Abstract
The magnetic surfaces of modern stellarators are characterized by complex, carefully optimized shaping and exhibit locally compressed regions of strong turbulence drive. Massively parallel computer simulations of plasma turbulence reveal, however, that stellarators also possess two intrinsic mechanisms to mitigate the effect of this drive. In the regime where the length scale of the turbulence is very small compared to the equilibrium scale set by the variation of the magnetic field, the strongest fluctuations form narrow bandlike structures on the magnetic surfaces. Thanks to this localization, the average transport through the surface is significantly smaller than that predicted at locations of peak turbulence. This feature results in a numerically observed upshift of the onset of turbulence on the surface towards higher ion temperature gradients as compared with the prediction from the most unstable regions. In a second regime lacking scale separation, the localization is lost and the fluctuations spread out on the magnetic surface. Nonetheless, stabilization persists through the suppression of the large eddies (relative to the equilibrium scale), leading to a reduced stiffness for the heat flux dependence on the ion temperature gradient. These fundamental differences with tokamak turbulence are exemplified for the QUASAR stellarator [G. H. Neilson et al., IEEE Trans. Plasma Sci. 42, 489 (2014)].
1 More- Received 12 February 2016
DOI:https://doi.org/10.1103/PhysRevX.6.021033
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Published by the American Physical Society
Physics Subject Headings (PhySH)
Synopsis
Good News for Stellarators
Published 7 June 2016
New simulations of an alternate fusion reactor design reveal that it can be stable against turbulent fluctuations.
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Popular Summary
Nuclear fusion, the power of our Sun, is an immense source of energy, which is a highly desirable source on Earth. The output using the lithium in one laptop battery combined with half a bath tub’s worth of water amounts to around 200,000 kw/h of electricity, or the equivalent of 40 tons of coal. Furthermore, there is no danger of greenhouse gas emissions, uncontrolled chain reactions, or environmental disasters associated with nuclear fusion. The endeavor to harness fusion power started in the 1950s in the former USSR where scientists discovered the “tokamak”; at the same time, scientists in the United States developed the “stellarator.” A key goal for fusion design is prevention of heat losses, which result when plasma manages to escape the magnetic-field confinement. One of the primary causes of these losses is turbulence. The understanding of this in stellarators is a scientific task at the forefront of high-performance computing. Using petaflop-scale simulations, we tackle this challenging problem.
Plasma turbulence in fusion devices is caused by low-frequency plasma instabilities. Our work relies on simulations of the yet-to-be-built QUASAR stellarator, using the massively parallel GENE code. We demonstrate and explain, for the first time, how a stellarator manages, thanks only to its intricately shaped magnetic field, to mitigate the turbulence produced at unstable regions within the device, thereby achieving “intrinsic turbulence stabilization.”
We expect that our results will inform the experimental operation of existing stellarators such as Wendelstein 7-X, as well as provide insights for the design of future fusion experiments.