  Propagation distances of intense laser pulses and high-charge electron beams
through the plasma are, respectively, limited by diffraction and
self-deceleration. This imposes severe constraints on the performance of the
two major advanced accelerator concepts: laser and plasma wakefield
accelerators. Using numerical simulations, we demonstrate that when the two
beams co-propagate in the plasma, they can interact synergistically and extend
each other's travel distances. The key interactions responsible for the synergy
are found to be laser channeling by the electron bunch, and direct laser
acceleration of the bunch electrons by the laser pulse. Remarkably, the amount
of energy transferred from the laser pulse to the plasma can be increased by
several times by the guiding electron bunch despite its small energy content.
Implications of such synergistic interactions for the high-gradient
acceleration of externally injected witness charges are discussed, and a new
concept of a Laser-pulse and Electron-bunch Plasma Accelerator (LEPA) is
formulated.
