Electrically charged particles constituting the fourth state of matter—the so-called plasma—are bound to magnetic field lines similar to beads on a string, whether they zip through a star or a fusion device on Earth.
Regrettably, the challenge faced by plasma physicists analyzing this phenomenon is that the magnetic field lines usually do not have simple shapes that can be easily modeled by equations. Mostly, they twist and knot similar to pretzels. At times, when the lines twist in a particular way, they crack apart and join together again, emitting blobs of plasma and energy in enormous amounts.
Currently, the outcomes of a study by an international team of researchers headed by the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) reveal that the twisted magnetic fields have the potential to evolve in only a stipulated number of ways, where the plasma within the fields follows a general rule. Provided that there exists a high pressure on the outer side of the plasma pressing inward, the plasma will impulsively take on the shape of a doughnut, or torus, and balloon out in a horizontal direction. Yet, the average amount of twisting in the plasma constrains the outward expansion, a feature called “helicity.”
“The helicity prevents the configuration from blowing apart and forces it to evolve into this self-organized, twisted structure,” stated Christopher Smiet, a physicist at PPPL and lead author of the paper, describing the outcomes in the Journal of Plasma Physics.
The outcomes are applicable to the whole spectrum of plasma phenomena and can enable researchers to understand the behavior of magnetic clouds, which are enormous masses of plasma discharged from the sun that can enlarge and collide with the Earth’s own magnetic field. The northern lights are caused by these collisions in their mild form. If they are sufficiently powerful, they can even disrupt the operations of satellites and interrupt cell phones, radio and television signals, and global positioning systems.
Since the effects are in part caused by topological properties like linking and twisting that are not affected by shape or size, the results apply both to outer space plasma plumes thousands of light years long and centimeter-long structures in Earth-bound fusion facilities.
Christopher Smiet, Physicist, PPPL.
Furthermore, “by studying the magnetic field in this more general framework, we can learn new things about the self-organizing processes within tokamaks and the instabilities that interfere with them,” stated Smiet.
Smiet’s goals for future research include the analyses of variations in the linking and connections of field lines in tokamaks that occur at the time of two types of plasma instabilities that could hamper fusion reactions. “It’s fascinating what you can learn when you study how knots unravel,” stated Smiet.
The team included researchers from Leiden University, the Dutch Institute for Fundamental Energy Research, and the University of California-Santa Barbara. This study was financially supported by the U.S. Department of Energy (Fusion Energy Sciences) and the Rubicon program that is partially funded by the Netherlands Organization for Scientific Research.