  The ionization dynamic of argon plasma irradiated by an intense laser is
investigated to understand transient physics in dynamic systems. This study
demonstrates that significant delayed ionization responses and stepwise
ionization processes are crucial factors in determining the ionization state of
such systems. When an intense laser begins to ionize an initially cold argon
plasma, the conditions change rapidly, leading to a delayed response in
ionization. Consequently, the dynamics do not reach a steady state, even if the
electron temperature and density appear unchanged, particularly when the atomic
transition process is not sufficiently rapid compared to the relevant time
scales. Furthermore, in this case, numerous highly excited states are created
primarily through collisional excitation. Thus, even low-energy photons can
predominantly ionize plasmas, challenging the conventional belief that such
photon energies insufficient to overcome the binding energy of bound electrons
typically contribute less to the ionization. These findings underscore the
necessity of incorporating these processes in ionization modeling within
radiation hydrodynamic simulations for various laser-plasma experiments.
