Scaling the Yield of Laser-Driven Electron-Positron Jets to Laboratory Astrophysical Applications

We report new experimental results obtained on three different laser facilities that show directed laser-driven relativistic electron-positron jets with up to 30 times larger yields than previously obtained and a quadratic ($\sim E_L^2$) dependence of the positron yield on the laser energy. This favorable scaling stems from a combination of higher energy electrons due to increased laser intensity and the recirculation of MeV electrons in the mm-thick target. Based on this scaling, first principles simulations predict the possibility of using such electron-positron jets, produced at upcoming high-energy laser facilities, to probe the physics of relativistic collisionless shocks in the laboratory.

Figure 1

Dependence of the measured positron yield on the laser energy, $E_L$, obtained at three different laser facilities: Omega EP, Orion, and Titan. The upper group is from shots with 1 ps laser pulse: (brown) triangles Titan and (green) diamonds Orion. The lower group is obtained with 10 ps laser pulse: (blue) squares Titan and (red) circles Omega EP.

Figure 2

Dependence of the positron yield, per kJ of incident laser energy, on the laser intensity. The experimental data are compared with analytical calculations based on the model of Ref. [20] including refluxing are shown in solid black circles (or diamonds) and with geant4 simulations shown in empty black circles (or diamonds). The calculations were made for using two different Te scalings with laser intensity: Pukhov scaling (circles) and Ponderomotive scaling (diamonds). The results from LSP simulations are shown for the cases with refluxing (solid red square) and without refluxing (a hollow square). The data are well fitted when refluxing is included, and we use Pukhov scaling for Te. The conversion efficiency from the laser energy to electrons calculated by LSP is about 40%, and the same conversion efficiency is used for both Te scalings in the analytical and geant4 calculations.

Figure 3

Particle-in-cell simulations of the counterstreaming of relativistic pair plasmas for laser-driven laboratory parameters. Magnetic field structure and transversely averaged density profile (inset) at the end of the interaction are shown for pair plasma flows corresponding to laser parameters of a) $E_L=7\mathrm{kJ}$, ${\tau }_0=10\mathrm{ps}$, and b) $E_L=22\mathrm{kJ}$, ${\tau }_0=25\mathrm{ps}$. The results illustrate the possibility of reaching (a) the saturation of the Weibel instability and (b) the formation of a shock with near-future laser systems.