Electron acceleration by laser wakefields in tapered plasma densities

When a laser wake wave passes through a sharp downward density transition in plasmas, a significant amount of plasma electrons are self-injected into the acceleration phase of the wakefield and accelerated to relativistic high energies over a very short distance. We report that the energies of the injected plasma electrons can be increased a few times if an upward density tapering is used. Although space-charge effect of the trapped electrons deforms the accelerating wakefield severely, it is demonstrated that density tapering is a very effective way to enhance the trapped electron energies.