Transition of electron beams between vacuum and plasma in the external injection into a laser wakefield accelerator

Laser wakefield acceleration is a remarkably efficient method for relativistic electron acceleration that ensures high electric field gradients generated by plasma waves. In this approach, an ultra-short, high-intensity laser pulse propagates through a plasma medium. It has already proven its potential by reaching gradients up to hundreds of GV/m. To further stabilize and control the process of acceleration, a separate source of electrons is widely considered. In order to address this problem, we have performed 3D particle-in-cell simulations using the Smilei code. Several plasma density profiles with different vacuum-plasma transition region and their effect on the external injection were analysed.