  We report an overall enhancement of a laser wakefield acceleration (LWFA)
using the ionization injection in a mixture of 0.3 % nitrogen gas in 99.7 %
helium gas. Upon the interaction of 30 TW, 30 fs laser pulses with a gas jet of
the above gas mixture, > 300 MeV electron beams were generated at a helium
plasma densities of 3.3-8.5*10^18 cm^{-3}. Compared with the electron
self-injection in pure helium gas jet, the ionization injection has led to the
generation of electron beams with higher energies, higher charge, lower density
threshold for trapping, and a narrower energy spread without dark current (low
energy electrons) or multiple bunches. It is foreseen that further optimization
of such a scheme is expected to bring the electron beam energy-spread down to 1
%, making them suitable for driving ultra-compact free-electron lasers
