  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 in
the order of nanocoulomb, the energy spread of $\sim 1\%$, and the normalized
emittance of $\rm \sim 0.1\ mm\cdot mrad$ can be generated in uniform plasma
using $\sim 100\ \rm TW$ laser pulses. An empirical formula is also given for
predicting the beam charge. This injection scheme, with a very simple setup,
paves the way towards practical high-quality laser wakefield accelerators for
table-top electron and radiation sources.
