Ion acceleration from microstructured targets irradiated by high-intensity picosecond laser pulses

Structures on the front surface of thin foil targets for laser-driven ion acceleration have been proposed to increase the ion source maximum energy and conversion efficiency. While structures have been shown to significantly boost the proton acceleration from pulses of moderate-energy fluence, their performance on tightly focused and high-energy lasers remains unclear. Here, we report the results of laser-driven three-dimensional (3D)-printed microtube targets, focusing on their efficacy for ion acceleration. Using the high-contrast ($\sim {10}^{12}$) PHELIX laser ($150\mathrm{J}$, ${10}^{21}\mathrm{W}/{\mathrm{cm}}^2$), we studied the acceleration of ions from $1\text{−}\mu \mathrm{m}$-thick foils covered with micropillars or microtubes, which we compared with flat foils. The front-surface structures significantly increased the conversion efficiency from laser to light ions, with up to a factor of 5 higher proton number with respect to a flat target, albeit without an increase of the cutoff energy. An optimum diameter was found for the microtube targets. Our findings are supported by a systematic particle-in-cell modeling investigation of ion acceleration using 2D simulations with various structure dimensions. Simulations reproduce the experimental data with good agreement, including the observation of the optimum tube diameter, and reveal that the laser is shuttered by the plasma filling the tubes, explaining why the ion cutoff energy was not increased in this regime.

Figure 1

Sketch of the experimental setup: The PHELIX laser was focused with ${15}^{\circ }$ incidence onto flat and microstructured $1\text{−}\mu \mathrm{m}$ foils. Accelerated ions were detected along the target normal with a Thomson parabola spectrometer. The inset images show microscope images of two structure types.

Figure 2

(a)–(c) Ion energy distribution along target normal measured by the TP diagnostic for flat $1\text{−}\mu \mathrm{m}$-thick Co foils and structured foils. In the notation $d\times h$, $d$ is the tube inner diameter or pillar diameter and $h$ the tube or pillar height. The blue and orange bands indicate the full range observed within all shots on the same target type in each energy bin for the flat reference and structured target, respectively. The yellow shading is the gain from flat to structured targets taken from the average spectra. (a) Proton spectrum from flat references and a micropillar target. (b), (c) Ion spectra from microtube targets, respectively (b) for protons and (c) for carbon-6+ ions. (d) plots the proton cutoff energy for the different types of targets. (e)/(f) show the total proton/carbon number enhancement for the different types of targets, normalized to the flat reference. The gray bands in each plot represent the full range variation (min/max) of the flat reference. The flat reference variations are taken into account in the error propagation when calculating the error bars (min/max) for the normalized plots (e) and (f). The number of shots used is 2 for the flat reference; 1 for the $8\text{−}\mu \mathrm{m}$-length pillars; 2 for the $15\text{−}\mu \mathrm{m}$-length pillars; 1 for the $19\text{−}\mu \mathrm{m}$-length pillars; 2 for the $2\mu \mathrm{m}\times 5\mu \mathrm{m}$, $3\mu \mathrm{m}\times 5\mu \mathrm{m}$, $2\mu \mathrm{m}\times 10\mu \mathrm{m}$, and $5\mu \mathrm{m}\times 10\mu \mathrm{m}$ tubes; and 3 for the $5\mu \mathrm{m}\times 5\mu \mathrm{m}$ and $3\mu \mathrm{m}\times 10\mu \mathrm{m}$ tubes.

Figure 3

Ion acceleration results from the 2D PIC simulations. Laser electric field (front side) and ion densities (rear side) at $t=465\mathrm{fs}$ ($\approx 100\mathrm{fs}$ before the laser peak reaches the substrate) for (a) a flat reference, (b) a $3\mu \mathrm{m}\times 5\mu \mathrm{m}$ microtube target, and (c) a $1\mu \mathrm{m}\times 8\mu \mathrm{m}$ micropillar target with an average gap size of $1\mu \mathrm{m}$. In (d) and (e) are shown respectively the proton and carbon signals normalized to the flat reference (black dashed line), for all the microtube targets explored during the experiment and for the pillar target of (c).

Figure 4

(a) Energy balance, divided between fields (black), ions (red), and electrons (blue) for the flat reference (solid line) and a $3\mu \mathrm{m}\times 5\mu \mathrm{m}$ microtube structured target (dashed line). (b) Density normalized to the critical density for the different microtubes' diameters ($2\mu \mathrm{m}$ in blue, $3\mu \mathrm{m}$ in red, and $5\mu \mathrm{m}$ in green) and heights ($10\mu \mathrm{m}$ in solid lines and $5\mu \mathrm{m}$ in dashed lines). The horizontal dashed line corresponds to the relativistically corrected critical density $\gamma n_{\mathrm{c}}$ and the vertical dashed line to the instant when the laser peak reaches the substrate.