Improving electron beam quality in laser wakefield acceleration by using a plasma channel with an up-ramp density profile

Laser wakefield acceleration (LWFA) is a promising way for producing GeV-scale electron beams within a tabletop size. Increasing acceleration energy and reducing energy spread are extremely important for many applications. Here, we experimentally demonstrate that using a plasma channel with a longitudinally up-ramp density profile can simultaneously boost the accelerated electron energy and lower the final beam energy spread. In a plasma channel with uniform plasma density, electron beams with a peak energy of 250 MeV and a large energy spread (∼ 40%) were obtained. In contrast, within a plasma channel of the same length with an up-ramp density profile, stable electron beams with energies up to 1 GeV and a small energy spread (∼ 20%) were observed. Particle-in-cell simulations show that the plasma channel not only suppresses laser diffraction, but also affects the self-injection and acceleration of the electron beam. In the up-ramp plasma channel, the continual electron injection is suppressed and electrons can be locked in the acceleration phase for a longer duration, which leads to the reduction of energy spread and the increase of electron energy. This method provides a relatively simple and reliable way toward compact, high-performance tabletop electron accelerators.