Cold optical injection producing monoenergetic, multi-GeV electron bunches.

A cold optical injection mechanism for a laser-plasma accelerator is described. It relies on a short, circularly polarized, low-energy laser pulse counterpropagating to and colliding with a circularly polarized main pulse in a low density plasma. Contrary to previously published optical injection schemes, injection is not caused here by electron heating. Instead, the collision between the pulses creates a spatially periodic and time-independent beat force. This force can block the longitudinal electron motion, leading to their entry and injection into the propagating wake. In a specific setup, we compute after acceleration over 0.6 mm, a 60 MeV, 50 pC electron bunch with 0.7 MeV rms energy spread, proving the interest of this scheme to inject electron bunches with a narrow absolute energy spread. Acceleration to 3 GeV with a rms spread smaller than 1% is computed after propagation over 3.8 cm in a plasma channel.