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.
Highlights
Energy absorption and coupling to electrons in dense targets irradiated by high intensity laser pulses is fundamentally important to the development of ultra-bright sources of high energy ions [1, 2], neutrons [3, 4], positrons [5, 6] and photons [7], to advanced schemes for inertial confinement fusion [8], and in the generation of transient states of warm dense matter [9, 10]
Consistent with the definition used in Ping et al [19], we define the laser absorption as EAbs = EL − ES − EBR, where EL, ES and EBR are the energies of the incident laser pulse, the total light measured in the sphere and laser light back-reflected through the aperture, respectively
We report the first measurements of total laser energy absorption as a function of target thickness in the transition from surface- to volume-dominated relativistic laser–foil interactions
Summary
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. S D R Williamson1, R J Gray1 , M King1 , R Wilson1 , R J Dance1, C Armstrong1,2 , D R Rusby1,2, C Brabetz3, F Wagner3, B Zielbauer3, V Bagnoud3, D Neely1,2 and P McKenna1 Keywords: intense laser–solid interactions, laser absorption in dense plasma, relativistic self induced transparency
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