Fig. 1. (a) Experimental setup. (b) Variation of the proton cutoff energy with target thickness. The diamond symbols represent the averages of the five best shots in the experiments, and the triangles represent the results of the PIC simulations. (c) Optimum energy spectra of protons from aluminum and plastic targets detected by the Thomson spectrometer. Solid lines represent experimental results and dashed lines PIC simulations. (d), (f), (g) and (e), (h), (i) Energy-resolved spatial distributions of proton beams detected by the RCF stack for the 2.5 µm aluminum and 1.2 µm plastic targets, respectively. The white dashed circles indicate the positions of protons with a deflection angle of 14°.

Fig. 2. Energy (transfer) as a function of time. Data are shown for laser energy entering the simulation box (black), electromagnetic energy (red), electron kinetic energy (green), and ion kinetic energy (blue). The results from plastic targets (dashed lines) are compared with those from aluminum targets (solid lines). Here, the thickness is 1.2 µm for both aluminum and plastic targets.

Fig. 3. Electromagnetic energy densities for (a) aluminum and (b) plastic targets at t = 53 fs (i.e., 20T₀). The solid lines indicate the initial positions of the solid targets, and the dashed lines mark the fronts of the pre-plasma. The thickness is 1.2 µm for both targets.

Fig. 4. Electron kinetic energy distributions for (a) and (b) aluminum and (c) and (d) plastic targets, both with thickness 8 µm, at t = 53 fs (i.e., 20T₀) and t = 80 fs (i.e., 30T₀).

Fig. 5. Magnitude of the electric field as a function of time and position for (a) aluminum and (b) plastic targets, both with thickness 8 µm. As indicated by the dashed line in Fig. 4, the electric field is along the normal direction of the back-side surface, starting at z = 17 µm and y = 0 µm.

Fig. 6. Magnitude of the electric field as a function of time and position for aluminum targets of thickness (a) 2 µm and (b) 8 µm. As indicated by the dashed line in Fig. 4, the electric field is along the normal direction of the back-side surface, starting at z = 17 µm and y = 0 µm.

Fig. 7. (a) and (b) Spatial distribution and (c) and (d) angular distribution of proton energies for aluminum and plastic targets, both of thickness 2 µm, at t = 320 fs (i.e., 120T₀). The angle ϕ is defined as ϕ = arctan(u_y/u_z) − θ_t, where θ_t is the target dip angle, which is 30°, and u_y and u_z are the proton momenta.