New discoveries about the nature of light could improve methods for heating fusion plasma

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An artist’s view of photons, the particles that make up light, that disrupt plasma. Credit: Kyle Palmer / PPPL Communications Department

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An artist’s view of photons, the particles that make up light, that disrupt plasma. Credit: Kyle Palmer / PPPL Communications Department

Light permeates the world both literally and figuratively. It dispels darkness, transmits telecommunications signals between continents and makes the invisible visible, from distant galaxies to the smallest bacteria. Light can also help heat the plasma in ring-shaped devices known as tokamaks, as scientists worldwide aim to harness the fusion process to generate green electricity.

Now scientists have made discoveries about light particles known as photons that could aid the search for fusion energy. By performing a series of mathematical calculations, the researchers discovered that one of the basic properties of a photon is topological, meaning it does not change even as 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 fundamental physical laws, a photon’s polarization helps determine the direction in which the photon travels and limits its motion. Therefore, a beam of light consisting only of photons with one type of polarization cannot spread to every 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 could lead scientists to design better light beams for heating and measuring plasma,” said Hong Qin, a principal research physicist at the U.S. Department of Energy’s (DOE) PPPL and co-author of a paper reporting the results Physical examination D.

Simplify a complicated problem

Although the researchers studied individual photons, they did so as a way to solve a larger, more difficult problem: how to use beams of intense light to induce long-lasting disturbances in the plasma that could help keep the high temperatures in necessary for merger.

These fluctuations are known as topological waves and often occur at the boundary of two different regions, such as plasma and the vacuum in outer edge tokamaks. They’re not exactly exotic: They occur naturally in Earth’s atmosphere, where they help produce El Niño, a collection of warm water in the Pacific Ocean that influences the weather in North and South America.

To produce these waves in plasma, scientists must have a better understanding of light – specifically the same type of radio frequency wave used in microwave ovens – that physicists already use to heat plasma. With greater understanding comes greater opportunity for control.

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

The technique is similar to ringing a bell. Just as using a hammer to hit a bell causes the metal to move in a way that creates sound, the scientists want to hit plasma with light to make it wobble in a certain way to create sustained heat.

Solving a problem by simplifying it happens throughout science. “When you learn to play a song on the piano, you don’t start by playing the entire song at full speed,” says Eric Palmerduca, a graduate student in the Princeton Program in Plasma Physics, which is based at PPPL, and lead author of the article.

“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 down a larger problem into smaller problems, which you solve one or two at a time , and then bring them back together to solve the big problem.”

Turn, turn, turn

In addition to discovering that a photon’s polarization is topological, the scientists also discovered that the spinning motion of photons could not be separated into internal and external components. Think of the Earth: it spins on its axis, producing day and night, and revolves around the sun, creating the seasons.

These two types of movements generally do not influence each other; For example, the Earth’s rotation on its axis does not depend on its revolution around the Sun. In fact, the rotational motion of all objects with mass can be separated in this way. But scientists aren’t so sure about particles like photons, which have no mass.

“Most experimentalists assume that the angular momentum of light can be broken down into spin and orbital angular momentum,” says Palmerduca. “However, there has been a long debate among theorists about the correct way to perform this splitting, or whether it is possible to perform this splitting at all. Our work helps settle this debate and shows that the angular momentum of photons is not can be broken down into spin and orbital components.”

Furthermore, Palmerduca and Qin determined that the two motion components cannot be split due to the topological, invariant properties of a photon, such as its polarization. This new finding has implications for the laboratory. “These results mean 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 better understanding of light rays, they hope to figure out how to create topological waves that could be useful for nuclear fusion research.

Insights for 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 Hairy Ball Theorem.

“The theorem states that if you have a ball covered in hair, you can’t comb all the hair flat without creating a crest somewhere on the ball. Physicists thought this implied that you couldn’t have a light source that sends photons in all directions. at the same time,” Palmerduca said.

However, he and Qin discovered that this is incorrect, because mathematically the theorem does not take into account the fact that photon electric fields can rotate.

The findings also modify 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 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 have shown that using topology,” Palmerduca said, “we can modify Wigner’s classification for massless particles, providing a description of photons acting in all directions simultaneously.”

A clearer insight for the future

In future research, Qin and Palmerduca plan to investigate how to create useful topological waves that heat plasma without creating useless manifolds that suck away the heat.

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

In the meantime, they are enthusiastic about the current findings. “We have a clearer theoretical understanding of the photons that can help generate topological waves,” Qin said. “Now it’s time to build something so we can use them in the search for fusion energy.”

More information:
Eric Palmerduca et al, Photon Topology, Physical examination D (2024). DOI: 10.1103/PhysRevD.109.085005. On arXiv: DOI: 10.48550/arxiv.2308.11147

Magazine information:
Physical examination D


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