  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.
