Caltech Physicists Discover “Double Helix,” a New Equilibrium State of Cosmic Plasma

Caltech experiments uncovered a stable double helix state in plasma flux ropes. The same principles explain cosmic structures such as the Double Helix Nebula.

Research into the Sun’s outer atmosphere has led Caltech applied physics professor Paul Bellan and his former graduate student Yang Zhang (PhD ’24) to identify a previously unknown equilibrium state of magnetic fields and the plasma they contain. The solar corona, the Sun’s outermost atmospheric layer, is far less dense than the surface, yet it reaches temperatures a million times higher.

This region is dominated by powerful magnetic fields that trap plasma, a hot mixture of charged particles (electrons and ions). The newly identified equilibrium, known as a double helix, is not limited to the corona but also appears in much larger cosmic systems, including the Double Helix Nebula near the center of the Milky Way.

Solar activity such as flares often takes the shape of magnetic flux ropes, which are twisted tubes of magnetic field filled with plasma. One way to picture a flux rope is to imagine a garden hose filled with plasma and wrapped in a spiral stripe. Along the hose runs an electric current, while the spiral represents the helical magnetic field surrounding it. Because plasma carries electric charge, it conducts current and becomes bound—or “frozen”—to the magnetic field.

These magnetic flux ropes appear across a remarkable range of scales, from small laboratory setups to massive solar flares spanning hundreds of thousands of kilometers. In astrophysics, similar rope-like structures can extend across hundreds or even thousands of light-years.

Creating plasma braids in the lab

Inside a large vacuum chamber, Bellan and Zhang (now a NASA Jack Eddy postdoctoral fellow at Princeton) created miniature versions of solar flares measuring just 10 to 50 centimeters in length. Yang describes the process: “We have two electrodes inside the vacuum chamber, which has coils producing a magnetic field spanning the electrodes. Then we apply high voltage across the electrodes to ionize initially neutral gas to form a plasma. The resulting magnetized plasma configuration automatically forms a braided structure.”

In this setup, two flux ropes entwine to form a double helix. Remarkably, this braided formation was found to be self-stabilizing—it held its shape without either tightening or unraveling. In their recent publication, Zhang and Bellan show that this equilibrium state can be mathematically described and predicted with precision.

While the physics of single flux ropes is well documented, far less was known about braided flux ropes, particularly in cases where the electric currents in both strands flow in the same direction. Previous studies had mainly modeled the alternative case, with currents moving in opposite directions, but this is believed to be uncommon in real astrophysical environments.

The same-direction current pattern is of special interest because it should, in theory, be vulnerable to distortion through kinking or expansion driven by hoop forces—effects that scientists have observed in both solar braids and lab-based plasma experiments. By contrast, when currents flow in opposite directions (a “no-net-current” state), these distortions are not expected to occur.

Rethinking assumptions about merging

Previously, scientists assumed that braided flux ropes where the strands have current flowing in the same direction would always merge, because parallel currents magnetically attract each another. However, in 2010, researchers at Los Alamos National Laboratory found that such flux ropes instead bounce off one another as they come closer together.

“There was clearly something more complicated going on when the flux ropes are braided, and now we have shown what that is,” Bellan says. “If you have electrical currents flowing along two helical wires that wrap around each other to form a braided structure, as seen in our lab, the components of the two currents flowing along the length of the two wires are parallel and attract, but the components of the two currents flowing in the wrapping direction are anti-parallel and repel. This combination of both attractive and repulsive forces means there will be a critical helical angle at which these opposing forces balance, producing an equilibrium. If the helical flux ropes twist tighter, there will be too much magnetic repulsion; if they twist more loosely, there will be too much magnetic attraction. At the critical angle of twist, the helical structure arrives at its lowest energy state, or equilibrium.”

Building a mathematical model

The next task was to create a mathematical model of this behavior—something not previously done. Using what Bellan describes as “brute force mathematics,” Zhang created a set of equations that could apply to multiple flux tubes in various configurations, including braided ropes, and showed there is indeed a state at which the attractive and repulsive forces balance each other, creating an equilibrium. “And as an unexpected bonus, Yang can calculate the magnetic fields inside and outside the flux ropes, and the current and pressure inside them,” Bellan says, “giving us a full picture of the behavior of these braided structures.”

Zhang tested his mathematical model against the Double Helix Nebula, an astrophysical plasma formation located 25,000 light-years from Earth that covers a 70 light-year swath of space, to see if the equations could describe a large model as well as it did the structures he and Bellan created in the lab. “What was rather amazing about this calculation is that Yang didn’t really need to know much about the nebula,” Bellan says.

“Just knowing the diameter of the strands and the periodicity of the twist, numbers that can be observed astronomically, Yang was able to predict the angle of twist that yielded an equilibrium structure, and that was consistent with observations of this nebula. One of the most exciting aspects of this research is that magnetohydrodynamics, the theory of magnetized plasmas, turns out to be fantastically scalable. When I first started looking into this, I thought the phenomena of magnetic structures at different scales were qualitatively similar, but because their sizes are so different, they couldn’t be described by the same equations. It turns out that this is not so. What we see in lab experiments and in solar and astrophysical observations are governed by the same equations.”

Reference: “Magnetic Double Helix” by Yang Zhang and Paul M. Bellan, 30 July 2025, Physical Review Letters.
DOI: 10.1103/sz9k-6l22

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