The paper discusses the observation of energetic protons in ultra-intense laser-solid interactions, specifically in PetaWatt experiments. The authors propose a mechanism where hot electrons generated by the laser prepulse interact with the target, ionizing the hydrogen layer on the backside of the target and accelerating the protons to energies of tens of MeV over distances of a few microns. This process, termed Target Normal Sheath Acceleration (TNSA), explains the observed proton energies, number, and angular spread. The paper also presents a 1-D model to explain the acceleration mechanism and 2-D simulations to validate the model, showing that the accelerating electric field is strongly dependent on the spatial distribution of electrons. Additionally, the authors suggest that curving the back of the target could create an ion lens, focusing the protons to a small spot size while maintaining high energy. The findings are consistent with experimental data and offer insights into the physics of ultra-intense laser interactions with solid targets.The paper discusses the observation of energetic protons in ultra-intense laser-solid interactions, specifically in PetaWatt experiments. The authors propose a mechanism where hot electrons generated by the laser prepulse interact with the target, ionizing the hydrogen layer on the backside of the target and accelerating the protons to energies of tens of MeV over distances of a few microns. This process, termed Target Normal Sheath Acceleration (TNSA), explains the observed proton energies, number, and angular spread. The paper also presents a 1-D model to explain the acceleration mechanism and 2-D simulations to validate the model, showing that the accelerating electric field is strongly dependent on the spatial distribution of electrons. Additionally, the authors suggest that curving the back of the target could create an ion lens, focusing the protons to a small spot size while maintaining high energy. The findings are consistent with experimental data and offer insights into the physics of ultra-intense laser interactions with solid targets.