Collisional radiative models with loss and recycling

We describe how the collisional-radiative model, which is usually used to simplify the treatment of excited states in the description of atomic processes in plasmas, must be extended if transport of the excited states is significant. We identify recycling of neutrals near the plasma edge as a circumstance where this is indeed the case. We show how the effective ionization and recombination rates appropriate to a correct 'nonlocal' collisional-radiative model depend on the recycling rate, as well as electron temperature and density. We are able to identify the specific shortcomings of the conventional collisional-radiative model, and show that its use leads to a spurious source of neutral particles. We compare results obtained for the atomic dynamics using both the conventional and extended collisional-radiative models with the exact solution of the appropriate nonlinear equations in which all the excited states are explicitly included, and demonstrate that whereas the conventional model fails to reproduce the exact result, the extended model does so. (C) 1998 American Institute of Physics.