Fig. 1. Schematic of three laser beams irradiating a wire-hemisphere (left, TWH) target. Four other cases are considered: a single laser beam irradiating a wire-hemisphere (SWH) target, three laser beams irradiating a hemisphere (TH) target, a single laser beam irradiating a hemisphere (SH) target and a single laser beam irradiating a planar (SP) target. The total laser energy is the same in all the five cases.

Fig. 2. (a) Evolution of the TNSA proton flux for the five cases in Figure 1. Here, only protons with energies higher than 0.5 MeV are counted. The blue dots and orange squares are for the peak flux and total number , respectively, of the protons. (b) Angular distributions of the protons behind the targets at . The blue dots and orange squares are for the averaged angular deviation (i.e., the average angle between the momentum direction of each proton and its direction with respect to the target center) of all protons, as well as the standard deviation of the proton angular distribution . For the SP case, the averaged angular deviation, standard deviation and proton number are divided by 2, 4 and 6, respectively. (c) Proton energy spectra at . The proton temperatures (obtained from the gradients of the curves) are also given. (d) The maximum laser-to-proton (with energies higher than 0.5 MeV) energy conversion efficiency for the five cases.

Fig. 3. (a) Hot-electron energy-density distributions in the region behind the wire-hemisphere structure at . The solid black curve shows a typical constant-energy-density contour relatively far away from its back surface. (b) Energy-density distribution of the wire electrons at and for the (b1), (b2) TWH and (b3), (b4) SWH cases, respectively. (c) Evolution of the total electron energy density in the hollow behind the targets. The red and yellow dashed curves are for the contribution of the wire electrons in the TWH and SWH cases. (d) Electron-energy spectra at . The blue and black dashed curves show the spectra of the wire electrons in the TWH and SWH cases. The inset shows the electron energy spectra in the (green and blue curves) and (black and red curves) regions of panels (b2) and (b4) for the TWH and SWH cases. (e) Laser-to-electron energy conversion efficiencies. The overlapping inner bars in the TWH and SWH cases are those of the laser-to-wire electrons only, from which one can clearly see the effect of the wires.

Fig. 4. (a) Distributions of the electric field strength behind the five targets at . The arrows show the electric field magnitudes and directions. (b) Profiles of the sheath electric field at a distance perpendicular to the local target-rear surface. (c) Standard deviation of the sheath electric fields shown in (b).

Fig. 5. Proton energy-density distribution in the circular region behind the five targets at .

Fig. 6. Dependence of the peak flux of protons on (a) the wire radius , angle between the two adjacent wires and (b) wire length in the TWH case. The values (in units of cm⁻² s⁻¹) marked in (a) are for .

Fig. 7. (a), (c), (e) Angular distribution and (b), (d), (f) peak flux (left) and total number (right) of the TNSA protons versus the laser incident angle , misalignment of the laser incident angle and transverse drift of the focal spot. The panels on the far left are for , with misalignment and transverse drift .

Fig. 8. Results of radiation hydrodynamic simulations. (a) Density distribution of preplasma produced by the prepulse at different laser power , and its (b) angular and (c) radial profiles along the white lines in (a). The density is in logarithmic color scale and these data are extracted 3 ps before the peak of the main pulse arrived. The black curves in (a) are the contour of the overcritical region. The above density distributions from FLASH simulations are then used in the PIC code EPOCH as input parameters of the wire-hemisphere target. Dependence of (d) the peak flux and (e) angular distribution of protons on both without and with preplasma from the simulation.