  By using Dirac-Heisenberg-Wigner formalism we study electron-positron pair
production for linear, elliptic, nearly circular and circular polarizations of
electric fields with symmetrical frequency chirp, and we obtain Momentum
spectra and pair yield. The difference of results among polarized fields is
obvious for the small chirp. When the chirp parameter increases, the momentum
spectra tend to exhibit the multiphoton pair generation that is characterized
by the multi-concentric ring structure. The increase of number density is also
remarkable compared to the case of asymmetrical frequency chirp. Note that the
dynamically assisted Schwinger Mechanism plays an important role for the
enhanced pair production in the symmetrical frequency chirp.
