Laser-driven ion acceleration from near-critical Gaussian plasma density profile

In this paper, we report on multiple phases of efficient laser-driven ion acceleration from near-critical density plasma of Gaussian density profile. Tracking of high-energy accelerated ions in multidimensional particle-in-cell simulations reveals the development of accelerating fields affecting the particles and the contribution of each acceleration phase to final ion energies. While the acceleration of ions occurs in a short time interval when a steep (infinite) density gradient is present, the accelerating field affecting the most energetic ions has unexpected local maxima about 50 fs after the moment when ultrashort (30 fs) laser pulse completely left the target with smooth density gradients. This field can be attributed to the apex of electron filament created behind the transmitted laser pulse. Full 3D simulation confirms the observations in 2D simulations in terms of ion acceleration mechanisms. However, it shows a substantial reduction of maximum achievable ion energies and a larger angular spread of accelerated ions compared with 2D approach, which demonstrates the necessity of using computationally demanding full 3D geometry for similar numerical studies.