Efficient laser-plasma acceleration of protons via near-critical mass limited targets

Isolated micro-targets are a promising avenue to high-performance laser-driven proton and ion accelerators due to their ability to confine the coupled laser energy to a small volume and small number of particles. Experimental results on proton emission from levitated plastic micro-spheres with an initial diameter of 1 µm are presented. For a laser with only 0.4 J contained within the central spot, proton energies of up to 27 MeV were observed with micro-spheres pre-expanded to near-critical density. Three-dimensional PIC simulations reveal that the combination of a ponderomotively driven process similar to radiation pressure 'hole-boring' generates a dense plasma bunch propagating at 0.1 c with the subsequent expansion of the moving bunch driven by hot electrons boosting the maximum proton energy E_{max} considerably. Simulations agree well with the observed experimental results and suggest a scaling of 54MeV/J^{1/2}, predicting that 100 MeV protons are possible with only 4 J in a Gaussian spot.