Energy absorption and coupling to electrons in the transition from surface- to volume-dominant intense laser–plasma interaction regimes

Abstract

The coupling of laser energy to electrons is fundamental to almost all topics in intense laser-plasma interactions, including laser-driven particle and radiation generation, relativistic optics, inertial confinement fusion and laboratory astrophysics. We report measurements of total energy absorption in foil targets ranging in thickness from 20 μm, for which the target remains opaque and surface interactions dominate, to 40 nm, for which expansion enables relativistic-induced transparency and volumetric interactions. We measure a total peak absorption of ∼80% at an optimum thickness of ∼380 nm. For thinner targets, for which some degree of transparency occurs, although the total absorption decreases, the number of energetic electrons escaping the target increases. 2D particle-in-cell simulations indicate that this results from direct laser acceleration of electrons as the intense laser pulse propagates within the target volume. The results point to a trade-off between total energy coupling to electrons and efficient acceleration to higher energies.

Optimization and control of synchrotron emission in ultraintense laser–solid interactions using machine learning

“…To produce synchrotron radiation in these interactions, highly relativistic electrons are required in an intense electromagnetic field, and as changes to the target thickness cause a transition between transparent and opaque targets, the coupling of the laser energy to the electrons also changes [ 84 ] . Therefore, the total electron energy sampled along the -axis ( nm) in the transparent plasma in front of the relativistic critical density surface ( , where ) and in the laser skin depth ( ), integrated over the period of synchrotron emission (estimated as , where the laser peak intensity reaches at ), was calculated for each of the simulations and is shown in Figure 2(b).…”

Optimisation of multi-petawatt laser-driven proton acceleration in the relativistic transparency regime

“…A greater fraction of the laser energy is transferred to the hot electron population, which produces electric fields that accelerate the protons, during the interaction when RSIT occurs on the falling edge of the laser pulse. Measurements using a laser system with τ L = 700 fs and a 0 = 9 have shown the overall absorption of the laser pulse peaks for the thinnest target that does not become relativistically transparent [59]. Although the overall trend is similar for all four laser intensities, the relative conversion efficiencies achievable are almost a factor of two higher for a 0 = 160 than a 0 = 16, with little increase from a 0 = 160 to a 0 = 310.…”

“…Figures 1(a) and (b) show the electron density, and (c) shows the laser intensity, illustrating the propagation of the laser light through the relativistically underdense (classically overdense) plasma. Here the laser pulse is volumetrically absorbed within the target, generating a larger number of high energy electrons which escape the target, when compared to targets that do not experience RSIT [59]. This is due direct laser acceleration (DLA) [60] of electrons, induced by strong longitudinal electric field structures that form as the laser pulse reflects off and self-focuses through the plasma aperture [61].…”

Self-Referencing Spectral Interferometric Probing of the Onset Time of Relativistic Transparency in Intense Laser-Foil Interactions

“…This can occur on the rising or falling edge of the laser pulse interaction with the foil. The fraction of the pulse that is transmitted will further heat the electron population over its propagation length, enhancing the electrostatic fields responsible for ion acceleration [26]. The time at which RSIT occurs with respect to the peak of the laser pulse determines the fraction of the transmitted laser pulse energy and the degree of enhancement in the electrostatic field strength.…”