  An optical injection scheme into the laser wakefield accelerator by preceding
injection pulse is investigated by means of 3D numerical particle-in-cell
simulations. Quasimonoenergetic hundred-pC electron bunches as short as 6 fs
can be generated. Optimal beam separation distance is found at the intersection
point of the injection beam bubble with the collection volume for transverse
injection into the accelerator beam bubble. It approximately corresponds to the
plasma wavelength. The main advantage of this scheme is the localized injection
of high charge. This injection mechanism can be useful for applications such as
ultrashort and relatively intense X-ray radiation sources such as a betatron
radiation or Thomson backscattering, time-resolved electron diffraction or for
seeding of further acceleration stages.
