Princeton scientists overcome key setback in achieving nuclear fusion
Physicists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have taken a critical step forward toward achieving nuclear fusion by pinpointing the source of the collapse of heat that precedes disruptions that can damage tokamak fusion facilities, according to a press release by the institution published this Tuesday. This development could overcome one of the most critical challenges that future fusion facilities face now and in the future.
A 3D disordering of magnetic fields
The scientists traced back the collapse to the 3D disordering of strong magnetic fields. “We proposed a novel way to understand the [disordered] field lines, which was usually ignored or poorly modeled in the previous studies,” said Min-Gu Yoo, a post-doctoral researcher at PPPL and lead author of the new study.
Magnetic fields are used in fusion facilities as substitutes for the powerful gravity that holds fusion reactions in place in celestial bodies. However, in laboratory experiments these fields are disordered by plasma instability resulting in a superhot plasma rapidly escaping confinement. The ensuing heat can damage fusion facility walls.
“In the major disruption case, field lines become totally [disordered] like spaghetti and connect fast to the wall with very different lengths,” said principal research physicist Weixing Wang, Yoo’s PPPL advisor and a coauthor of the paper. “That brings enormous plasma thermal energy against the wall.”
Scientists everywhere are working to capture and direct the atomic fusion process on Earth in order to develop a clean, carbon-free and possibly inexhaustible source of power that can generate electricity.
One setback that previously remained unknown was the 3D shape, or topology, of the disarrayed field lines caused by turbulent instability. This topology was behind the creation of tiny hills and valleys where some particles were trapped while others rolled down the hills and affected the walls of the facility.
“The existence of these hills is responsible for the fast temperature collapse, the so-called thermal quench, as they allow more particles to escape to the tokamak wall,” Yoo said. “What we showed in the paper is how to draw a good map for understanding the topology of the field lines. Without magnetic hills, most electrons would have been trapped and could not produce the thermal quench observed in experiments.”
Simulating the thermal quench topology
To reach their conclusions, PPPL scientists simulated the thermal quench topology as a complex 3D structure avoiding the over-simplifications that so often mislead physicists. This topology is notoriously difficult to understand because of the complex interaction between the electric and magnetic fields. PPPL researchers used the Laboratory’s GTS code to understand this topology.
This code simulates the effect of turbulent instability on particle motion revealing that the electric field produced in facilities acts to kick particles among magnetic field lines and then facilitates the resulting movement of trapped particles that leads to a thermal quench.
“This research provides new physical insights into how the plasma loses its energy towards the wall when there are open magnetic field lines,” Yoo said. “The new understanding would be helpful in finding innovative ways to mitigate or avoid thermal quenches and plasma disruptions in the future.”
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