Assessment of Multiplet Splitting and Line Radiation Imprisonment Effects during Discharge Quenching by Intense Argon Injection in ITER

Abstract

One of the conditions of safe operation for the experimental tokamak reactor ITER is the possibility of mitigating disruption instability by massive injection of inert gases, in particular, of argon and neon. Here we present the results of assessing the influence of multiplet splitting and line radiation imprisonment during the discharge quenching by intense argon injection in ITER. In this paper, the fine structure of energy levels and the noncoronal collisional-radiative kinetics for the radiating excited state are used. For the radiation of two argon ions, Ar⁺¹⁵ and Ar⁺³, which have spectral lines of high intensity and could be used for plasma diagnostics, it is shown that the optical thickness for the ionic strongest lines has no significant effect on the total power losses of plasma radiation in the considered quenching scenario (massive argon injection in the 15 MA, Q ~ 10 basic scenario in ITER, carried out at the quasi-stationary stage of the discharge, flat-top of the current). The most significant effect appears to be the multiplet splitting of atomic levels, which provides an increase in the radiative losses, e.g., by a factor of ~2 for low-ionized atoms at low temperatures, because the resolution of the fine structure of atomic levels for Δn = 0 transitions leads to a contribution of lower excitation energy than that in the model of multiplet-average radiative transitions.