Toward high-energy laser-driven ion beams: Nanostructured double-layer targets

The development of novel target concepts is crucial to make laser-driven acceleration of ion beams suitable for applications. We tested double-layer targets formed of an ultralow density nanostructured carbon layer ($\sim 7{\mathrm{mg}/\mathrm{cm}}^3$, $8–12\mu \mathrm{m}$–thick) deposited on a $\mu \mathrm{m}$–thick solid Al foil. A systematic increase in the total number of the accelerated ions (protons and ${\mathrm{C}}^{6+}$) as well as enhancement of both their maximum and average energies was observed with respect to bare solid foil targets. Maximum proton energies up to 30 MeV were recorded. Dedicated three-dimensional particle-in-cell simulations were in remarkable agreement with the experimental results, giving clear indication of the role played by the target nanostructures in the interaction process.

Figure 1

Setup of the experiment. The laser pulse is tightly focused by an off-axis parabolic mirror (angle of incidence 30°) onto 8 or $12\mu \mathrm{m}$–thick carbon foam deposited on a $0.75\mu \mathrm{m}$–thick Al foil. Ion spectra are recorded using two Thomson parabola spectrometers (TPS) and electron spectra are collected with an electron spectrometer.

Figure 2

Energy spectra of the protons obtained with a laser intensity of 4.1, 3.5, $3.7\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$ for $s$-, $p$- and $c$–polarization respectively. The targets are DLT with $8\mu \mathrm{m}$–thick foam (spectra a, b, c respectively) and ST (spectra d, e and f). The spectra are collected along the target normal direction. The inset shows the electron energy spectra when using DLT with $12\mu \mathrm{m}$–thick foam with $s$–, $p$- and $c$–polarization, (spectra a, b, c respectively) and using ST for $s$–and $c$–polarization (spectra d and f).

Figure 3

Role of laser polarization and intensity. $E_p^{\max }$ as a function of laser intensity with $s$- (blue squares), $p$- (red triangles), and $c$-polarization (black circles) for ST (empty symbol) and DLT with $8\mu \mathrm{m}$–thick foam (full symbol). Horizontal error bars are reported only for the ST case to improve the clarity of the graph.

Figure 4

3D–PIC simulations of the electron density dynamics in the interaction between a laser pulse and a nanostructured foam at different times: (a) 0 fs, (b) 67 fs, and (c) 134 fs. The red cone represents the incident laser, and the yellow layer indicates Al substrate.

Figure 5

Calculated proton energy spectra from homogeneous foam, nanostructured foam, and Al foil using $p$- and $c$- polarized laser pulse. The inset shows the corresponding experimental spectra.