Transport study of trace tungsten in rotating fusion plasmas through development of imaging spectrometer and analysis of impurity density profile

Abstract

Future fusion device producing high particle and energy fluxes toward the plasma wall will need high-Z plasma facing components (PFCs) to avoid unintended impurity penetration into the plasma through plasma-wall interaction, which might degrade the plasma confinement. Owing to a relatively low sputtering yield and erosion rate as well as a high melting point and a low retention of hydrogen isotopes, tungsten (W, atomic number 74) is considered to be a promising wall material, which is currently determined to be employed in the divertor of the International Thermonuclear Experimental Reactor (ITER). However, high-Z elements like W can lead to severe detrimental effects on tokamak operation once they accumulate inside the plasma, thereby resulting in high radiation cooling. Therefore, it is essential issue to study the behavior of tungsten impurity in plasmas and relevant researches are actively underway for several fusion devices. In this dissertation, the analysis of the trace tungsten impurities demonstrated through experimental observations in two tokamaks, one being an externally injected tungsten impurity in the Korea Superconducting Tokamak Advanced Research (KSTAR) and the other being an intrinsic tungsten impurity in the Tungsten Environment in Steady-state Tokamak (WEST) in France. The aim of the study is to monitor the tungsten ions in fusion plasma, to deduce their densities from a forward model and reconstruction method, and to understand transport phenomena. These are achieved by developing a particle injection system and a space-resolved extreme ultraviolet (EUV) spectrometer followed by the development of an empirical model of the tungsten transport and a new tomographic algorithm for diagnostics with a limited viewing angle. In this study, firstly, the W impurity is injected into the fusion plasma either by using an external injection system or intentionally striking the W target inside the vessel. Accordingly, experiments are conducted both in KSTAR with a developed gun-type particle injector after the performance tests and in WEST with its tungsten wall. The injector used is made of SUS-316 (stainless steel) and equipped with a piezoelectric motor covered by graphite. These features enable effective operation close to the plasma.The charge states of W27+ to W45+ that exist in the core region of a typical fusion plasma of several keV of electron temperature emit radiations of a few nanometers in wavelength in the EUV range. The second step of this study was to observe the existing W ions and to track the distribution of each charge state over the plasma. Therefore, a diagnostic tool for monitoring the W impurities requires spectroscopic feature including the spatial and temporal information of the emissions, for developing a compact advanced EUV spectrometer system for requirement satisfaction. The main difference of the system from other imaging spectrometers is the separated entrance and the space-resolved slits on both sides of the grating to achieve a wide field of view (FoV) and to become a relatively small chamber to permit portability. In addition, an improved back-illuminated charge-coupled device (BI-CCD) detector is adopted for good spatiotemporal resolution. The spectrometer was compatible at KSTAR and WEST. The one for KSTAR has been operated since 2016 and the one for WEST was installed in 2019.When spatially resolved W spectra are obtained, further physical interpretation is essential. The existing impurity transport codes are usually a one-dimensional (flux-averaged) models that are utilized to analyze the behavior of the impurities. To support the obtained space-resolved spectra, however, development of a spectral model that allows (weighted) two-dimensional (2D) calculation in the W transport study. A new simulation code aims at finding appropriate diffusion and convection coefficients along with the time evolution of the distributed W concentration on the poloidal cross-section of the tokamak and its fractional ion abundance by comparing it with the space-resolved spectroscopic data. The code solves the poloidally averaged continuity equation similar to the aforementioned impurity codes. Further, it also solves an additional equation on centrifugal force (CF) of the impurity in rotating plasma that generally gives poloidally asymmetric density distribution. Therefore, a 2D weighted density profile of the impurity can be obtained with flux-surface averaged transport coefficients minimizing the chi-square (χ^2) value by comparing with the measured brightness of the line emission integrated along the viewing chords of the spectrometer.Contrary to the fact that a forward model requires several assumptions and free parameters, an inversion method serves to be a good candidate for finding the density profile of an impurity species as it aims at solving only an inversion problem. To make a concrete ground for the tungsten transport, a new tomography code was developed that could be utilized for space-resolved spectrometers such as the ones at KSTAR and WEST and another one at ITER that have limited field of views. Performance tests and experimental results showed that this technique is feasible for the above-mentioned spectrometers, and even the light impurity species, such as carbon and oxygen, experience significant CF in rapidly rotating plasmas, thereby illustrating poloidally asymmetric distribution. Based on the development of the injection system, space-resolved spectrometer, impurity transport and reconstruction codes, the density analysis of trace tungsten impurities has been studied through experiments in KSTAR and WEST. This work is expected to be the core foundation for analyzing the effects of tungsten impurity in fusion devices such as KSTAR and WEST as well as ITER in the future. In addition to the known facts of spectrometer operation transport analysis experience, this framework will be extended to contribute to stable plasma operation in next-generation fusion devices.