Topology is the mathematical study of how objects can be deformed by stretching or twisting but not tearing or breaking. Almost fifty years ago, Brown University physicist Michael Kosterlitz and his team employed topology to elucidate puzzling phase changes in particular types of matter. That work won a share of the Nobel Prize in Physics for Kosterlitz in 2016. Topological phenomena have been discovered in many different kinds of systems extending from thin films that conduct electricity only around their edges to strange waves that propagate in oceans and the atmosphere at the Earth’s equator.
    Now a new team of researchers including another Brown physicist has developed a theory of a new topological phenomenon to add to that huge list. In their research, the team has shown that electromagnetic waves of topological origin should be found on the surfaces of plasmas. If their theory is true, such waves could provide a new way for scientists to explore the properties of plasmas which are found in many things from fluorescent lightbulbs to stars including our own sun.
     Jeffrey Parke is a research scientist at Lawrence Livermore National Laboratory. He led the research in collaboration with Brad Marston, a professor of physics at Brown, and others. They have published a report on their findings in Physical Review Letters.
     These new theorized waves are called gaseous plasmon polaritons. It is thought that they propagate along the interface of a plasma and its surroundings when the system is enclosed in a strong magnetic field. Marston says that one of the most interesting things about these waves is that they are “topologically protected”. This means that they are inherently present in the plasma and cannot be scattered by impurities in the plasma.
     Marston said, “Any time you have a wave that’s protected against scattering, it means they can stay intact over a long distance. As a practical matter, we’re hoping that these can be used to diagnose plasma states. One of the big problems in plasma physics is to figure out the state of a plasma without disturbing it. If you stick in a probe, you’re going to disrupt the system. We might be able to use these waves to discern the state of a plasma without disturbing it.”
     Marston says that one way to consider topological protection is something referred to as the hairy ball theorem. If you had a ball covered with long hair, you could try to comb those hairs lie flat on the surface but there would always be at least one spot where the hair would not lie flat. Marston said, “This spot will always be there. You can move it around, but the only way to get rid of it is to tear some hair out. But barring something violent like that, if you’re just manipulating it continuously without tearing anything, there’s always going to be a vortex.” The vortex on the surface of the hairy ball is mathematically analogous to the waves on the surface of the plasma. He said, “In this case, there’s always a vortex but it’s in the wave-number space, wavelengths of the different waves. It’s a little more abstract than in real space, but the math is largely similar.”
     Now that there is a theoretical basis for these waves, the next step will be to carry out experiments to confirm that they really do exist. Recently, Marston and his collaborators obtained a seed grant from Brown University which will permit them to perform the needed experiments in collaboration with the Basic Plasma Physics Facility at UCLA.
     Marston is optimistic that the experimental confirmation of the waves could be very useful for plasma physics research. They could aid scientists in better understanding and control of plasma systems such as fusion reactors. He said, “If we can use these waves to discern the states of plasmas, it might help in designing a fusion reactor that’s stable and able to produce energy. If we can demonstrate these things experimentally, people in the plasma community will hopefully start paying closer attention to this idea.”