Description
The interaction of high-power laser pulses with low-density porous materials is of significant interest for inertial confinement fusion (ICF) and high-energy-density physics. In such targets, the foam homogenization timescale and the resulting plasma conditions strongly affect laser–plasma coupling. We report on an experimental study of laser interaction with ultra-low-density SiO₂ foam targets performed at the Prague Asterix Laser System (PALS). The third harmonic of a kilojoule-class iodine laser (λ = 438 nm, τ ≈ 350 ps) was focused to intensities >10¹⁵ W/cm². The targets consisted of closed-pore foams made of ultra-thin SiO₂ shells with an average density of ~40 mg/cm³.
Plasma formation and expansion were diagnosed using three-frame interferometry with a synchronized Ti:Sapphire probe laser (λ = 800 nm, τ ≈ 40 fs). The ionization wave propagation and plasma expansion dynamics were recorded with a soft X-ray streak camera. Bremsstrahlung emission and the energy distribution of escaping electrons were measured using a bremsstrahlung cannon and electron spectrometers. The delay between the 50 J auxiliary beam at the first harmonic (λ = 1315 nm) and the main laser pulse was varied to control pre-plasma formation, evaluate its role in foam pre-homogenization, and generate long- scale-length plasma for the study of laser–plasma instabilities.
Hydrodynamic simulations performed with the FLASH code were used to interpret the experimental observations and clarify the homogenization mechanism. In over-dense, closed-pore SiO₂ foams, a shock wave forms and X-ray preheating ahead of the shock front initiates homogenization. As a result, the main laser pulse interacts with a pre-homogenized low-density plasma. The simulations indicate a shock-driven homogenization velocity of ≈310 μm/ns and an under-dense plasma density scale length of ~125 μm. The simulated plasma parameters are consistent with interferometric and X-ray diagnostics, providing a coherent description of the foam homogenization and the shock ignition ICF schemes.