Fig. 1. Schematic of the experimental setup. R1–R7 are high-reflection dielectric mirrors; OAP1–OAP3 are off-axis parabola high-reflection mirrors.

Fig. 2. Energy ratio of the reflected and incident laser beams of the PM system.

Fig. 3. Measured temporal contrasts without and with PM on the (a) ns time scale and (b) ps time scale.

Fig. 4. (a)–(f) Proton beam spatial-intensity distribution for HC (a)–(c) and for LC (d)–(f) collected by the RCF stacks. The small hole in the middle of the RCF lets part of the particles pass through to be measured by ion spectroscopy. (g), (h) Ion imaging trajectories collected by the energy spectrometer for (g) HC and (h) LC laser conditions, respectively; (i) proton energy spectra solved from (g) and (h).

Fig. 5. Energy spectrum of the proton with the different pre-ionized expanded targets from PIC simulations at ∼107 fs after the laser arrived at the front surface of the target. The thickness of the ideal unexpanded target was 50 nm (blue line) with uniform density distribution. The density of the Gaussian function distribution was considered for expanded target cases with thickness (FWHM) of 0.1 µm (green line), 0.5 µm (light blue line), 1.5 µm (red line) and 5 µm (black line).