  The idea of using a gas-mixture comprising atoms with high mass number in
order to increase proton energies in laser induced plasma acceleration at under
critical density is investigated by means of 2D PIC (Particle-In-Cell)
simulations. Comparing and discussing the case of a pure hydrogen plasma, and
that of a plasma containing higher mass number species with a small percentage
of hydrogen, we demonstrated that the mixture enhances the energies of the
accelerated protons. We also show that using a gas-mixture introduces the
possibility of using the densities ratio to change the relative acceleration of
the species.
