New discoveries about the nature of light may improve methods for heating fusion plasmas

Both literally and figuratively, light permeates the world. It dispels darkness, carries telecommunication signals between continents, and makes visible the invisible, from distant galaxies to the tiniest bacteria. The light can also help heat the plasma inside ring-shaped devices known as tokamaks as scientists around the world try to harness the fusion process to generate green electricity.

Now, scientists have made discoveries about particles of light known as photons that could help the search for fusion energy. By performing a series of mathematical calculations, the researchers discovered that one of the fundamental properties of a photon is topological, meaning that it does not change even when the photon moves through different materials and environments.

This property is polarization, the direction—left or right—that electric fields take as they move around a photon. Due to the fundamental laws of physics, the polarization of a photon helps determine the photon’s direction and constrains its motion. Therefore, a light beam consisting only of photons with one type of polarization cannot propagate in any part of a given space. These findings demonstrate the strengths of the Princeton Plasma Physics Laboratory (PPPL) in theoretical physics and fusion research.

“Having a more precise understanding of the fundamental nature of photons can lead scientists to design better light beams for heating and measuring plasma,” said Hong Qin, a principal research physicist at the US Department of Energy’s PPPL. (DOE) and co-authored a paper reporting the results in Physical examination D.

Simplifying a complicated problem

Although the researchers were studying individual photons, they were doing so as a way to solve a larger, more difficult problem—how to use intense light beams to induce long-term disturbances in plasma that could help maintain temperatures. high required for fusion. .

Known as topological waves, these motions often occur at the boundary of two different regions, such as the plasma and vacuum in a tokamak at its outer edge. They’re not particularly exotic—they occur naturally in Earth’s atmosphere, where they help produce El Niño, a buildup of warm water in the Pacific Ocean that affects weather in North and South America.

To produce these waves in plasma, scientists need a greater understanding of light—specifically, the same kind of radio-frequency waves used in microwave ovens—that physicists already use to heat plasma. With greater understanding comes greater control.

“We are trying to find similar waves for fusion,” Qin said. “They don’t stop easily, so if we could create them in the plasma, we could increase the heating efficiency of the plasma and help create the conditions for fusion.”

The technique resembles the ringing of a bell. Just as using a hammer to strike a bell causes the metal to move in a way that creates sound, scientists want to hit the plasma with light so that it vibrates in a certain way to create consistent heat.

Solving a problem by simplifying it occurs throughout science. “If you’re learning to play a song on the piano, you don’t start out by trying to play the whole song at full speed,” said Eric Palmerduca, a graduate student in the Princeton Program in Plasma Physics, which is based at PPPL, and the main author of the paper.

“You start playing it at a slower pace; you break it down into small parts; maybe you learn each hand separately. We do this all the time in science—breaking a bigger problem into smaller problems , solving them one or two at a time, and then bringing them together to solve the big problem.”

Come back, come back, come back

In addition to discovering that the polarization of a photon is topological, scientists discovered that the rotational motion of photons could not be separated into intrinsic and extrinsic components. Think about the Earth: How it spins on its axis, producing day and night, and revolves around the sun, producing the seasons.

These two types of movements do not usually affect each other; for example, the rotation of the Earth on its axis does not depend on its rotation around the sun. In fact, the reciprocating motion of all objects with mass can be divided in this way. But scientists haven’t been so sure about particles like photons, which have no mass.

“Most experimentalists assume that the angular momentum of light can be separated into spin and orbital angular momentum,” Palmerduca said. “However, among theorists, there has been a long-standing debate about the proper way to do this separation, or whether it is possible to do this separation. Our work helps resolve this debate by showing that the angular momentum of photons cannot splits into spin. and orbital components.”

Furthermore, Palmerduca and Qin proved that the two components of motion cannot be separated due to topological, invariant properties of a photon, such as its polarization. This new finding has implications for the laboratory. “These results mean that we need a better theoretical explanation of what’s going on in our experiments,” Palmerduca said.

All these findings about photons give researchers a clearer picture of how light behaves. With a greater understanding of light rays, they hope to understand how to create topological waves that could be useful for fusion research.

Insights into Theoretical Physics

Palmerduca notes that the photon findings demonstrate PPPL’s ​​strengths in theoretical physics. The findings relate to a mathematical result known as the Fur Ball Theorem.

“The theorem says that if you have a hairy ball, you can’t brush all the hairs flat without creating a cow’s oscillation somewhere in the ball. Physicists thought that meant you couldn’t have a light source that sends photons to all directions at the same time,” said Palmerduca.

However, he and Qin found that this is not correct, because the theorem does not take into account, mathematically, that photonic electric fields can rotate.

The findings also reverse the research of former Princeton University physics professor Eugene Wigner, whom Palmerduca described as one of the most important theoretical physicists of the 20th century. Wigner realized that by using principles derived from Albert Einstein’s theory of relativity, he could describe all possible elementary particles in the universe, even those that had not yet been discovered.

But while his classification system is accurate for particles with mass, it produces inaccurate results for massless particles such as photons. “Qin and I showed that by using topology,” Palmerduca said, “we can modify Wigner’s classification for massless particles, giving a description of photons that operate in all directions simultaneously.”

A clearer understanding of the future

In future research, Qin and Palmerduca plan to explore how to create useful topological waves that heat the plasma without creating useless heat-removing species.

“Some harmful topological waves can be inadvertently excited, and we want to understand them so they can be removed from the system,” Qin said. “In this sense, topological waves are like new breeds of insects. Some are beneficial to the garden, and some of them are pests.”

Meanwhile, they are excited about the current findings. “We have a clearer theoretical understanding of photons that can help excite topological waves,” Qin said. “Now is the time to build something so that we can use them in the search for fusion energy.”