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Abstract
Low-density structured materials, or foams, became in the recent years of great interest for the research in Inertial Confinement Fusion (ICF) [1]. The internal structure constituted by solid parts and voids allows the laser beam to scatter on the solid parts and into the voids, thus penetrating the material in depth, even with an average density larger than the critical density for the given laser wavelength [2]. The inhomogeneous plasma created in this way allows for a volumetric absorption of the laser energy, which contribute to smooth the spatial non-homogeneity of the laser beam [3] and to increase the absorption efficiency [4], in turn reducing the development of parametric instabilities [5]. All these features lead to an improved overall coupling between the laser and the plasma, which represents one of the critical issues for ICF. A fusion capsule having an ablator layer constituted by low-density low-Z structured material could then show an enhanced laser-plasma coupling and therefore an increased efficiency in the compression, thus leading to a higher gain from fusion reactions initiated in the plasma. On the other hand, foams constituted of high-Z materials, such as gold, allow to obtain a higher efficiency of conversion of the laser energy into X-rays compared to a homogeneous material of the same chemical composition [6]. The efficient production of X-rays with high-power lasers is of great importance for the indirect drive scheme of ICF, where the fusion capsule is suspended at the center of a gold cylinder, called the hohlraum, whose inner walls are heated by powerful lasers to produce X-rays, which in turn ablate and compress the capsule. Structured materials in this context have the two-fold advantage of allowing the increase of the absorption efficiency of the laser energy and the conversion efficiency into X-rays.
Thanks to its compatibility with virtually every kind of substrate and its capability of controlling material density, morphology and composition down to the nanoscale, Pulsed Laser Deposition (PLD) could represent an ideal tool for the production of foam-based ICF targets, although its potential for this application is largely unexplored. In PLD, laser pulses are shot on a target placed in a vacuum chamber, causing the evaporation of target surface layers. The ablated species expand in a controlled background atmosphere and are finally collected on a substrate. The resulting PLD nanofoams are composed by nanoparticles (typical size from 10 nm to 100 nm) arranged in a void-rich, fractal-like structure. PLD synthesis offers a number of potential advantages compared with standard chemical techniques, such as the capability of tailoring the density and composition profile along the target thicknesses by suitably acting on the deposition parameters. Moreover, the morphological differences between standard plastic foams and PLD nanofoams at the micro- and nano-scale might offer an additional degree of freedom to control and improve laser-target interaction. In previous works we have shown how to exploit the PLD to tune the average density of carbon nannofoams from solid density down to few mg/cm3, a value close to the critical plasma density for visible laser wavelength [7,8,9]. We have demonstrated the potential of near-critical PLD nanofoam targets to enhance the particle energy and number in laser-driven acceleration with super-intense ultra-short (pulse duration < 1 ps) laser pulses [10, 11, 12].
Here we discuss the potential of PLD nanofoams in ICF-relevant laser-matter interaction. We present the results concerning the deposition of carbon- , boron- , copper- and gold-based PLD nanofoams having near-critical density. In particular we show the potential of an unconventional PLD setup exploiting ultrashort pulsed (namely fs-PLD) to obtain thick (>100 μm) metallic nanofoams. The irradiation of nanofoams with energetic (~ 100 J) and nanosecond laser pulsed is simulated with the 1D hydrodymanic code MULTI-FM, which has been recently applied to the study of laser absorption and plasma behavior with porous targets [6, 13]. MULTI-FM parameters have been suitably adapted to the simulation of nanostrucured fractal-like materials as the PLD foams. Finally, we discuss a the design of a proposed experiment concerning the irradiation of PLD nanofoam targets in ICF-relevant conditions, to be realized at the ABC laser facility at ENEA Frascati, Italy.
References
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2. S. Y. Gus’kov and V. B. Rozanov, Quantum Electron. 27, 696 (1997).
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4. M. Cipriani et al., High. Pow. Laser Sci. Eng. 9, e40 (2021).
5. D. A. Mariscal et al., Physics of Plasmas 28, 013106 (2021).
6. C. Kaur, Plasma Phys. Control. Fusion 61, 084001 (2019).
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10. M. Passoni et al., Plasma Phys. Control. Fusion 62 014022 (2019)
11. A. Pazzaglia et al., Commun. Phys. 3 133 (2020)
12. I. Prencipe et al., New J. Phys. 23 (9) 093015 (2021)
13. M. Cipriani et al., JINST 15 C10003 (2020)
Acknowledgements
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 – EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.
