The measurements reported here provide scaling laws for the acceleration process in the ultra-short regime and access ion acceleration conditions never investigated before. The scaling of accelerated ion energies was studied by varying a number of parameters such as target thickness (down to 10 nm), target material (C and Al) and laser light polarization (circular and linear) at normal laser incidence. A pronounced increase in the C6+ ion energy up to ~238 MeV has been observed for ultrathin (10-100 nm) carbon targets. Furthermore, it is seen that measured peak proton energies of about 20 MeV are observed almost independently from the target thickness over a wide range (10 nm-10 µm), and the target material (insulator and conductor) and laser polarisation doesn’t play a significant role on the maximum proton energy for target thicknesses < 50 nm. The results can be explained by the specific electron dynamics at ultra-high contrast and ultra-intense laser target irradiation. 2D PIC simulations are in good agreement with the experimental findings
