| Laser driven sources of high energy ions have applications in plasma and fusion science as an electromagnetic field probe and may find other applications in medical science and laboratory astrophysics. These sources commonly use thin solid foils, and ions are accelerated at their rear side in the electrostatic field created by hot electrons. Gaseous targets can also produce ion beams with characteristics comparable to those obtained with solid targets. By adjusting the laser and plasma parameters, a two-step acceleration process can be triggered using low density plasmas: first, ions are accelerated in volume by electric fields generated by hot electrons, second, the ion energy is boosted in a strong electrostatic shock propagating along the descending density profile. Very efficient ion acceleration can be achieved and this regime constitutes a promising alternative to schemes involving high density targets. Using Particle-In-Cell simulations and Vlasov-Poisson simulations, we have studied in detail ion acceleration with high intensity laser pulses interacting with low density plasmas. Scaling laws for the maximum proton energy, maximum number of accelerated protons and beam divergence in this regime were obtained and are used to discuss the possibility to highlight this regime experimentally using nowadays laser setups. The influence of the laser wavelength has also been investigated and the possibility to use CO2 lasers to study this regime is discussed. The strong electrostatic shock launched during this process is easy to control and can be applied to study low velocity astrophysical shocks relevant to supernovae explosions and gamma ray bursts. |
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