Evaluation of tungsten influx rate using line emissions from W$^{5+}$ ions in EAST Tokamak

TL;DR

This study determines tungsten influx in the EAST Tokamak by analyzing specific EUV line emissions of W$^{5+}$ ions, employing advanced collisional-radiative modeling and atomic codes to improve impurity influx measurements.

Contribution

The paper introduces a comprehensive collisional-radiative model for W$^{5+}$ ions using three atomic codes, enhancing accuracy in tungsten impurity influx estimation in tokamak plasmas.

Findings

S/XB ratios are nearly independent of electron density.

S/XB ratios vary significantly with electron temperature.

Method improves tungsten impurity influx measurement accuracy.

Abstract

The S/XB ratios (ionization 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 EUV (Extreme Ultraviolet) line emissions at 382.13 \AA and 394.07 \AA, corresponding to the $4f 14 5f \rightarrow 4f 14 5d$ radiative transitions of the W$^{5+}$ ion. The ground configuration of W$^{5+}$ consists of the ground level and a metastable level, with the 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 reactor, the S/XB ratios are almost independent of electron density but exhibit significant variation with electron temperature.