Manipulation of laser-accelerated proton beam profiles by nanostructured and microstructured targets

Nanostructured and microstructured thin foils have been fabricated and used experimentally as targets to manipulate the spatial profile of proton bunches accelerated through the interaction with high intensity laser pulses ($6\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$). Monolayers of polystyrene nanospheres were placed on the rear surfaces of thin plastic targets to improve the spatial homogeneity of the accelerated proton beams. Moreover, thin targets with grating structures of various configurations on their rear sides were used to modify the proton beam divergence. Experimental results are presented, discussed, and supported by 3D particle-in-cell numerical simulations.

Figure 1

Top view of the experimental setup (a), sketch showing the plastic target covered with nanospheres (b), and side view of the grating targets (c) used in the experiment. Here $\overrightarrow{x}$ is the axis normal to the target surface, $\overrightarrow{y}$ is the direction perpendicular to the orientation of grating ridges and $\overrightarrow{z}$ is the direction parallel to the orientation of horizontal grating ridges.

Figure 2

Photo (a) and electrical scheme (b) of the SCDD detector, and sketch of the magnetic spectrometer (c) used during the experiment.

Figure 3

Proton energy distributions measured by the magnetic spectrometer for PET and ${\mathrm{NS}}_{\mathrm{rear}}$ (a) and for SiN and ${\mathrm{SiN}}_{2\mu \mathrm{m}}$ (c); the corresponding TOF spectra from the SSCD detector are shown in (b) and (d).

Figure 4

Examples of radiochromic films (obtained accumulating 2 shots) showing spatial profiles of proton beams generated from PET target (a) and from ${\mathrm{NS}}_{\mathrm{rear}}$ (b). (c) and (d) represent the lateral profile of the proton beam respectively for PET and ${\mathrm{NS}}_{\mathrm{rear}}$ targets.

Figure 5

Examples of radiochromic films showing the spatial profiles of proton beams emerging from SiN (obtained accumulating 2 shots) (a), from a ${\mathrm{SiN}}_{1\mu \mathrm{m}}$ (1 shot) (b) and from a ${\mathrm{SiN}}_{2\mu \mathrm{m}}$ (1 shot) (c) targets. (d),(e),(f) show the lateral profile of the proton beam for the different targets in (a)–(c).

Figure 6

Angular distribution of accelerated protons from 3D PIC simulations for SiN (a) and ${\mathrm{SiN}}_{2\mu \mathrm{m}}$ (b). (a), (b) Horizontal axis: deflection angle from the target normal direction ($x$ axis) in the plane $x-y$; vertical axis: deflection angle from the target normal direction ($x$ axis) in the plane $x-z$; colorbar: number of numerical macroparticles per spatial angle in simulations. Electric field perpendicular to the grating orientation for SiN (c) and ${\mathrm{SiN}}_{2\mu \mathrm{m}}$ (d) and electric field parallel to the grating orientation for SiN (e) and ${\mathrm{SiN}}_{2\mu \mathrm{m}}$ (f). The target front (laser-irradiated) surface is at $x=0$, slightly outside the area shown in (c)–(f) (the laser comes from the bottom).