Laser-driven ion acceleration in the presence of increasing heating of relativistic electrons at steep overdense plasma interfaces

The role of the density gradient in the electron acceleration process by intense laser pulses for a plasma profile with a steep interface between vacuum and a strongly overdense plasma is investigated via particle-in-cell simulations with the Emi2d code. Laser pulses at relativistic intensities interacting with finite gradients at the laser–plasma interface favor collective electron motion in the underdense plasma provided that the pulse duration is long enough to form a standing wave structure. It is shown that the steepness of the gradient influences the evolution of the distribution of electrons that are injected into the dense plasma. Heating mechanisms of the electron bulk and a very energetic electron tail are identified. The heating of the targets evolves each time when bunches of electrons accelerate to relativistic energies and return to the laser–plasma interface. The heating dynamics have consequences on the ion front motion at the rear of the target. This is elaborated by determining the predominant hot electron populations and their temperatures that govern the expansion of the rear density profile. The role of the temporal dynamics of the hot electron populations is investigated with respect to the known analytic models for ion acceleration at the rear target, showing that those models are robust despite the important temporal increase in hot electron temperatures.