How Fusion Energy Turns Into Spaghetti to Smash up Tokamaks

Scientists think they have found the reason why fusion reactions often fall apart inside reactors—the intense magnetic-field lines scatter like spaghetti.

This scattering means that the intensely hot plasma being confined by the magnetic fields is able to escape and make contact with the reactor walls, causing damage as well as ruining the reaction.

Plasma illustration
A stock illustration depicts a fiery plasma field. Tokamaks enable nuclear fusion by containing very hot plasma with magnetic fields. sakkmesterke/Getty

For decades, physicists have been attempting to create stable nuclear-fusion reactions—in which atomic nuclei fuse together under intense heat and pressure—since it is possible to harness these reactions to generate electrical power.

Doing so would be an energy-technology breakthrough, since fusion power is theoretically more powerful, safer, and easier to fuel than existing nuclear power plants, which rely on fission, namely splitting atoms apart.

However, no one has yet been able to create a fusion reactor that is capable of generating more power than it requires to operate, and it has also proved difficult to maintain a reaction for extended amounts of time.

One of the leading reactor types in the world of fusion is the tokamak, a donut-shaped machine that uses powerful magnets to control a circular flow of super-hot plasma in which fusion can occur. Unfortunately, these reactors are prone to a sudden and puzzling drop in heat.

Researchers at the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) created a 3D model of the disarrayed magnetic-field lines to see how their shape affects the reaction, rather than the oversimplified, one-dimensional models previously used.

The 3D model was difficult to understand due to complex interactions between the electric and magnetic fields in the reactor, but the researchers were able to use a PPPL-developed particle simulation code to unravel it.

They found that tiny hills and valleys would form in the topology of the field lines, and these would allow the plasma particles to escape confinement and hit the walls of the reactor with enormous amounts of heat energy.

"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," said Min-Gu Yoo, a post-doctoral researcher at PPPL and author of a study outlining the 3D modelling, in a PPPL press release. "What we showed in the paper is how to draw a good map for understanding the topology of the field lines."

Yoo's colleague, principal research physicist Weixing Wang, said: "In the major disruption case, field lines become totally [disordered] like spaghetti and connect fast to the wall with very different lengths."

Identifying these hills in the magnetic-field lines is an important step towards avoiding these plasma disruptions, which would allow fusion reactions to run for longer periods of time—a crucial factor in having fusion energy on the grid.

The Princeton scientists' study, titled 'The 3D magnetic topology and plasma dynamics in open stochastic magnetic field lines', was published in the journal AIP Physics of Plasmas in July.