Enhanced laser-driven ion acceleration from a low-density-PMMA coated metal-foil

Strong enhancement in proton energy was investigated from a two-dimensional particle-in-cell simulation where an ultraintense laser pulse irradiates a 2-μm thick metal foil coated with a low density, 1-μm thick PMMA (polymethylmathacrylate - C5H8O2) on the rear surface. The reduction of PMMA density effectively increases resistivity of hot electrons, which results in the generation of a strong electrostatic field at the metal-PMMA interface in addition to the sheath electrostatic field at the PMMA-vacuum boundary. The interaction of each proton beam accelerated by the two electrostatic fields leads to the enhancement of energy for the protons originated from the PMMA-vacuum side. With a laser intensity of 1×1020 W/cm2, maximum proton energy of 80 MeV was investigated with a modulation in energy spectrum, which is 2.2 times higher than those from a metal-contamination layer target or a metal-high density PMMA target. It is also interesting that there is an energy peak around 18 MeV, which is caused by an interaction with heavier ions.