  In laser-driven wakefield, ionization induced injection is an efficient way
to inject electrons in the plasma wave. A detailed study on the beam dynamics
under the influence of beam loading effects, which can be controlled by the
concentration of nitrogen impurity introduced in the hydrogen gas was
conducted. For a specific value of this percentage, the final energy of the
high-energy electron bunch becomes nearly independent of the trapped positions,
thus leading to a small energy dispersion. We also show that the final beam
emittance is mainly determined by the injection process.
