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Laser driven sources of high energy ions commonly use thin solid foils. A gaseous target can also produce ion beams with characteristics comparable to those obtained with solid targets. Using Particle-In-Cell simulations, we have studied in detail ion acceleration with high intensity laser pulses interacting with low density plasmas. A two-step acceleration process can be triggered: first, ions are accelerated in volume by electric fields generated by hot electrons, second, the ion energy is boosted in a strong electrostatic shock. 2D and 3D simulations show the potential of this regime. It is possible to model separately these two steps. In the first step a hot electron population and a descending density profile are necessary, and the second step develops if a fast proton wave enters in a low density plasma.
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