Nanometer-scale proton focusing via density-gradient-induced intrinsic focusing electric fields

By tailoring the target density to a radial gradient distribution, we propose a proton focusing mechanism. This mechanism is based on the focusing radial electric field self-induced by the radial density gradient. This field, different from the defocusing field of a uniform density target, focuses near the longitudinal axis of the target and is capable of producing a proton source with a nanometer-scale focal spot size, and an emittance on the order of 10^{-3}mmmrad. This type of proton source offers further improvements in both focal spot size and collimation over other proton focusing enhancement mechanisms.