Generation of quasi-monoenergetic proton beam by using ultrathin double- layer target

The quasi- monoenergetic proton beam generated from ultra- short ultra- intense laser pulse interacting with ultrathin double- layer target (both the thickness of the substrate layer and that of the acceleration layer are in nanometer scale) is examined by one- dimensional particle-in-cell simulation. The thickness of the substrate and the acceleration layer is crucial to the proton spectrum. The quasi-static electric field becomes stronger as the substrate layer thickness decreases, which results in the enhancement of the maximum proton energy. Reducing the acceleration layer thickness induces a more uniform quasi- static electric field. As a result, the proton spectrum becomes more monoenergetic with nearly unchanged mean energy. The quasi- monoenergetic proton with an energy spread of a