This paper investigates the interaction of an ultra-short, high-intensity laser with a near-critical density hydrogen target, offering valuable insights into how laser intensity, plasma density, and target thickness influence the generation of high-energy proton beams. We report that optimizing target parameters at a fixed laser intensity results in a significantly greater increase in proton energy compared to simply increasing the laser intensity from \(6 \times 10^{20}\) to \(6 \times 10^{21}\ \mathrm{W/cm}^2\) . Specifically, simulation results show that the maximum proton energy rises from 45 to 94 MeV with optimized target parameters, whereas it only increases from 45 to 68 MeV with higher laser intensity on a near-critical density target of thickness 100 nm. By carefully selecting the laser and target parameters, we successfully exploit the Directed Coulomb Explosion (DCE) mechanism for ion acceleration, where both Radiation Pressure Acceleration (RPA) and Coulomb Explosion (CE) contribute to achieving such high proton energies. The optimal density and thickness are found to satisfy the condition for DCE proposed by Brantov et al. (IEEE Trans Plasma Sci 44:364–368, 2015) and the energy obtained matches with the theoretically predicted energy for DCE (Bulanov et al. in Phys. Rev. E-Stat. Nonlinear Soft Matter Phys. 78: 026412, 2008). Protons with energies around 100 MeV hold significant potential for practical applications, including cancer therapy, fusion energy, and other advanced technologies.
