Speaker
Description
Low-density foams have a wide variety of applications in the fields of inertial confinement fusion and high energy density physics. However, direct simulations of laser interaction with foam targets are difficult and computationally expensive due to the necessity to spatially resolve the density differences in the foam microstructure in order to capture the underlying physical phenomena. Unfortunately, low-density foams also cannot be modeled as a uniform material of an equivalent mean density as such results overestimate the propagation speed of the laser-driven ionization wave.
Recent interests in foam simulations led to the development of two-scale models [1,2,3] where a simplified interaction model is computed on the scale of an individual foam pore in addition to the conventional macroscale hydrodynamics. Existing models describe the laser-foam interaction in terms of volumetric heating and expansion of planar/cylindrical foam microstructure. However, further analysis of laser absorption in sub-wavelength objects shows that laser is absorbed mostly at the surface of the overcritical foam elements and that ablation plays an important role in the overall dynamics. The mass ablated from the surface layer rapidly fills the empty space in the pores and creates a high-temperature, low-density plasma background. A significant portion of the absorbed laser energy is converted to an ion kinetic energy due to ion acceleration by the ablation process.
The subsequent ion-ion collisions with the ambient plasma contribute to the dissipation of this energy into an ion thermal component, effectively acting as an ion heating.
We present a novel approach to the foam modeling that combines a self-similar expansion of cylindrical elements with a surface ablation by laser. In our microscale model, each foam pore is divided into two regions with separate masses, densities and temperatures - the central cylinder represents the expanding solid element and the outer plasma region acts as the ablated plasma in the pore. The movement of the boundary between these two environments is controlled by the self-similar expansion while the mass transfer between regions is given by a stationary ablation model. Ordinary differential equations for the temporal advancement of the state variables are solved on the microscale and connected to the macroscale hydrodynamics using the conservation of energy. Pores are considered to be homogenized when the cylinder and plasma reach the same density. The cross-section for laser deposition and scattering is calculated according to the Mie theory of electromagnetic scattering on cylindrical particles.
The proposed model is implemented in the PALE and FLASH hydrodynamic codes for laser-plasma interaction and the results are compared to the available experiments. The comparison shows that the model is sufficiently flexible and produces results compatible with observations.
References
1. J. Velechovský et al., Plasma Phys. Control. Fusion 58, 095004 (2016)
2. M. Cipriani et al., Laser and Particle Beams 36, 121 (2018)
3. M.A. Belyaev et al., Physics of Plasmas 27, 112710 (2020)