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We propose to use tightly focused lasers to generate high quality electronbeams in laser wakefield accelerators. In this scheme, the expansion of thelaser beam after the focal position enlarges the size of wakefield bubble,which reduces the effective phase velocity of the wake and triggers injectionof plasma electrons. This scheme injects a relatively long beam with highcharge. The energy spread of the injected beam can be minimized if an optimalacceleration distance is chosen so that the beam chirp is suppressed.Particle-in-cell simulations indicate that electron beams with the charge inthe order of nanocoulomb, the energy spread of $\sim 1\%$, and the normalizedemittance of $\rm \sim 0.1\ mm\cdot mrad$ can be generated in uniform plasmausing $\sim 100\ \rm TW$ laser pulses. An empirical formula is also given forpredicting the beam charge. This injection scheme, with a very simple setup,paves the way towards practical high-quality laser wakefield accelerators fortable-top electron and radiation sources.

High quality beam produced by tightly focused laser driven wakefield accelerators