Three-dimensional particle-in-cell simulations of laser-driven multiradiation sources based on double-layer targets

Double-layer targets (DLTs), made of a low-density foam on top of a solid substrate, can efficiently convert the energy of a high-intensity laser to provide sources of photons and protons. We investigate a 30-fs pulse with a peak intensity of $I\sim 8.7\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$ and a peak power of $\sim 120$ TW interacting with a DLT using three-dimensional (3D) particle-in-cell simulations. We focus on providing quantitative results in full 3D geometry on the foam thickness dependence; on the competition between two photon-generating processes in DLTs, i.e., nonlinear inverse Compton scattering (NICS) and bremsstrahlung (BS); and on the acceleration of protons via enhanced target-normal sheath acceleration. We discuss conversion efficiency, average energy, and angular distributions of such multiradiation sources. We find that NICS can prevail over BS if the DLT's substrate is thin enough $(\sim \mu \mathrm{m})$ and that the optimal foam thickness that maximizes the conversion efficiency in NICS and BS photons and the proton cutoff energy, among those considered, is the same $\left(15\mu \mathrm{m}\right)$. These results show that DLTs constitute an excellent tool for developing relatively compact and optimized laser-driven multicomponent radiation sources.

Figure 1

Snapshots from the 3D PIC simulations of photon emission with the $15\text{−}\mu \mathrm{m}$-thick foam DLT: (a) bremsstrahlung with epoch and (b) nonlinear inverse Compton scattering with warpx. The variables depicted are electron number density (gray scale), contours of $|B_z|=10$ kT (red-blue), macro-electrons (red dots), macro-protons (blue dots), and photons (green lines with length proportional to the photon energy).

Figure 2

Photon spectra for all the simulations (top row) and angular distributions of photons with energy $>0.1$ MeV for BS (central row) and NICS (bottom row). $\varphi =arctan(p_y/p_x)$ and $\theta =arctan(p_z/\sqrt{p_x^2+p_y^2\phantom{{}^1}})$ are the azimuthal and polar angles, respectively.

Figure 3

(a) Time evolution of the conversion efficiency of laser energy into photon energy. (b) Final value of the conversion efficiency as a function of the foam thickness. (c) Photon spectra for the $15\text{−}\mu \mathrm{m}$-foam DLT comparing NICS and BS with $1\text{−}\mu \mathrm{m}$ substrate vs 1-mm substrate.

Figure 4

Proton spectra for all the simulations (top row) and angular distribution of protons with energy between 2 and 10 MeV (central row) and between 10 and 50 MeV (bottom row) from NICS simulations. $\varphi =arctan(p_y/p_x)$ and $\theta =arctan(\sqrt{p_x^2+p_y^2\phantom{{}^1}}/p_z)$ are the azimuthal and polar angles, respectively.