Tight focusing proton beam with radius in nanometer scale generation based on channeled solid target

An efficient scheme of generating ultra-tightly focused proton bunch with radius in nanometer scale is proposed. A needlelike proton filament of transverse size in nanometer scale with the density of and charge quantity is obtained based on multi-dimension Particle-in-Cell (PIC) simulations. The regime is achieved via laser irradiating on a solid target with pre-channeled density profile. The theoretical analysis mentions that the transverse electric field dramatically transits from a defocusing dipole to double dipoles structure with the change of the initial target density distribution from uniform to pre-channeled. The inner dipole of the electric field tightly focuses the proton beam into the order of magnitude of nanometer. 3D simulations verify the scheme in the realistic condition. Various pre-channeled density profiles including linear, parabolic and arbitrary steeped prove to work well for the regime, which declares the robustness and the performability of the scheme in experiment.