directly depend not only on the final
number density of created particles, but also on the external electric field
parameters. Moreover, it is found that the momentum spectrum presents complex
interferences after considering the backreaction. These results give us the
safety range of external electric fields for taking no account of the
backreaction effect and deepen our understanding of the pair production with
the backreaction effect.
