New insights into the nature of light could improve methods for heating fusion plasma

Light permeates the world, both literally and figuratively. It dispels darkness, carries telecommunications signals between continents, and makes the invisible visible, from far-flung galaxies to the smallest bacteria. Light can also help heat the plasma in ring-shaped devices called tokamaks, as scientists around the world work to use the fusion process to generate green electricity.

Now, scientists have made discoveries about particles of light known as photons that could help in the search for fusion energy. Through a series of mathematical calculations, the researchers found that one of the fundamental properties of a photon is topological, meaning it doesn’t 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 laws of physics, a photon’s polarization helps determine the direction the photon is moving 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.

“A more precise understanding of the fundamental nature of photons could help scientists develop better beams of light for heating and measuring plasma,” said Hong Qin, senior research physicist at the U.S. Department of Energy’s (DOE) PPPL and co-author of a paper reporting the results in Physical examination D.

Simplifying a complicated problem

Although the researchers studied individual photons, they wanted to solve a larger and more difficult problem: How can intense beams of light be used to induce long-lasting disturbances in the plasma that help maintain the high temperatures required for fusion?

These fluctuations, known as topological waves, often occur at the boundary between two different regions, such as the plasma and vacuum in tokamaks at their outer edge. They are not particularly exotic – they occur naturally in the Earth’s atmosphere, where they contribute to the creation of El Niño, a buildup of warm water in the Pacific Ocean that affects the weather in North and South America.

To create these waves in the plasma, scientists need to better understand light – the exact kind of radio frequency waves used in microwave ovens – and which physicists already use to heat plasma. With a better understanding comes greater possibilities for control.

“We’re trying to find similar waves for fusion,” Qin said. “They’re not so easy to stop, so if we could create them in the 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 hitting a bell with a hammer moves the metal to produce a sound, scientists want to use light to excite plasma to move in a certain way, creating sustained heat.

Solving a problem by simplifying it is common throughout science. “When you learn to play a song on the piano, you don’t immediately try to play the whole song at full speed,” said Eric Palmerduca, a doctoral student in Princeton’s Program in Plasma Physics, which is based at PPPL, and lead author of the paper.

“You start playing it at a slower tempo; you break it down into little pieces; maybe you learn each hand individually. We do this all the time in science – we break down a bigger problem into smaller problems, solve them individually or in pairs, and then put them back together to solve the big problem.”

Turn, turn, turn

Scientists discovered not only that the polarization of a photon is topological, but also that the rotational motion of photons cannot be divided into internal and external components. Think of the Earth: on the one hand, it rotates on its axis, creating day and night, and on the other hand, it orbits the Sun, creating the seasons.

Normally, these two types of motion do not influence each other. For example, the rotation of the Earth on its axis does not depend on its rotation around the Sun. In fact, the rotational motion of all objects with mass can be separated in this way. However, scientists were not so sure about particles such as photons, which have no mass.

“Most experimental physicists assume that the angular momentum of light can be split into spin and orbital angular momentum,” Palmerduca said. “However, there has been a long debate among theorists about how to properly perform this splitting, or whether it is even possible. Our work helps to end this debate by showing that the angular momentum of photons cannot be split into spin and orbital components.”

In addition, Palmerduca and Qin found that the two components of motion cannot be separated due to the topological, unchanging properties of a photon, such as its polarization. This novel discovery has implications for the laboratory. “These results mean that we need a better theoretical explanation for what is going on in our experiments,” Palmerduca said.

All of this knowledge about photons is giving 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 fusion research.

Insights for theoretical physics

Palmerduca notes that the photon results demonstrate the strengths of the PPPL in theoretical physics. The results relate to a mathematical result known as the hairy ball theorem.

“The theorem states that if you have a ball covered with hair, you cannot comb all the hairs straight without creating a vortex somewhere on the ball. Physicists thought this meant that you couldn’t have a light source that sent photons in all directions at the same time,” Palmerduca said.

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

The results also complement 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 he could use principles from Albert Einstein’s theory of relativity to describe all possible elementary particles in the universe, including 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 to get a description of photons that works in all directions at once.”

A clearer understanding of the future

In future research, Qin and Palmerduca plan to investigate how to create favorable topological waves that heat plasma without creating unfavorable variations that dissipate heat.

“Some harmful topological waves can be excited unintentionally, and we want to understand them so we can remove them from the system,” Qin said. “In this sense, topological waves are like new species of insects. Some are useful for the garden, others are pests.”

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