The source of the sudden and puzzling collapse of heat that precedes disruptions that can damage tokamak fusion facilities has been proposed by physicists. One of the most important challenges that future fusion facilities will face is the production of fusion energy that drives the sun and stars.
The collapse was caused by the 3D disordering of the strong magnetic fields. "We proposed a novel way to understand thedisordered field lines, which was usually ignored or poorly modeled in the previous studies," said Min-Gu Yoo, the lead author of a Physics of Plasmas paper. General Atomics is located in San Diego.
The fusion facilities hold fusion reactions in place due to the immense gravity. The field lines allow the superhot plasma heat to escape. When fusion facility walls are damaged by million- degree heat, they can be torn apart.
"In the major disruption case, field lines become completely disorganized like spaghetti and connect quickly to the wall with very different lengths," said Wang, a coauthor of the paper. "That brings a lot of heat against the wall."
The hot, charged state of matter composed of free electrons and atomic nuclei is generated by fusion. 99% of the visible universe is made up of free electrons and atomic nucleus. The fusion process on Earth can be captured and controlled to create a clean, carbon-free and virtually inexhaustible source of power to generate electricity.
The hills and valleys are steep.
The disarrayed field lines caused by turbulent instability had not been known. Some particles are trapped in valleys and unable to escape confinement while others roll down the hills and impact the walls of the facility as a result of the topology forming small hills and valleys.
The thermal quench is caused by the existence of hills that allow more particles to escape. The paper shows how to draw a good map to understand the field lines. Most electrons wouldn't have been trapped and wouldn't have produced the thermal quench seen in experiments without magnetic hills.
The thermal quench topology was modeled as a complex 3D structure rather than as a simple 1D structure as it had been depicted. The researchers avoided over-simplifications that could cause confusion.
The complex interaction between the electric and magnetic fields made it difficult to understand. The laboratory's GTS code was used to model the effect of turbulence on particle movement. The code shows that the electric field produced in facilities kicks particles among spaghetti-like magnetic field lines and then facilitates the motion of trapped particles along the field lines that give rise to the thermal quench.
When there are open magnetic field lines, the plasma loses its energy. It would be helpful to find innovative ways to mitigate or avoid thermal quenches in the future.
More information: Min-Gu Yoo et al, The 3D magnetic topology and plasma dynamics in open stochastic magnetic field lines, Physics of Plasmas (2022). DOI: 10.1063/5.0085304 Journal information: Physics of Plasmas
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