Numerical Investigation of Ionization Dynamics in Intense Laser-heated Argon Plasmas using the NLTE Model*

Ionization dynamics of laser-produced plasma play a pivotal role in comprehending phenomena in larger macroscopic systems, including thermal transport and plasma instabilities. This study investigates the ionization dynamics of Argon gas plasma (N_e: 10¹⁹ ~ 2.5x10¹⁹ cm^-3) irradiated by intense laser beams using the widely used NLTE model, CRETIN [1, 2]. Our objective is to accurately determine the ionization process. Under normal circumstances, the photon energy of an optical laser (~3 eV) is typically insufficient to overcome the binding energy of bound electrons, resulting in a low probability of photoionization. However, when a high-energy laser with an intensity of approximately 660 kJ (60 kJ × 11 beams) is focused on the plasma, the abundance of photons leads to multi-photon ionization. This process generates numerous highly excited states through dielectric capture (or inverse process of Auger ionization) of the free electrons as well as various excitation processes. Consequently, even with low photon energy, the result demonstrates the photoionization process can be dominant throughout the experimental time range and dynamics may not reach a steady state. This research enhances our understanding of ionization phenomena in laser-produced plasma and provides valuable insights for various applications.