Pioneering explanation of the mysterious onset of a universal process


Luca Comisso. (Photo by Elle Starkman/ PPPL)

Researchers at PPPL and Princeton University have proposed a groundbreaking solution to a mystery that has puzzled physicists for decades. At issue is how magnetic reconnection, a universal process that sets off solar flares, northern lights and cosmic gamma-ray bursts, occurs so much faster than theory says it should. The answer could aid forecasts of space storms, explain several high-energy astrophysical phenomena, and improve plasma confinement in doughnut-shaped magnetic devices called tokamaks designed to obtain energy from nuclear fusion.

Understanding the source of fast reconnection can be fundamental to achieving magnetic fusion energy, since controlling the speed will help stabilize the plasma that fuels fusion reactions. Fast reconnection can have detrimental effects on plasma stability and confinement and can also lead to complete plasma disruption.

Magnetic reconnection takes place when the magnetic field lines embedded in a plasma — the hot, charged gas that makes up 99 percent of the visible universe — converge, break apart and violently reconnect. This process takes place in thin sheets in which electric current is strongly concentrated.

The new mathematical findings, reported in the journal Physics of Plasmas, focuses on a phenomenon called “plasmoid instability” to explain the onset of rapid reconnection. This instability breaks up the current sheets into small magnetic islands called plasmoids that can evolve over time.

The research, led by physicist Luca Comisso of Princeton and PPPL, shows how the plasmoid instability begins in a slow linear phase that goes through a period of quiescence before accelerating into an explosive phase that triggers a dramatic increase in the speed of magnetic reconnection. The researchers adapted a variant of the 17th century mathematician Pierre de Fermat’s “principle of least time” to determine the key features of this process. The approach led to a quantitative formula for the onset time of fast magnetic reconnection and the physics behind it.

For magnetic fusion energy, the model could lead to design of a system to regulate fast reconnection. Just as the physics of the friction between tires and the road led to anti-lock braking systems, so knowledge of the physics of fast reconnection could help achieve a globally stable fusion plasma.