Stable laser-ion acceleration in the light sail regime

We present experimental results on ion acceleration with circularly polarized, ultrahigh contrast laser pulses focused to peak intensities of $5\times {10}^{19}\mathrm{W}{\mathrm{cm}}^{-2}$ onto polymer targets of a few 10 nanometer thickness. We observed spatially and energetically separated protons and carbon ions that accumulate to pronounced peaks around 2 MeV containing as much as 6.5% of the laser energy. Based on particle-in-cell simulation, we illustrate that an early separation of heavier carbon ions and lighter protons creates a stable interface that is maintained beyond the end of the radiation pressure dominated acceleration process.

Figure 1

Measured proton (blue) and ${\mathrm{C}}^{6+}$ (red) spectra generated using circularly polarized laser pulses with durations 34 fs (a)–(d), 70 fs (e)–(h), and target thicknesses of ranging from $l=15\mathrm{nm}$ to $l=50\mathrm{nm}$. To compare the velocities of the individual ion species, the energy scale is given in MeV per atomic mass unit $u$, the ion interface is indicated as the ${\mathrm{C}}^{6+}$ cutoff (black dashed line) with its standard deviation of three laser shots (grey area). The low energy cutoff of the proton spectra is due to the detector edge which was modified in the case of (a), (c), and (e) to resolve the low energy part of the proton trace.

Figure 2

Data from 3D-PIC simulation. (a) Spatial proton and carbon ion density distribution in the $x\mathrm{\text{−}}y$ plane at $z=4{\lambda }_0$ and $t=54\mathrm{fs}$ after the arrival of the laser pulse. Obviously, the protons move ahead of the carbon ions. (b) Proton and carbon ion density distribution in the $y\mathrm{\text{−}}z$ plane at $x=2.3{\lambda }_0$ (top) and $x=2.4{\lambda }_0$ (below) at the same time to indicate that the protons are always located outside of the carbon ion distribution. (c) Spectral distribution of protons (black) and carbon ions (red). The dashed red line in (a) and (b) illustrates the relative position of the proton with respect to the carbon ions.

Figure 3

Data from 2D-PIC simulation. (a)–(d) Spatial proton (blue) and carbon (red) ion density distribution in the $x\mathrm{\text{−}}y$ plane at different times (given in the figure caption) after the arrival of the peak of the Gaussian (in space and time) laser pulse. The spectral evolution during the laser-target interaction of protons (e), carbon ions (f), and after the interaction (g). In (h) the proton phase space during the interaction is revealing the characteristic spiral structure for RPA.