Proton Acceleration with High-Intensity Ultrahigh-Contrast Laser Pulses

We report on simultaneous measurements of backward- and forward-accelerated protons spectra when an ultrahigh intensity ($\sim 5\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$), ultrahigh contrast ($>{10}^{10}$) laser pulse interacts with foils of thickness ranging from 0.08 to $105\mu \mathrm{m}$. Under such conditions, free of preplasma originating from ionization of the laser-irradiated surface, we show that the maximum proton energies are proportional to the $p$ component of the laser electric field only and not to the ponderomotive force and that the characteristics of the proton beams originating from both target sides are almost identical. All these points have been corroborated by extensive 1D and 2D particle-in-cell simulations showing a very good agreement with the experimental data.

Figure 1

Variation of maximum detectable proton energy as a function of target thickness. The FWD and BWD emissions for a laser contrast of ${10}^{10}$ (${10}^6$) and intensity of $5\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$ (${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$) are represented, respectively, by open (solid) circles and squares. Lines are a guide for the eye.

Figure 2

Radiochromic films profiles in the FWD (left) and BWD (right) direction for the same shot. The estimated divergence along the dashed lines is around 4.5° for both proton beams.

Figure 3

Comparison of experimental data with numerical predictions for FWD and BWD proton emission of the 1D and 2D PIC simulations.

Figure 4

Electron energy distribution inside the target at the laser peak for a 400 nm and a semi-infinite target.

Figure 5

(a) Proton energy distribution in the FWD and BWD direction for a 400 nm target and (b) related electron phase space plot at the laser peak.

Figure 6

Plot of maximum proton energy as a function of the laser polarization in the HC case in the FWD (left) and BWD (right) direction. The laser $p$ ($s$) polarization corresponds to the 0° (45°) wave plate angle. The solid line is the best fit using the function defined in the text. The horizontal dashed lines show the detection thresholds for FWD (0.33 MeV) and BWD (0.14 MeV) emissions. The full symbols represent the points obtained from 2D PIC simulations, normalized to the experimental mean values for $p$ polarization.