In a significant stride toward understanding the unique properties of spherical tokamaks (STs), a recent study published in the journal “Nuclear Fusion” (translated from the original title) has shed light on the pedestal stability in highly elongated plasmas and its variation with decreasing aspect ratio. This research, led by J.Y. Kim from the Korea Institute of Fusion Energy in Daejeon, Republic of Korea, could have profound implications for the future of fusion energy, particularly in the design and optimization of ST devices.
The study focuses on the H-mode pedestal, a critical region in tokamak plasmas where the temperature and density gradients are steepest. Understanding the stability of this region is crucial for maintaining the performance and safety of fusion reactors. Kim and his team investigated how the pedestal stability varies with the aspect ratio, a key geometric parameter that distinguishes STs from conventional tokamaks.
One of the most intriguing findings of the study is the behavior of the peeling–ballooning mode (PBM) eigenvalue spectrum in highly elongated plasmas. “When the elongation is very large, the PBM eigenvalue spectrum shifts to the n = 1 limit, making the mode sensitive to the edge safety-factor,” explains Kim. This shift results in an oscillating behavior of the threshold pedestal height as the edge safety-factor increases, a behavior expected for peeling-type modes.
The study also reveals that as the aspect ratio decreases, the pedestal stability characteristics are maintained, but the threshold pedestal height exhibits different behaviors depending on whether the edge safety-factor is fixed or varies with the aspect ratio. “When the edge safety-factor is fixed, the threshold pedestal height increases with decreasing aspect ratio, primarily due to the increase in plasma current,” Kim notes. “However, when the edge safety-factor varies with the aspect ratio, the threshold pedestal height exhibits an oscillating behavior, as expected for the n = 1 peeling-dominant mode.”
These findings are not just of academic interest. They have significant implications for the design and operation of future ST devices. By understanding how the pedestal stability varies with the aspect ratio and elongation, engineers can optimize the geometry of STs to enhance their performance and safety. This could lead to more efficient and cost-effective fusion reactors, bringing us closer to the realization of commercial fusion energy.
Moreover, the study highlights the importance of considering the coupling between different modes, such as the peeling–ballooning mode and the external kink mode, in the design of ST devices. This could help prevent potential instabilities and improve the overall stability of fusion plasmas.
While the study provides valuable insights into the pedestal stability in STs, it also identifies a discrepancy between the modeling results and experimental measurements in contemporary ST devices. “This discrepancy warrants further investigation and could have important implications for the design of future ST devices,” Kim concludes.
As the world looks to fusion energy as a clean, sustainable, and virtually limitless source of power, research like this is crucial. By deepening our understanding of the complex physics governing fusion plasmas, we can pave the way for a future powered by fusion energy.