Enhanced laser-driven electron beam acceleration due to ionization-induced injection

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