The energy spread in laser wakefield accelerators is primarily limited by the energy chirp introduced during the injection and acceleration processes. Here, we propose the use of longitudinal density tailoring to reduce the beam chirp at the end of the accelerator. Experimental data sustained by quasi-3D particle-in-cell simulations show that broadband electron beams can be converted to quasimonoenergetic beams of $\le 10\%$ energy spread while maintaining a high charge of more than 120 pC. In the linear and quasilinear regimes of wakefield acceleration, the method could provide even lower, subpercent level, energy spread.

• Received 31 January 2017

DOI:https://doi.org/10.1103/PhysRevLett.121.074802