Role of target material in proton acceleration from thin foils irradiated by ultrashort laser pulses

We report on the proton acceleration studies from thin metallic foils of varying atomic number ($Z$) and thicknesses, investigated using a 45 fs, 10 TW Ti:sapphire laser system. An optimum foil thickness was observed for efficient proton acceleration for our laser conditions, dictated by the laser ASE prepulse and hot electron propagation behavior inside the material. The hydrodynamic simulations for ASE prepulse support the experimental observation. The observed maximum proton energy at different thicknesses for a given element is in good agreement with the reported scaling laws. The results with foils of different atomic number $Z$ suggest that a judicious choice of the foil material can enhance the proton acceleration efficiency, resulting into higher proton energy.

Figure 1

Schematic diagram of the experimental setup.

Figure 2

Typical ion spectra recorded using the TPIS, for a 6.5 μm thick Al foil. The inset shows a proton beam profile from the same foil, recorded on an RCF.

Figure 3

(a) Proton energy spectrum recorded using Al foils of different thicknesses. The inset shows the variation of the maximum proton energy with foil thickness. (b) 1D hydrodynamic simulation of the foil density evolution and the shock wave caused by a 1.5 ns ASE prepulse with intensity $2\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ for Al foils of different thicknesses. Numbers next to the curves indicate the foil thicknesses in micrometers. (c) Proton energy spectra recorded from 6.5 μm Al foil for different laser prepulse conditions, which are shown in the inset.

Figure 4

(a) 1D hydrodynamic simulation of density evolution and the shock wave caused by a 1.5 ns ASE prepulse with intensity $2\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ for Ni foils of different thicknesses. Numbers next to the curves indicate the foil thicknesses in micrometers. (b) Proton energy spectra from Al and Ni foils of 1.5 and 5 μm thicknesses. The inset shows fast electron spectra measured at the target rear for Al and Ni foils of 5 μm thickness. The solid lines are the exponential fit.

Figure 5

(a) The proton energy spectra from Al, Ti, Ni, and Ag foils of 25 μm thickness and (b) for Al, Ti, and Cu foils of 12.5 μm thickness.

Figure 6

Dependence of the maximum proton energy on the foil thickness for different foil materials. The thick, solid curve (red) is the one calculated for maximum proton energy using Mora's model (see the text for details). The other curves are for the eye only.

Figure 7

Dependence of the maximum proton energy with the target density for two foil thicknesses. The solid and dashed curves are calculated by fitting an exponential decay function.