Development of ultralow density double layer targets for laser-plasma interaction

The development of ultra-low-density targets has become an important research topic due to their applications in experiments with high power lasers that aim to accelerate particles or generate short bursts of radiation. In the last years there has been strong interest in optimizing the laser-plasma interaction to achieve enhanced particle acceleration. Our approach is focused on developing specially structured foil targets which can improve laser absorption. We present a detailed methodology for utilizing Pulsed Laser Deposition (PLD) to fabricate ultra-low density carbon targets, within a density range of a few mg/cc, on aluminum foil substrates, along with their characterization. Furthermore, we conducted experimental validation of the laser-target interaction in vacuum using a millijoule level, femtosecond (fs) pulsed laser in the range of intensities of about ⁓10¹³ W/cm² at ELI-NP to evaluate the plasma plume formation at the target’s surface. A comparative analysis with flat aluminum foils was conducted using an ultra-fast ICCD imaging camera. The findings reveal a clear difference between the kinetic energy of the plasma plume ejected from the structured targets compared to the simple Al foils. The experimental data were compared with the radiative-hydrodynamic simulations results obtained with the FLASH code. In the simulation the evolution of the plasma plume in time was monitored and the mechanism of plasma heating and expansion was evidenced. The solid Al-foils are heated principally near their surface and the generated plasma plume has a low and localized expansion. In contrast, in the case of a low-density targets the radiation absorption is volumetric leading to an explosion-like plasma expansion.