Ionization lag in laser ionized mid-Z plasmas*

Ionization dynamics of laser-produced plasmas play a pivotal role in comprehending phenomena in larger macroscopic systems, including thermal transport and plasma instabilities. Simulations using the Non-local Thermodynamic Equilibrium (NLTE) model Cretin, show when intense laser beams are used to produce and heat a mid-Z plasma, the rapidly changing conditions lead to a lag in ionization. The ionization state in these systems does not reach the steady state predictions even after the plasma conditions appear stable due to the rapid changes in the plasma. The characteristic time for these plasmas to reach steady state ionization was found to be 1-2ns. Experimental measurements of the ionization state were performed using ion-acoustic wave Thomson scattering in argon-hydrogen plasmas where the mass difference produces two sets of ion-acoustic wave features. These measurements show the ionization state can remain below the steady state predictions over 2ns.