Pulsations of the electron-positron plasma in the field of optical lasers

The possibility to observe vacuum electron-positron pair creation due to a powerful optical laser pulse is discussed. We employ a quantum kinetic formulation of the problem with a source term describing the vacuum pair production in a homogeneous electric field with arbitrary time dependence (dynamical Schwinger mechanism). For a periodic field weak in comparison with the critical value E_{cr}=m^2/|e|, the electron-positron plasma density changes approximately periodically with twice the field frequency. Under these conditions, the mean value $<n>$ for the density per period in the volume lambda^3 is a more appropriate characteristic quantity than the residual density n_r which is taken over an integer number of field periods and calculated using the imaginary time method. The value <n> is proportional to the squared field intensity and does not depend on the frequency. We show that in terms of the parameter <n> an optical laser can be more effective than a X-ray one. We expect that it is possible to observe the vacuum creation effect not only by means of the planned X-ray free electron lasers but already at present-day optical lasers.