Stellarators are a type of fusion reactor design that rely on twisted magnetic fields to compress and heat a plasma. They are considered to be a major contender for the development of commercial fusion reactors. Investigators have discovered a possible critical issue for stellarators. They have clarified the potential impact of a concern that has been largely overlooked.
The research carried out at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) demonstrates how periodic changes in the strength and shape of stellarator magnetic fields can facilitate the rapid loss of confinement of high energy plasma particles that fuel fusion reactions.
Roscoe White is a senior physicist at PPPL and the lead author of a Physics of Plasmas paper. His paper identifies a new type of energetic particle loss according to Felix Parra Dias who is the head of the Theory Department at PPPL. He said, “Studies have so far focused on controlling other types of energetic losses that are dominant, and we are now trying to reduce energetic particle losses even more. The paper on which these findings are based identifies 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 singled out as a problem that needed to be addressed. We cannot use detailed analysis for stellarator optimization due its computational cost. This is why Roscoe's paper is important: It identifies the problem and proposes an efficient way to evaluate and optimize the stellarator shape to avoid it. This gives us the opportunity to develop stellarator configurations that are even better than existing ones.”
The plasma mechanisms that create this issue are referred to as “resonances”. They describe the paths that particles follow as they orbit the magnetic fields that run around the reaction chamber. When particles are resonant, they return again and again to the point they started from. These returns allow instabilities, or modes, in the hot charged plasma gas to create what are called islands in the path of orbits. These islands allow the particles and their energy to escape confinement.
White utilized a high-speed software code to search for instabilities called “Alfven modes” that can create islands in donut shaped tokamak. Tokamaks are more widely used in experimental fusion laboratories than stellarators. White said, “So I thought, 'Okay,' I'll go look at stellarators too.” He found that in stellarators, “something very different is happening.”
White went on to say that it "Turns out that in a stellarator you don't need modes. In stellarators, when the number of periodic changes in the orbit of resonant high-energy particles matches the number of periodic changes in the magnetic field, particle losses can occur. It's like pushing a child on a swing. When you want the child to swing higher and higher, every time the swing comes back to you, you push it again, and that's a push in resonance.” He went on to say, “The problem up until now is that people have been focusing on the form of the magnetic field. But high energy orbiting particles drift across the field, so you must also consider the particle orbits." He added that “seeing whether particle resonances in stellarators match the magnetic field period has got to enter into design conditions for finding a good reactor.”