A COMPARISON OF TIME-DEPENDENT IONIZATION MODELS FOR LASER-PRODUCED PLASMAS

Models for calculating time-dependent ionization are compared in simul ations of a germanium plasma for conditions which are relevant to coll isionally pumped x-ray lasers. A simple two-level model, which treats each ion stage with a ground state and single excited state, is compar ed with the collisional radiative model in the hydrogenic approximatio n and the time-dependent average atom model in simplified single Lagra ngian test cell problems. A method for determining ionic populations f rom average atom level populations (Djaoui and Rose 1992) is investiga ted. The results show good overall agreement between the three models, but indicate that the two-level model underestimates stepwise ionizat ion via excited states. The importance of this limitation is considere d in more detail in full simulations of a collisionally pumped system with a 1.5D hydrodynamic code. The two-level model is tested against t he hydrogenic collisional radiative approach and the importance of det ailed modelling of the sodium- and neon-like ion stages is also consid ered. The simulations indicate that the discrepancies between the two- level and hydrogenic models are small. A detailed description of the n eon-like ion stage, considering both enhanced ionization from the meta stable levels and dielectronic recombination, is found to be more sign ificant when evaluating the neon-like population.