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Two-dimensional particle-in-cell simulations show that when an intense picosecond laser pulse irradiates a target with steep but smooth density profile, the target protons can be accelerated to high energies with small divergence by a combination of target normal sheath acceleration and radiation pressure acceleration. The effects of plasma density profile on proton acceleration and collimation are investigated. In general, smaller(larger) density gradients lead to larger(smaller) self-generated azimuthal magnetic fields and smaller(larger) target-back electrostatic sheath fields, and thus proton beams with smaller(larger) divergence angle as well as cutoff energy. Accordingly, within limits, proton beams with desired peaked spectrum, energy, and divergence angle can be obtained by tailoring the target density profiles. It is also demonstrated that target tailoring can be achieved by having two suitable nanosecond lasers separately irradiating the front and back sides of a uniform plane slab. [ABSTRACT FROM AUTHOR]

We propose a hybrid laser-driven ion acceleration scheme using a combination target of a solid foil and a density-tailored background plasma. In the first stage, a sub-relativistic proton beam can be generated by radiation pressure acceleration in intense laser interaction with the solid foil. In the second stage, this sub-relativistic proton beam is further accelerated by the laser wakefield driven by the same laser pulse in a near-critical-density background plasma with decreasing density profile. The propagating velocity of the laser front and the phase velocity of the excited wakefield wave are effectively lowered at the beginning of the second stage. By decreasing the background plasma density gradually from near critical density along the laser propagation direction, the wake travels faster and faster, while it accelerates the protons. Consequently, the dephasing between the protons and the wake is postponed and an efficient wakefield proton acceleration is achieved. This hybrid laser-driven proton acceleration scheme can be realized by using ultrashort laser pulses at the peak power of 10 PW for the generation of multi-GeV proton beams. [ABSTRACT FROM AUTHOR]