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Description
The laser-target interaction at high laser intensity results in the generation of a population of fast electrons which penetrate through and spread across the target, forming an electron sheath at the target surface. The quasi-static sheath electric field creates a potential barrier close to the target surface that repels hot electrons with energies below the barrier height. The hot electrons with the energies above the barrier escape the target, which results in the electric polarization of the target [1]. The hot electron spot evolution may last for periods orders of magnitude longer than the laser pulse duration [2], [3] and extend to the size orders of magnitude larger than the laser spot [4]. The electric polarization of the target is essential for estimates of the target neutralization currents which are important in the studies of electromagnetic pulse emission, post-acceleration ion guiding and generation of extreme magnetic fields. The hot electron spot evolution is also essential for the studies of x-ray generation relevant for x-ray backlighting at large laser facilities. The first-principles modeling of the hot electron spot evolution is a formidable simulation task [1], hence for estimates in practical situations one must revert to simplified models. An interesting model of the hot electron spot evolution and target charging was proposed in [5], where high computability was achieved at the price of severe simplifications. Nevertheless, the model of [5] proved useful in estimating target charging at the conditions characteristic for the IPPLM 10 TW fs laser facility [6]. However, the model has some shortcomings: the early-phase hot electron spot evolution seems to be oversimplified; it seems that the model overestimates the sensitivity of the target charge to the target material; and it is likely that the model does not account properly for the target charge dependence on the laser pulse duration. In this contribution we take a closer look at this model and propose some improvements.
References
[1] J.-L. Dubois et al., “Target charging in short-pulse-laser–plasma experiments,” Phys. Rev. E, vol. 89, no. 1, pp. 013102, 1–15, Jan. 2014, doi: 10.1103/PhysRevE.89.013102.
[2] J. Myatt et al., “High-intensity laser interactions with mass-limited solid targets and implications for fast-ignition experiments on OMEGA EP,” Physics of Plasmas, vol. 14, no. 5, p. 056301, May 2007, doi: 10.1063/1.2472371.
[3] H. Chen et al., “Fast-electron-relaxation measurement for laser-solid interaction at relativistic laser intensities,” Phys. Rev. E, vol. 76, no. 5, p. 056402, Nov. 2007, doi: 10.1103/PhysRevE.76.056402.
[4] P. M. Nilson et al., “Time-Resolved Measurements of Hot-Electron Equilibration Dynamics in High-Intensity Laser Interactions with Thin-Foil Solid Targets,” Phys. Rev. Lett., vol. 108, no. 8, p. 085002, Feb. 2012, doi: 10.1103/PhysRevLett.108.085002.
[5] A. Poyé et al., “Thin target charging in short laser pulse interactions,” Phys. Rev. E, vol. 98, no. 3, pp. 033201, 1–12, Sep. 2018, doi: 10.1103/PhysRevE.98.033201.
[6] P. Rączka et al., “Target Charging, Strong Electromagnetic Pulse Emission and Proton Acceleration from Thin Foils at 10 TW IPPLM Femtosecond Laser Facility,” Acta Phys. Pol. A, vol. 138, no. 4, pp. 593–600, Oct. 2020, doi: 10.12693/APhysPolA.138.593.
