Ion Ring Formation after Laser-driven Ion Acceleration in Thin Film Targets

High-intensity lasers are able to accelerate electrons to high energies in less than one optical cycle and these hot electrons can efficiently accelerate ion beams to energies exceeding tens of MeV/nucleon. Understanding the characteristics (energy, divergence, etc.) of these beams is key to interpreting experimental results and developing potential applications. In this work, we perform 3D particle-in-cell simulations using the code EPOCH to examine the resulting ion beams that are formed when an intense, short-pulse laser (I ∽ 10²¹ W/cm², τ ∼ 30 fs) interacts with a thin liquid crystal target. We show that ion rings with a relatively large opening angle (∼ 5^○ – 10^○) can form in the angular distribution far from the target. We compare our results to a recent experiment performed at the Scarlet Laser Facility at The Ohio State University and find reasonable agreement. We describe how quasi-static electric fields forming after the laser-plasma interaction lead to ring formation.