A collisional-radiative model for lithium impurity in plasma boundary region of Experimental Advanced Superconducting Tokamak

A green emission layer caused by lithium impurity is universally observed in plasma boundary region of Experimental Advanced Superconducting Tokamak (EAST) via a visible-light camera, where lithium coating is normally adopted as a routine technique of wall conditioning. In this article, in order to estimate the spatial distribution of green light intensity of this emission layer according to the given real parameter distributions of edge plasmas, a practicable method is proposed based on a collisional-radiative model. In this model, a finite number of energy levels of lithium are taken into account, and proper simplifications of convection-diffusion equations are made according to the order-of-magnitude analysis. We process the atomic data collected from the OPEN-ADAS database, and develop a corresponding program in Mathematica 10.4.1 to solve the simplified one-dimensional problem numerically. Estimation results are obtained respectively for the two sets of edge plasma profiles of EAST in L-mode and H-mode regimes, and both clearly show a good unimodal structure of the spatial distribution of green light intensity of this emission layer. These analyses actually provide the spatial distributions of lithium impurities at different energy levels, not only indicating the spatial distribution of the intensity of this emission layer induced by lithium impurity but also revealing the physical processes that lithium experiences in edge plasma. There are some different and common characteristics in the spatial distribution of the intensity of this emission layer in these two important cases. This emission layer is kept outside the last closed magnetic surface in both cases while it becomes thinner with a higher intensity peak in H-mode case. Besides, the sensitivity of this algorithm to the measurement error of edge plasma profile is also explored in this work. It is found that the relative errors of the numerical results obtained by our proposed method are comparable to those of edge plasma profiles. This work provides important theoretical references for developing a new practical technique of fast reconstructing edge plasma configurations in EAST based on the emission of lithium impurity, and may further contribute a lot to the studies of edge plasma behaviors when three-dimensional perturbation fields are adopted.