Comparison of Laser Ion Acceleration from the Front and Rear Surfaces of Thin Foils

The comparative efficiency and beam characteristics of high-energy ions generated by high-intensity short-pulse lasers ($\sim 1–6\times {10}^{19}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$) from both the front and rear surfaces of thin metal foils have been measured under identical conditions. Using direct beam measurements and nuclear activation techniques, we find that rear-surface acceleration produces higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. Our observations are well reproduced by realistic particle-in-cell simulations, and we predict optimal criteria for future applications.

Figure 1

(a) Schematic of the experiment showing a double proton beam, (b) $\sim 2\mathrm{M}\mathrm{e}\mathrm{V}$, and (c) $\sim 4.5\mathrm{M}\mathrm{e}\mathrm{V}$ RCF layers. (d)–(f) Same as (a)–(c) except that here the target is tilted by $30°$. The observed structures in the proton beam are imprinted by the copper mesh.

Figure 2

(a) Simulated angularly integrated ${\mathrm{H}}^{+}$ spectra overlaid with the experimental spectrum (limited by the number of used RCF layers). The RCF sensitivities used are indicated in color. The RSA spectrum low-energy cutoff is due to ${\mathrm{H}}^{+}$ contaminant depletion in the simulation. (b) Simulated phase space distribution of the FSA and RSA ${\mathrm{H}}^{+}$ at $t=400\mathrm{f}\mathrm{s}$ after the peak of the interaction. The target is tilted by $25°$. Each contour line corresponds to every ${10}^{0.3}$ of the ion number in a log scale.

Figure 3

(a) Setup of the activation experiment at ${10}^{19}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$. (b) Simulated ${\mathrm{D}}^{+}$ spectra for a $20\mu \mathrm{m}$ foil. The RSA ${\mathrm{D}}^{+}$ have lower energy than if the ${\mathrm{H}}^{+}$ layer would be absent since, due to their smaller mass, the ${\mathrm{H}}^{+}$ are preferentially accelerated by RSA. (c) Experimental and simulated ${}^{11}\mathrm{C}$ yield for FSA ${\mathrm{D}}^{+}$. (d) Simulated ${\mathrm{H}}^{+}$ spectra overlaid with the RCF inferred spectrum, both for a $20\mu \mathrm{m}$ foil. All spectra are angularly integrated.

Figure 4

Activation experiment at $6\times {10}^{19}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$. (a) Simulated ${\mathrm{D}}^{+}$ spectra for a $20\mu \mathrm{m}$ foil, (b) RCF inferred experimental ${\mathrm{H}}^{+}$ spectra for different Al foil thicknesses (the lines are guides for the eye), (c) ${}^{11}\mathrm{C}$ yield for FSA ${\mathrm{D}}^{+}$, experimental (filled circles) and simulated with (empty circles) or without (crosses) the contribution from ${\mathrm{H}}^{+}$ in ${}^{11}\mathrm{B}$ inferred from the spectra shown in (b). All spectra are angularly integrated.