Bubble Structure Evolution and Electron Injection Controlled by Optical Cycles in Wakefields

The evolution of bubble structure and electron injection in laser wakefield acceleration with different optical cycles is investigated through three-dimensional particle-in-cell simulations. Under fixed transverse and longitudinal ponderomotive force, the effect of optical cycles on the evolution of bubble structure and electron injection is studied by changing the laser wavelength. For a multi-cycle laser, electron acceleration is dominated by the ponderomotive force that produces symmetrical bubble and continuous injection. As the optical cycles decrease, the dominant effect of the electron acceleration can transition from the ponderomotive force to the carrier wave, and the carrier envelope phase shift can cause transverse oscillation of the bubble and periodic electron injection in the direction of laser polarization. The criterion for the dominant acceleration mechanism and the dependence of transition distance on the optical cycles and pulse width are obtained. The results are beneficial for manipulating electron acceleration and betatron radiation generation.