Negative triangle shape – positive for tokamak fusion reactors
Tokamaks, like the Tokamak Configuration Variable (TCV) shown here, are donut-shaped devices that trap plasma to create fusion reactions. The shape of the plasma cross-section influences the quality of the containment. Credit: CRPP-EPFL, Suisse-Euratom Association
Tokamak devices use strong magnetic fields to confine and shape the plasma, which contains the fuel that causes the fusion. The shape of the plasma affects the ease or difficulty of obtaining a useful fusion current source. In a conventional tokamak, the cross-section of the plasma is in the shape of the capital letter D. When the straight part of the D faces the “donut hole” side of the donut-shaped tokamak, this shape is called a positive triangle shape. If the plasma cross-section has a rearward D-shape and the curved portion of the D faces the “donut hole” side, then this shape is called a negative triangle shape. New research shows that a negative triangle reduces the interaction of the plasma with the plasma-facing material surfaces of the tokamak. This finding indicates decisive advantages for the achievement of nuclear fusion energy.
One of the challenges in the science and technology of fusion energy is building future power plants that control plasmas that are many times hotter than the sun. At these extreme temperatures, interactions between the plasma and the material walls of the power reactor must be controlled and minimized. Unwanted interactions occur due to turbulence in the edge area of the plasma. This research shows that the limit turbulence in plasmas with negative triangle shape is greatly reduced compared to plasmas with positive triangle shape. This also greatly reduces the undesired interactions with the walls facing the plasma, which in principle leads to longer lifetimes of the wall and a reduction in the risk of wall damage which could bring a reactor to a standstill.
Scientists know that in tokamak fusion devices, negative triangle shaped nuclear plasma forms have a significant increase in energy restriction compared to positive triangle shaped plasmas. Negative triangle shape plasma shapes also show reductions in fluctuation levels in core electron temperature and density. This alone makes negative triangular plasmas promising candidates for a future fusion reactor.
The new research reported here shows that the sign and degree of triangularity also have a large impact on plasma edge dynamics and performance and particle exhaust properties, but scientists know relatively little about such effects. These experiments on the Tokamak à Configuration Variable (TCV) at the École polytechnique fédérale de Lausanne (EPFL) in Lausanne, Switzerland, showed a strong reduction in boundary plasma fluctuations and the plasma interaction with the attachment wall with sufficiently negative triangular values. The researchers observed the effects over a wide range of densities in both inner-wall-confined and diverted plasmas. This strong reduction in the plasma-wall interaction with sufficiently negative triangularity increases the prospects of plasmas with negative triangularity as a potential reactor solution.
The research was published in Nuclear Fusion.
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W. Han et al., Suppression of First Wall Interaction in Negativ Triangularity Plasmas on TCV, Nuclear Fusion (2021). DOI: 10.1088 / 1741-4326 / abdb95 Provided by the US Department of Energy
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