Two-dimensional energy transfer and plasma formation under laser beam irradiation of a subcritical-density material

Based on two-dimensional numerical simulations, a study was made of the radiation absorption, the energy transfer, and the plasma formation upon the interaction of a laser beam with a homogeneous medium consisting of light elements with a density not exceeding the critical plasma density, which corresponds to the plasma resonance for the wavelength of the driving laser radiation. The calculations were performed by the HEAT-3D code, which involved the solution of a two-dimensional heat conduction equation with an energy source describing the absorption of laser radiation due to inverse bremsstrahlung in the material. The simulations were performed of the interaction of laser beams of radii 10^–2 — 6×10^–2 cm, 10¹⁴ — 5×10¹⁵ W cm^–2 in intensity, and with wavelengths of 1.053 and 0.527 μm with materials composed of light elements with densities of 1 — 10 mg cm^–3. An analysis of the simulations showed that the spatial temperature distribution of the resultant plasma is determined by the anisotropy of energy transfer. In its turn, the degree of anisotropy depends on the relation between the beam radius and the laser radiation absorption length, which is a function of the density and the temperature of the resultant plasma. The results of simulations are compared with the findings of experiments on laser irradiation of targets composed of low-density materials.