Lateral confinement of fast electrons and its impact on laser ion acceleration

In intense laser-plasma interactions, maximizing the density of fast electrons in the laser spot area is key to achieving plasma heating and particle acceleration. We find that when the laser spot size is large compared with the target foil thickness, fast electrons circulating in the foil show a random walk in the lateral direction due to the scattering by fluctuating fields at the plasma surface inside the spot area. We model the lateral motion as a diffusion, and find the resulting diffusion velocity is much slower than the speed of the ballistic transport. Hence, fast electrons accumulate in the spot region, and over time their density becomes typically 10 times greater than the laser-accelerated fast electron density. The enhancement of fast electron density in the target pushes the ion acceleration to more efficient regime. The model is planned to be tested by a NIF Discovery Science experiment.