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.
Highlights
A high-intensity laser pulse interacting with a plasma on the front surface of a solid target will generate a population of hot electrons that propagates into the target
Mackinnon et al.[21] showed an increase in proton energies that was suggested to be due to reacceleration of hot electrons as the laser pulse duration is longer than the recirculation time through the target
We identify the refluxing and escaping electron populations using particle tracking and show that the temperature of each is lower than that of the initial hot-electron population accelerated by the laser
Summary
A high-intensity laser pulse interacting with a plasma on the front surface of a solid target will generate a population of hot electrons that propagates into the target. Mackinnon et al.[21] showed an increase in proton energies that was suggested to be due to reacceleration of hot electrons as the laser pulse duration is longer than the recirculation time through the target These re-accelerated electrons are able to increase the field strength on the rear surface as they return multiple times. La Fontaine et al.[28] studied the X-ray source size for two different target thicknesses, 20 and 100 μm, using a penumbral imaging detector, showing that the source size increases by a factor of two for the thinner target This was likely due to the electrons being able to reflux through the target for an increased number of times, which was modelled using particle-in-cell (PIC) simulations. Both the refluxing (Section 4) and escaping (Section 5) electron populations are discussed separately after sections describing the simulation methodology (Section 2) and the analysis of the simulated electric field (Section 3)
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