Kinetics of highly excited states in Ar17+ charge exchange recombination fusion plasma spectroscopy

Charge exchange (CX) between impurities and injected neutral particles, which results in line emission from highly excited states of impurity ions, is the basis of an important technique for spectroscopic diagnostics of fusion plasmas. In this paper, we study the temporal kinetics of highly excited nl states (n ≤ 20) of H-like argon under typical conditions of a CX recombination experiment in tokamaks. A two-zone model is implemented to describe the tokamak plasma with and without a neutral beam. A detailed time-dependent collisional- radiative model describing ions of Ar15+ through Ar18+ and the atomic processes that affect the excited levels is used to determine the level densities. The relative contributions of atomic processes to population influx and outflux with and without CX are discussed in detail. The simulations show that after injection of a neutral beam, the populations of excited states oscillate between two quasi steady-state conditions, and the relaxation of ionization equilibrium is reached on times much longer than a typical plasma rotation time in a tokamak.