A field-particle correlation analysis of laser-driven proton acceleration across the opaque–relativistically transparent transition

High-energy proton beams are essential for a wide range of applications, from fusion energy and medicine, to laboratory astrophysics. While several laser-driven acceleration mechanisms, such as Target Normal Sheath Acceleration (TNSA), have been studied extensively, understanding the transition to optimal acceleration regimes remains a challenge. Relativistically induced transparency has demonstrated to produce significantly more energetic proton beams than conventional TNSA. In this work, we use the field-particle correlation (FPC) analysis to explore proton acceleration in particle-in-cell simulations of the interaction between high intensity short-pulse lasers and solid density plasmas, both opaque and relativistically transparent. By comparing the characteristic correlation-space signatures of TNSA and RIT, we reveal localized energization mechanisms responsible for the enhanced performance of RIT. These findings offer new insight into the transition between acceleration regimes and optimization of laser-plasma interactions for high-energy proton acceleration.