Surprises in the Ionization Dynamics of Intense Laser-Produced Plasmas Revealed by NLTE Modeling*

Understanding the ionization dynamics in laser-produced plasmas is crucial for deciphering large-scale phenomena such as thermal transport and plasma instabilities. This study explores the new insights gained from the point example of the ionization behavior of an Argon plasma subjected to intense laser beams [1], analyzed using the well-established Non-local Thermodynamic Equilibrium (NLTE) model, Cretin [2]. Targeting the plasma with a high-energy laser, delivering approximately 2.2 kJ (200 J × 11 beams), induces rapid changes in the plasma conditions, results in remarkable delayed ionization responses. Consequently, the dynamics fail to stabilize even when electron temperatures and densities seem constant, necessitating time dependent NLTE computations. This situation is to be contrasted with standard steady-state modeling. Additionally, this interaction generates numerous highly excited states mainly via collisional excitation. Surprisingly, even photons with low energy (around 3.5 eV) effectively ionize the medium, contradicting the traditional view that such energies are too low to disrupt the binding of electrons, thereby challenging the assumed rarity of ionization aided by photons under these conditions. Building on our analysis of this system, we have gained new, critical insights into when and how to use NLTE models and the necessity for time-dependent modeling [3, 4].