Numerical study of 1.1 GeV electron acceleration over a-few-millimeter-long plasma with a tapered density

We present two-dimensional particle-in-cell simulations of laser wakefield electron acceleration up to 1.1 GeV over a-few-millimeter-long plasma with the help of density tapering. We observed that, in a uniform plasma, the electron beam reaches the dephasing state not only by the slow phase velocity of the wakefield but also by the relativistic prolonging of the plasma wavelength. Such a dephasing between the wakefield and beam can be mitigated by an upward density taper. By employing a parabolically increasing plasma density, we obtained a significant enhancement of the beam energy from 850 MeV (uniform) to 1.1 GeV (tapered). However, the similar relativistically promoted dephasing was observed again in the environment of tapered density. Over a few millimeters the driving laser pulse was well self-guided without any externally prepared channel. Thus, this parameter regime is suitable for the gas-jet laser wakefield electron acceleration experiments.open6