  We show through experiments that a transition from laser wakefield
acceleration (LWFA) regime to a plasma wakefield acceleration (PWFA) regime can
drive electrons up to energies close to the GeV level. Initially, the
acceleration mechanism is dominated by the bubble created by the laser in the
nonlinear regime of LWFA, leading to an injection of a large number of
electrons. After propagation beyond the depletion length, leading to a
depletion of the laser pulse, whose transverse ponderomotive force is not able
to sustain the bubble anymore, the high energy dense bunch of electrons
propagating inside bubble will drive its own wakefield by a PWFA regime. This
wakefield will be able to trap and accelerate a population of electrons up to
the GeV level during this second stage. Three dimensional (3D) particle-in-cell
(PIC) simulations support this analysis, and confirm the scenario.
