Schematics of the proposed method. A laser pulse with intensity and duration is injected onto a Cu target, which has a small hole. Target thickness and hole diameter .
(a) Typical proton energy spectra from several target models at the time after the start of laser-target interaction [(1) 500 nm single-layer without hole, (2) double-layer without hole, (3) double-layer without hole, (4) double-layer without hole, (5) double-layer without hole, (6) double-layer with hole]. (b) Magnetic field distribution at time after the start of laser-target interaction. The dashed line is the initial target surface. (c) Electron acceleration by the effective combination of transverse motion driven by the laser field and the longitudinally driving forces (ponderomotive force and force). (d) The effect of hole diameter on the accelerated proton energy taken at the time after the start of laser-target interaction. The target thickness is .
(a) Angular distribution of the energetic proton velocity and proton numbers . (b) Proton phase space and isocurves of magnetic field -component (, blue: −, red: ) at the time after the start of laser-target interaction. Protons are plotted in rainbow color according to energy (blue to red). The hole diameter is .
(a) Modified target design to reduce the angular spread of the proton velocity and enhance the maximum achievable proton energy. (b) Simulation result of the proton velocity angular distribution and number density angular distribution from the modified target. The hole diameter is .
The maximum proton energy when linearly increasing preplasma of length exists at the front of the target. is the maximum density of the preplasma in the unit of . For the planar target, the maximum proton energy is increased while it is decreased for the hole target by preplasmas. The hole diameter is .
PIC simulation results and fitting curves for the hole target and the planar target. (open square) PIC simulation results, (dashed) fit-curve for simulation results, (open circle) model calculation results, (dotted) fit-curve for model calculation results. The hole diameter is .
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