Electron self-injection into the phase of a wake excited by a driver laser in a nonuniform density target

Abstract

It is well known that if electrons are externally injected into a density upramp, then their dephasing lengths will be extended greatly, and thus these electrons will gain more energy. However, we find that a density upramp can also be used to control the beam's collimation and the emittance that occurs by self-injection in the gradient. When electrons self-inject into the wakefield in a density gradient, an electron filtering mechanism is found to occur in the injection process. Electrons with high transverse velocities are scattered and only electrons with high longitudinal to transverse velocity ratios can be candidate electrons for self-injection. This causes the trapped electrons to be more highly collimated. In addition, the injection occurs near the axis, which causes the accelerated electron beam to have reduced emittance. An ultra-collimated electron beam with an angle spread of ∼1° and emittance of ∼0.01 mm mrad is generated by a 2.5-dimensional particle-in-cell (2.5-D PIC) simulation.