High quality beam produced by tightly focused laser driven wakefield accelerators

We propose to use tightly focused lasers to generate high quality electron beams in laser wakefield accelerators. In this scheme, the expansion of the laser beam after the focal position enlarges the size of wakefield bubble, which reduces the effective phase velocity of the wake and triggers injection of plasma electrons. This scheme injects a relatively long beam with high charge. The energy spread of the injected beam can be minimized if an optimal acceleration distance is chosen so that the beam chirp is suppressed. Particle-in-cell simulations indicate that electron beams with the charge on the order of nanocoulomb, the energy spread of ∼1%, and the normalized emittance of ∼0.1 mm mrad can be generated in uniform plasma using ∼100 TW laser pulses. An empirical formula is also given for predicting the beam charge. This injection scheme, with a very simple setup, paves the way toward practical high-quality laser wakefield accelerators for table-top electron and radiation sources.