Generation of monoenergetic proton beams by a combined scheme with an overdense hydrocarbon target and an underdense plasma gas irradiated by ultra-intense laser pulse

An optimization scheme for the generation of monoenergetic proton beams by using an overdense hydrocarbon target, followed by an underdense plasma gas, irradiated by an ultra-intense laser pulse is presented. The scheme is based on a combination of a radiation pressure acceleration mechanism and a laser wakefield acceleration mechanism, and is verified by one-dimensional relativistic particle-in-cell (1D PIC) simulations. As compared to the pure hydrogen (H) target, protons in the hydrocarbon target can be pre-accelerated to higher energy and compressed in space due to the existence of the heavy carbon atoms, which provides a better injection process for the successive laser wakefield acceleration in the underdense plasma gas, resulting in the generation of a monoenergetic, tens-of-GeV proton beam. Additionally, for the first time, it is found that the use of the hydrocarbon target can reduce the requirement for laser intensity to generate proton beams with the same energy in this combined scheme, as compared to the use of the pure H target.