Stabilized Radiation Pressure Acceleration and Neutron Generation in Ultrathin Deuterated Foils

Premature relativistic transparency of ultrathin, laser-irradiated targets is recognized as an obstacle to achieving a stable radiation pressure acceleration in the “light sail” (LS) mode. Experimental data, corroborated by 2D PIC simulations, show that a few-nm thick overcoat surface layer of high $Z$ material significantly improves ion bunching at high energies during the acceleration. This is diagnosed by simultaneous ion and neutron spectroscopy following irradiation of deuterated plastic targets. In particular, copious and directional neutron production (significantly larger than for other in-target schemes) arises, under optimal parameters, as a signature of plasma layer integrity during the acceleration.

Figure 1

(a) Schematic of the experimental setup. (b) On-axis deuteron spectra obtained from a target thickness scan (shots taken with different target thickness, as labeled in the legend) under similar laser conditions ($E_L=(160\pm 20)\mathrm{J}$, $I_0=(2.1\pm 0.3)\times {10}^{20}{\mathrm{W}\mathrm{cm}}^{-2}$). (c) Proton and deuteron energies at their spectral peaks obtained for different target thicknesses, plotted against the LS scaling parameter, $a_0^2{\tau }_p/\chi$. The error bars along the $y$ (energy) axis refer to the width of the spectral bunch. The solid line represents the predicted ion energy according to the LS scaling reported in Ref. [7].

Figure 2

Neutron spectra measured simultaneously to the results in Fig. 1, along (a) the laser axis, and (b) 135° off the laser axis, respectively. The error bars, only plotted over two representative cases in (a) for clarity, represent the uncertainty in measuring neutron spectra by the TOF detectors in the current setup.

Figure 3

(a) 3D- and 2D-projection plots showing neutron (blue) production at different times (as labeled above) from a dense sail of deuterium ions (red) moving along the $x$ axis with a spectral distribution peaked at 20 MeV and FWHM width of 1 MeV, obtained from VSIM simulations. (b) Angular distribution of neutrons produced from sails of deuterium ions of narrow spectral distribution of 1 MeV FWHM, peaked at different energies as shown in the figure. The color bar represents neutron flux normalized for each case. (c) Deuteron vs neutron energies obtained in different shots shown in Fig. 1, compared with the energies expected from reaction kinematic (analytical) and VSIM simulations for sails of a dense bunch of deuterium ions. (d) Peak neutron flux along the $x$ axis (emitted within a small cone of 5°) from sails of deuterium ions of spectral distribution peaked at 20 MeV for different values of the spectral width ($\mathrm{\Delta }{E}_{\mathrm{long}}$).

Figure 4

Laser electric field (*,1), proton ($n_p$) and deuterium ion ($n_d$) number density as the peak intensity reaches the target (*,2) and at the end of the simulation ($\sim 200\mathrm{fs}$ after the peak intensity) (*,3), obtained by PIC simulations, for three different thickness of the target: 320 nm (a,*), 90 nm (b,*) and 90 nm $\mathrm{CD}+\mathrm{Au}$ (c,*). Color bars for proton and deuteron densities are common to (*,2) and (*,3).