Fig. 1. (a) Schematic diagram of an NWA target; L, D, and S are the NW length, diameter, and spacing, respectively. (b) Schematic showing plasma heating and ion acceleration in an NWA target.

Fig. 2. 2D PIC simulation results. (a) Spatial distribution of the electric field E_y and electron density n_e at t = 150 fs. (b) Total laser absorption ratio and the fraction of the laser energy converted into ions and electrons over the simulation time for the planar and NWA targets. (c) Spatial distribution of the electron density at t = 210 fs; the black line shows the sheath field E_sx along the x axis at y = 0. (d) Spatial distribution of the sheath field E_sy at t = 210 fs; the red line shows the sheath fields E_sy along the y axis at x = 4 μm (e) and (f) Spatial distributions of the proton densities at t = 210 fs and t = 460 fs, respectively.

Fig. 3. Confined and emitted protons. (a) x–p_x and x–p_y phase diagrams of the protons at t = 460 fs. (b) Angular distribution of the protons at t = 460 fs; the green dashed line shows the integral of the energies at different angles. (c) Energy spectra of the confined and emitted protons at t = 460 fs; the dashed lines show the fitted temperatures of the protons.

Fig. 4. Spatial distributions and line plots (y = 0) of (a) EED and (b) PED for a planar target and a typical NWA target at a simulation time of t = 460 fs.

Fig. 5. Spatial distribution of PED for NWA targets with (a) L = 5 μm, (b) L = 2 μm, (c) L = 1 μm (D = 200 nm, S = 450 nm), and (d) L = 5 μm (D = 300 nm, S = 450 nm) (e) and (f) PEDs of different targets along the x and y directions, respectively.

Fig. 6. Simulation results as functions of NW diameter: (a) laser absorption ratio and confined electron temperature, (b) confined proton temperature and number of confined protons (>1 keV), and (c) energy proportions of emitted and confined protons. Panels (d)–(f) show the same results as functions NW length. Other NWA parameters were S = 450 nm, L = 5 μm in panels (a)–(c) and D = 200 nm, S = 450 nm in panels (d)–(f).

Fig. 7. (a) Schematic of the experimental setup. SEM images: (b) top and (c) side views of an NWA target.

Fig. 8. Experimental and simulated maximum proton energy results for varying the (a) diameters and (b) lengths of the NWs in the array. The other parameters of the NWA are S = 450 nm/S = 800 nm, L = 5 μm in (a) and D = 200 nm, S = 450 nm in (b). Panels (c) and (d) depict the proton spectra in (a) and (b), respectively, in which an NWA target with D = 200 nm, S = 450 nm, and L = 5 μm is abbreviated as 200-450-5.

Fig. 9. (a) Angular distribution of protons; error bars represent the standard deviations of the statistics. Panels (b)–(d) show the raw data of CR39 surfaces with the same sizes at angles of 20°, 55°, and 80° with respect to the target normal, respectively.

Fig. 10. Ion spectra: TPS raw data (a) without and (b) with 20 μm aluminum foil. (c) Projected range in aluminum foil for protons and deuterium and carbon ions. Energy spectra of ions with (d) CMR = 1 (proton) and (e) CMR = 1/2 (deuterium or carbon ion).

Fig. 11. (a) Deuterium energy density at a simulation time of t = 460 fs. (b) Spatial distribution of the density of helium ions at t = 460 fs. Simulated helium (neutron) yield for different NW (c) lengths and (d) diameters at a simulation time of t = 3 ps. The error bars represent the statistical fluctuations. The theoretical value of neutron yield from an NWA target with D = 200 nm, S = 450 nm, and L = 5 μm and the average neutron yield in the experiments from different NWA parameters are also shown in panel (c).

Table 1. Parameters of all NWA and planar targets.