The potential impact of a largely overlooked concern has been clarified by the investigation of a possibly critical issue with twisty magnetic stellarators.
Changes in the strength and shape of stellarator magnetic fields can facilitate the loss of confinement of high-energy particles that fuel.
It's high energy.
"If you want to do nuclear fusion, you have to have high energy," said the lead author of a physics paper that editors have selected as a "Scilight."
A new type of energetic particle loss has been identified in his paper. The studies have so far focused on controlling other types of energetic losses that are dominant. There is a mechanism that we need to include when designing the optimal shape of stellarator magnet fields.
While this mechanism is included in our more detailed analyses of stellarator configurations among many other effects, it had not been identified as a problem that needed to be fixed. Due to its cost, we can't use detailed analysis. Roscoe's paper proposes an efficient way to evaluate and improve the stellarator shape to avoid it. We have the chance to make stellarator configurations that are even better.
The mechanisms that created this issue are called "resonances" and they describe the paths that particles follow as they travel around the machine. Particles come back to the point they started from. Such returns allow instabilities, or modes, in the hot, charged plasma gas to create islands in the path of orbits, allowing the particles and their energy to escape.
A high-speed software code was used by White to search for instabilities that can create islands in tokamaks. He said that he would look at stellarators as well. Something very different is happening in stellarators.
Modes don't need to be used.
In a stellarator, you don't need modes, according to White. Particle losses can occur when the number of periodic changes in the magnetic field matches the number of periodic changes in the high-energy particles. It is similar to pushing a child on a swing. He said that when you want the child to swing higher and higher, every time the swing comes back to you, you push it again.
The problem has been that people have focused on the magnetic field. High energy particles move across the field.
If particle resonances in stellarators match the magnetic field period, it will have to be entered into design conditions for finding a good reactor.
More information: Roscoe White et al, Poor confinement in stellarators at high energy, Physics of Plasmas (2022). DOI: 10.1063/5.0094458 Journal information: Physics of Plasmas
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