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Description
Low density foams have interesting properties that make them attractive for fundamental studies of laser plasma interaction and for various applications such as inertial confinement fusion and bright sources of X-ray emission. However, the process of transformation of a cold foam into a hot plasma is complicated and not well-known. Experiments and numerical simulations show that the ionization of foam by laser proceeds slower than ionization of a homogeneous material of the same average density. Qualitatively, it is explained by a delay needed for the foam solid elements to expand and to mix.
The existent numerical models [1, 2, 3] have difficulties in modeling of the propagation of ionization wave in the foam. It is related to the fact that the size of solid elements in the foam is smaller than the laser wavelength. The laser absorption efficiency depends strongly on the shape of the structure and its orientation with respect to the laser polarization. Macroscopic models do not describe accurately how much energy can be absorbed and reflected and how the absorbed energy is distributed between the cold wire and surrounding plasma.
In this work we present an analytical model of laser absorption and scattering in foams and numerical simulations of ablation and expansion of solid elements and plasma formation with a kinetic particle-in-cell code. Since foams are typically composed of randomly distributed wire-like solid elements, we model a single pore with a straight cylindrical wire of a sub-wavelength size in the center. Analytical model and numerical simulations show that the resonance laser absorption in an expanding, radially inhomogeneous wire is the dominant process, which depends weakly of the collisional dissipation. Transformation of a solid wire into a hot plasma is controlled by a competition of expansion and ablation processes which proceed on a 10 ps time scale for laser intensity of $\sim 10^{14}$ W/cm$^2$. The single-cell microscopic model is implemented in a hydrodynamic code as a sub-grid module describing the foam ionization and homogenization.
- J. Velechovsky et al., Plasma Phys. Control. Fusion 58, 095004 (2016)
- M. Cipriani et al., Laser Part. Beams 36, 121 (2018)
- M. A. Belyaev et al., Phys.Plasmas 27, 112710 (2020)
