Optical shaping of plasma cavity for controlled laser wakefield acceleration

Laser wakefield accelerators rely on relativistically moving micrometer-sized plasma cavities that provide extremely high electric field >100 GV/m. Here, we demonstrate transverse shaping of the plasma cavity to produce controlled sub-GeV electron beams, adopting laser pulses with an axially rotatable ellipse-shaped focal spot. We showed the control capability on electron self-injection, charge, and transverse profile of the electron beam by rotating the focal spot. We observed that the effect of the elliptical focal spot was imprinted in the profiles of the electron beams and the electron energy increased, as compared to the case of a circular focal spot. We performed three-dimensional particle-in-cell simulations which reproduced the experimental results and revealed dynamics of an asymmetric self-injection process. This simple scheme offers a control method on laser wakefield acceleration to produce tailored electron beams and x rays for various applications.