New studies show photon polarization is constant across environments and may be improving plasma heating methods for the advancement of fusion energy.
Light, both literally and figuratively, permeates our world. It eliminates darkness, carries telecommunications signals across continents and reveals the invisible, from distant galaxies to microscopic bacteria. Light can also help heat the plasma in ring-shaped devices known as tokamaks, as scientists work to harness the fusion process to produce green electricity.
Recently, researchers at the Princeton Plasma Physics Laboratory discovered that one of the fundamental properties of photons – polarization – is topological, meaning that it remains constant even as the photon passes through different materials and environments. These findings, published in Physical examination D, could lead to more effective plasma heating techniques and advances in fusion research.
Implications for fusion research
Polarization is the direction (left or right) that electric fields follow as they travel around a photon. Due to fundamental laws of physics, a photon’s polarization dictates the direction in which it travels and limits its path. Consequently, a beam of light consisting solely of photons with a single type of polarization cannot spread to every part of a given space.
“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, chief research physicist at the US Department of Energy (DOE). PPPL and co-author of the study.
Simplify complex problems
The study of photons serves as a means to solve a larger, more difficult problem: how to use beams of intense light to induce long-lasting perturbations in the plasma that could help create the high temperatures needed for fusion in to stand firm.
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, the same type of radio frequency wave used in microwave ovens – which physicists already use to heat plasma.
“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.
Unraveling the nature of photon motion
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 rotates on its axis and produces day and night, but it also 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.
However, it was unclear whether this was true for particles such as photons, which have no mass. “Most experimentalists assume that the angular momentum of light can be broken down into spin and orbital angular momentum,” said Eric Palmerduca, lead author of the paper and a graduate student in Princeton’s Program in Plasma Physics. “Among theorists, however, there has been a long debate about the correct way to do this split, and whether it is even possible to do this split. Our work helps resolve this debate and shows that the angular momentum of photons cannot 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 is happening in our experiments,” Palmerduca said.
These findings provide insight into the behavior of light, furthering the researchers’ goals of creating topological waves for 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 simultaneously,” 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 research from former Princeton University Physics professor Eugene Wigner, who described Palmerduca 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 by using topology,” Palmerduca said, “we can modify Wigner’s classification for massless particles, providing a description of photons acting simultaneously in all directions.”
Future directions
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 that sense, topological waves are like new species of insects. Some are beneficial to the garden, others 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.”
Reference: “Photon topology” by Eric Palmerduca and Hong Qin, April 2, 2024, Physical examination D.
DOI: 10.1103/PhysRevD.109.085005