Effect of rear surface fields on hot, refluxing and escaping electron populations via numerical simulations

Abstract

After a population of laser-driven hot electrons traverses a limited thickness solid target, these electrons will encounter the rear surface, creating TV/m fields that heavily influence the subsequent hot-electron propagation. Electrons that fail to overcome the electrostatic potential reflux back into the target. Those electrons that do overcome the field will escape the target. Here, using the particle-in-cell (PIC) code EPOCH and particle tracking of a large population of macro-particles, we investigate the refluxing and escaping electron populations, as well as the magnitude, spatial and temporal evolution of the rear surface electrostatic fields. The temperature of both the escaping and refluxing electrons is reduced by 30%–50% when compared to the initial hot-electron temperature as a function of intensity between $10^{19}$ and $10^{21}~~\text{W}/\text{cm}^{2}$ . Using particle tracking we conclude that the highest energy internal hot electrons are guaranteed to escape up to a threshold energy, below which only a small fraction are able to escape the target. We also examine the temporal characteristic of energy changes of the refluxing and escaping electrons and show that the majority of the energy change is as a result of the temporally evolving electric field that forms on the rear surface.

Laser produced electromagnetic pulses: generation, detection and mitigation

“…Moreover, a relatively small proportion of the electrons are accelerated to energies much above the average and may leave the target [28] , thus charging it positively. The total number of escaping electrons is defined by dynamical competition between the high energy of escaping electrons and the electric potential increase due to electron escape [29] . We describe numerical methods for the charge evaluation in Sections 2.3 and 2.4.…”

Scaling of laser-driven electron and proton acceleration as a function of laser pulse duration, energy, and intensity in the multi-picosecond regime

“…The measured electron distributions presented throughout this paper represent the escaping electron population, which is distinct from the internal electron distribution of electrons trapped within the target. Although, previous work [36][37][38] has demonstrated these two distributions to be linked.…”

HPLSE editorial tribute to Professor David Neely

“…An example of the latter is that the investigation into the effect of the electronic sheath field, responsible for ion acceleration, on fast electron dynamics within foil targets is reported in Ref. [7]. In that paper, it is shown that energetic electrons that do not overcome the electrostatic potential formed at the target rear are reflected and reflux within the foil.…”