Selective Ion Acceleration by Intense Radiation Pressure

We report on the selective acceleration of carbon ions during the interaction of ultrashort, circularly polarized and contrast-enhanced laser pulses, at a peak intensity of $5.5\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$, with ultrathin carbon foils. Under optimized conditions, energies per nucleon of the bulk carbon ions reached significantly higher values than the energies of contaminant protons ($33\mathrm{MeV}/\mathrm{nucleon}$ vs 18 MeV), unlike what is typically observed in laser-foil acceleration experiments. Experimental data, and supporting simulations, emphasize different dominant acceleration mechanisms for the two ion species and highlight an (intensity dependent) optimum thickness for radiation pressure acceleration; it is suggested that the preceding laser energy reaching the target before the main pulse arrives plays a key role in a preferential acceleration of the heavier ion species.

Figure 1

Maximum ion energy from 0° TPS for ${\mathrm{C}}^{6+}$ (solid line and filled circles) and ${\mathrm{H}}^{+}$ (solid line and filled squares) for CP. 2D PIC simulations are dotted lines with the same markers but empty. Errors bars represent the fluctuations in the measured maximum energy due to shot to shot variation in laser energy for intensities greater than $4\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$ throughout the experiment.

Figure 2

(a) Maximum ion energy as a function of laser intensity for experimental measurements of ${\mathrm{C}}^{6+}$ (filled blue circles) and ${\mathrm{H}}^{+}$ (filled black squares). 2D PICs with expansion included are also shown with the same markers but empty. The solid line represents the best fit line to the ${\mathrm{C}}^{6+}$ experimental data, $\propto I^{1.2\pm 0.2}$. (b) Correlation between ${\mathrm{C}}^{6+}$ and ${\mathrm{H}}^{+}$ maximum energies for experimental and simulation data (marker colors indicate the laser intensity). Dotted line indicates where ${\mathrm{C}}^{6+}$ and ${\mathrm{H}}^{+}$ energies are equal. The dashed line at $y=8\mathrm{MeV}$ in (a) and (b) represents the detection threshold for protons, and the maximum proton energy did not meet this threshold for the two lowest intensity shots. Crosses indicate simulations which included a prepulse with a peak intensity of ${10}^{17}\mathrm{W}/{\mathrm{cm}}^2$ arriving on target 2.5 ps before the peak of the main pulse.

Figure 3

Cross-correlator data of the GEMINI laser pulse profile without the double plasma mirror (DPM) arrangement in blue (dashed) alongside a modeled, contrast enhanced pulse profile used as a simulation input (solid black) and based on [34]. The pulse profiles have a peak intensity of $5.5\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$ and $1.1\times {10}^{21}\mathrm{W}/{\mathrm{cm}}^2$ with and without plasma mirrors, respectively. Three annotated areas (divided by dotted black lines) mark the parts of the interaction highlighted in the text. The inset graphs show the parameter space for preferential species acceleration using a prepulse (left) and the coherent contrast (right) based on 2D PIC simulations. The intensities on the vertical axes indicate, respectively, the peak intensity of the prepulse (left frame) and the intensity of the flat pedestal (right frame), while the horizontal axes indicate the time separation between the prepulse and main pulse (left) and the start of the pedestal and the main pulse (right). The main pulse remains unchanged. The blue shaded areas indicate where preferential ${\mathrm{C}}^{6+}$ acceleration by RPA is observed. The white area is where ${\mathrm{H}}^{+}$ is favored by RPA and the grey area is where ${\mathrm{n}}_e<\gamma {\mathrm{n}}_c$ and proton acceleration is favored in the RIT regime.

Figure 4

Average density (left logarithmic axes) for ${\mathrm{C}}^{6+}$ (solid black), protons (red), and electrons (blue) for 10 nm [(a),(b)], 15 nm [(c),(d)], and 25 nm [(e),(f)] along the laser axis (averaged between $y=\pm 100\mathrm{nm}$). The right axes display the cycle-averaged longitudinal electric field ($E_x$, dashed black line and linear axis). The left panels capture the interaction at close to the peak of the pulse ($-3\mathrm{fs}$) while the panels on the right refer to a time towards the end of the pulse ($+37\mathrm{fs}$).