Abstract
The absorption of laser energy and dynamics of energetic electrons in dense plasma is fundamental to a range of intense laser-driven particle and radiation generation mechanisms. We measure the total reflected and scattered laser energy as a function of intensity, distinguishing between the influence of pulse energy and focal spot size on total energy absorption, in the interaction with thin foils. We confirm a previously published scaling of absorption with intensity by variation of laser pulse energy, but find a slower scaling when changing the focal spot size. 2D particle-in-cell simulations show that the measured differences arise due to energetic electrons recirculating within the target and undergoing multiple interactions with the laser pulse, which enhances absorption in the case of large focal spots. This effect is also shown to be dependent on the laser pulse duration, the target thickness and the electron beam divergence. The parameter space over which this absorption enhancement occurs is explored via an analytical model. The results impact our understanding of the fundamental physics of laser energy absorption in solids and thus the development of particle and radiation sources driven by intense laser–solid interactions.
Highlights
Laser energy absorption by electrons is central to high intensity laser–plasma interaction physics
In the absence of experimental techniques to resolve the various coupling processes occurring on ultrashort timescales, we can progress our understanding of the overall absorption physics by determining the total laser absorption from measurements of the reflected and scattered laser light [16, 17], and its dependency on laser and plasma parameters
We report on the first measurements of total absorption scaling as a function of laser intensity in which the pulse energy and focal spot size are separately varied, for the case of relativistic laser–foil interactions
Summary
Laser energy absorption by electrons is central to high intensity laser–plasma interaction physics. These results were utilised to develop an empirical scaling law [19] and to develop a theoretical model which identifies upper and lower bounds on laser-coupling for a given laser intensity [20] Whilst this demonstrates that absorption depends strongly on laser intensity by variation of pulse energy, the influence of focal spot size and pulse duration on total energy absorption in the relativistic regime is not well characterised. We report on the first measurements of total absorption scaling as a function of laser intensity in which the pulse energy and focal spot size are separately varied, for the case of relativistic laser–foil interactions. The results highlight the importance of considering changes to absorption caused by the recirculating population of relativistic electrons in experiment design and in the development of laser-driven particle and radiation sources
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