Ion Acceleration in Multispecies Targets Driven by Intense Laser Radiation Pressure

The acceleration of ions from ultrathin foils has been investigated by using 250 TW, subpicosecond laser pulses, focused to intensities of up to $3\times {10}^{20}\mathrm{W}{\mathrm{cm}}^{-2}$. The ion spectra show the appearance of narrow-band features for protons and carbon ions peaked at higher energies (in the $5–10\mathrm{MeV}/\mathrm{\text{nucleon range}}$) and with significantly higher flux than previously reported. The spectral features and their scaling with laser and target parameters provide evidence of a multispecies scenario of radiation pressure acceleration in the light sail mode, as confirmed by analytical estimates and 2D particle-in-cell simulations. The scaling indicates that monoenergetic peaks with more than $100\mathrm{MeV}/\mathrm{\text{nucleon}}$ are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.

Figure 1

(a) Schematic of the experimental setup. (b) Ion spectra obtained from 100 nm Cu target irradiated by a $LP$ laser pulse at $I_0=\sim 3\times {10}^{20}\mathrm{W}{\mathrm{cm}}^{-2}$. Different line colors correspond to different ion species (see the figure legend); solid and dotted lines represent spectra obtained on TP1 and TP2, respectively. (c) Ion spectra obtained on TP1 from 50 nm Cu target irradiated by a CP pulse at $I_0\simeq 1.25\times {10}^{20}\mathrm{W}{\mathrm{cm}}^{-2}$. The proton spectra obtained with RCF, in the same shot, at different positions in the beam, corresponding to $\sim 2°$, $\sim 7°$, and $\sim 13°$ from the laser axis are also shown (dashed lines). RCF spectra were determined by using an iterative algorithm which calculates the spectrum yielding the closest fit to the observed dose, by evaluating the energy response function of each RCF layer and subtracting the dose contribution to a given layer resulting from ions stopping farther in the detector, similar to Ref. 21.

Figure 2

(a) Graph showing comparison between three ion ($Z/A=0.5$) spectra, where the position of the spectral peaks is plotted in (b) as a function of $a_0^2{\tau }_p/\chi$. The experimental parameter set [$a_0$, target material, target thickness ($\mu \mathrm{m}$)] for the data points 1–7 is [15.5, Cu, 0.1], [10, Cu, 0.05], [13.8, Cu, 0.1], [7.5, Al, 0.1], [6.9, Al, 0.1], [13.6, Al, 0.5], and [14.1, Al, 0.8], respectively. The black solid line in (b) represents the ion energy estimated from the analytical modeling described in the text. The red circle represents the data from Henig et al. in Ref. 15.

Figure 3

(a) Ion spectra obtained from 2D multilayered PIC simulation corresponding to the data point “1” in Fig. 2a and experimental spectra of Fig. 1b. (b) and (c) show 2D density profiles of ions at 66 and 165 fs, respectively. In all frames the Cu ($Z/A=0.42$), C ($Z/A=1/2$) and proton ($Z/A=1$) spectra (a) and distributions (b,c) are ordered from left to right. In frames (b,c) each color map ranges from zero to the value shown at the top of the corresponding color bar. The color bars refer to Cu (top), C (middle), and protons (bottom), respectively.

Figure 4

Ion energy scaling (solid, black line) as shown in Fig. 2b, extrapolated to higher $a_0^2{\tau }_p/\chi$, assuming $R=1$. Square points are the experimental data shown in the same figure. The (red) circles represent the spectral peaks reported in the literature, as labeled, from multispecies PIC simulations for stable LS acceleration. The dot-dashed (black) line shows the $E_{\mathrm{ion}}\propto (a_0^2{\tau }_p/\chi {)}^2$ scaling valid for nonrelativistic ion energies. Dotted and dashed (blue) lines show the ion energy trend predicted by the rigid model by varying $\chi$ for 45 fs FWHM laser pulses at $I_0=5\times {10}^{20}\mathrm{W}{\mathrm{cm}}^{-2}$ and 450 fs FWHM laser pulses at $I_0=5\times {10}^{19}\mathrm{W}{\mathrm{cm}}^{-2}$, respectively. The inset shows the spectra obtained from a PIC simulation for a laser fluence increased by a factor of 2 and target density decreased by a factor of 2.5 with all other parameters identical to the run shown in Fig. 3.