Influence of a preplasma on electron heating and proton acceleration in ultraintense laser-foil interaction

Two-dimensional particle-in-cell simulations are performed to study laser-induced proton acceleration from solid-density targets in the presence of laser-generated preformed plasma. The preplasma generation and hydrodynamics are described using a one-dimensional Lagrangian code. The electron acceleration mechanism is shown to depend on the plasma scale length, exhibiting a transition from j × B heating to standing wave heating as smoother and smoother profiles are considered. Accordingly, the relativistic electron temperature and the cutoff proton energy are found to increase with the preplasma characteristic length.