Collimated proton beam from an ultra-intense laser pulse irradiating parallel to the plasma interface

Recently, there has been some works on surface plasma waves excited by a laser obliquely irradiating on a thin foil target, which can cause stronger target normal sheath acceleration of protons but cannot be excited by a grazing incidence laser. Here, we demonstrate that a large amplitude Interface Plasma Wave (IPW) can be excited by a relativistic laser pulse irradiating parallel (or grazing incidence) to the interface of a solid aluminum and low density hydrogen layer. This IPW markedly enhances the sheath electric field to accelerate protons and reduce reflection of the laser pulse to improve the coupling efficiency. As a result, a collimated high energetic and lower energy spread proton beam can be efficiently achieved.