Evaluation of tungsten influx rate using line emissions from W 5+ ions in EAST tokamak

Abstract

The S/XB ratios (ionization events per emitted photon) allow one to relate spectroscopic emissivity measurements to the impurity influx from a localized source. In this work, we determine the tungsten influx by examining two dominant extreme ultraviolet line emissions at 382.13 Å and 394.07 Å, corresponding to the 4f14 5f → 4f14 5d radiative transitions of the W5+ ion. The ground configuration of W5+ consists of the ground level and a metastable level, withthe latter having a higher population than the ground state. Therefore, a simple approach assuming that the transitions are independent, i.e. only populated by a unique level source, requires correction. To address this, we have developed a fully collisional-radiative modeling in which 430 levels contribute to the ionization. We have utilized three advanced computational codes – hullac (Hebrew University—Lawrence Livermore Atomic Code, AS (AutoStructure), and FAC (Flexible Atomic Code) – for the atomic structure calculations. These codes provide the necessary information such as wavelengths, collisional and radiative transition rate coefficients. The FAC code was also used to calculate the direct electron-impact ionization under the distorted-wave approximation. We also included contributions to total ionization from excitation-autoionization processes up to n = 15 manifolds from the distorted-wave calculations. Subsequently, we used these results to ascertain the tungsten impurity influx in a dedicated discharge of the EAST tokamak, which operates with full tungsten divertors. In our findings, we observed that for the density range relevant to the edge region of a tokamak plasma, the S/XB ratios are almost independent of electron density but exhibit significant variation with electron temperature.