In the quest to harness the power of nuclear fusion, scientists are continually refining their understanding of plasma behavior within tokamaks, the most promising devices for achieving sustained fusion reactions. A recent study published in Nuclear Fusion, the journal of the International Atomic Energy Agency, has shed new light on the dynamics of tungsten influx in tokamak plasmas, a critical factor in the design and operation of future fusion reactors. The research, led by Fengling Zhang of the Institute of Plasma Physics at the Hefei Institutes of Physical Science, Chinese Academy of Sciences, and the University of Science and Technology of China, delves into the complexities of tungsten impurity influx, a phenomenon that can significantly impact the performance and longevity of fusion reactors.
Tungsten, a high-melting-point metal, is often used in tokamak divertors—the components that handle the exhaust of plasma. However, tungsten can erode and enter the plasma, leading to impurities that can cool the plasma and reduce fusion efficiency. Zhang and her team focused on two specific extreme ultraviolet (EUV) line emissions from tungsten ions, at wavelengths of 382.13 Å and 394.07 Å, to understand the influx rates of tungsten impurities. These emissions correspond to radiative transitions in the W^5+ ion, providing a spectroscopic window into the plasma’s behavior.
The researchers developed a sophisticated collisional-radiative model that accounts for 430 energy levels contributing to ionization. This model is a significant advancement over simpler approaches that assume transitions are independent. “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,” Zhang explains. “Therefore, a simple approach assuming that the transitions are independent, i.e. only populated by a unique level source, requires correction.”
To achieve this, the team utilized three advanced computational codes: HULLAC (Hebrew University—Lawrence Livermore Atomic Code), AS (AutoStructure), and FAC (Flexible Atomic Code). These tools provided essential data on wavelengths, collisional and radiative transition rate coefficients, and direct electron-impact ionization under the distorted-wave approximation. The FAC code was particularly instrumental in calculating the contributions to total ionization from excitation-autoionization processes up to n = 15 manifolds.
The findings reveal that the S/XB ratios, which relate spectroscopic emissivity measurements to impurity influx, are almost independent of electron density but exhibit significant variation with electron temperature. This insight is crucial for optimizing tokamak operations, as it allows for more precise control over plasma conditions to minimize tungsten influx.
The implications of this research are far-reaching for the energy sector. As the world races to develop commercially viable fusion power, understanding and mitigating tungsten influx is essential for enhancing reactor performance and reducing operational costs. By providing a more accurate model of tungsten behavior in tokamak plasmas, Zhang’s work paves the way for improved design and operational strategies, bringing us one step closer to harnessing the power of the stars here on Earth. This research, published in Nuclear Fusion, underscores the importance of detailed spectroscopic analysis in advancing fusion energy technologies.
