Compact sources of high-energy protons (with magnitude larger than MeV) generated by the target normal sheath acceleration (TNSA) scheme  have a wide range of scientific applications, such as inertial confinement fusion, fast ignition, medical therapy, and proton imaging. However, the effectiveness of TNSA is lost as the foil becomes too thin, mainly because of its sensitivity to laser prepulses.

Researchers at Shanghai Institute of Optics and Fines Mechanics (SIOM/China) report experiments using a high-contrast PW laser pulse to irradiate thin solid foils. [Applied Physics Letters,102,224101,2013] Here noncollinear optical-parametric amplification (NOPA) and second harmonic generation (SHG) processes are used to obtain～10¹¹ contrast at 100 ps before the peak of the main pulse,while plasma mirrors are not used. A 7MeV proton beam is observed when this ultrahigh-contrast laser with intensity of ～2*10¹⁹ W/cm² irradiates an Al foil with the optimum thickness (2.5µm). The maximum proton energy decreases to 2.9MeV as the low contrast (～10⁸) laser is employed.Energetic protons are not detected when the saturable absorber is removed, which implies that the laser contrast at a ns scale is critical in this case. Two-dimensional (2D) particle-in-cell (PIC) simulations and MULTI simulations are performed to interpret the experimental results.