Nav: Home

No more zigzags: Scientists uncover mechanism that stabilizes fusion plasmas

July 17, 2018

Sawtooth swings -- up-and-down ripples found in everything from stock prices on Wall Street to ocean waves -- occur periodically in the temperature and density of the plasma that fuels fusion reactions in doughnut-shaped facilities called tokamaks. These swings can sometimes combine with other instabilities in the plasma to produce a perfect storm that halts the reactions. However, some plasmas are free of sawtooth gyrations thanks to a mechanism that has long puzzled physicists.

Researchers at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have recently produced complex simulations of the process that may show the physics behind this mechanism, which is called "magnetic flux pumping." Unraveling the process could advance the development of fusion energy.

Fusion drives the sun and stars

Fusion, the power that drives the sun and stars, is the fusing of light elements in the form of plasma -- the hot, charged state of matter composed of free electrons and atomic nuclei -- that generates massive amounts of energy. Scientists are seeking to replicate fusion on Earth for a virtually inexhaustible supply of power to generate electricity.

The flux pumping mechanism limits the current in the core of the plasma that completes the magnetic field that confines the hot, charged gas that produces the reactions. This development, found in some fusion plasmas, keeps the current from becoming strong enough to trigger the sawtooth instability.

Spearheading the research that uncovered the process was physicist Isabel Krebs, lead author of a Physics of Plasmas paper describing the mechanism that was published last September and made into a DOE Office of Science highlight in June that summarizes the findings. Krebs, a post-doctoral associate, used the PPPL-developed M3D-C1 code to simulate the process on the high-performance computer cluster at PPPL, working closely with theoretical physicists Stephen Jardin and Nate Ferraro, developers of the code. "The mechanism behind magnetic flux pumping had not been understood," Jardin said. "Isabel's paper describes the process."

Hybrid scenarios

In the PPPL simulations, magnetic flux pumping develops in "hybrid scenarios" that exist between standard regimes -- which include high-confinement (H-mode) and low-confinement (L-mode) plasmas -- and advanced scenarios in which the plasma operates in a steady state. In hybrid scenarios, the current remains flat in the core of the plasma while the pressure of the plasma stays sufficiently high.

This combination creates what is called "a quasi-interchange mode" that acts like a mixer that stirs up the plasma while deforming the magnetic field. The mixer produces a powerful effect that maintains the flatness of the current and prevents the sawtooth instability from forming. A similar process maintains the magnetic field that protects the Earth from cosmic rays, with the molten liquid in the iron core of the planet serving as mixer.

The mechanism also regulates itself, as the simulations show. If the flux pumping grows too strong, the current in the core of the plasma stays "just below the threshold for the sawtooth instability," according to Krebs. By remaining below the threshold, the current keeps the plasma temperature and density from zigzagging up and down.

The simulations could lead to measures to avoid the troublesome swings. "This mechanism may be of considerable interest for future large-scale fusion experiments such as ITER," Krebs said. For ITER, the major international fusion experiment under construction in France, creation of a hybrid scenario could produce flux pumping and deter sawtooth instabilities

One way to develop the hybrid scenario will be for operators of ITER to experiment with the timing of the neutral beam power that will heat the ITER plasma to fusion temperatures. Such experiments could lead to the combination of plasma current and pressure that produces sawtooth-free operation.
Support for this research comes from the DOE Office of Science.

PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas -- ultra-hot, charged gases -- and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy's Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit

DOE/Princeton Plasma Physics Laboratory

Related Magnetic Field Articles:

Earth's last magnetic field reversal took far longer than once thought
Every several hundred thousand years or so, Earth's magnetic field dramatically shifts and reverses its polarity.
A new rare metals alloy can change shape in the magnetic field
Scientists developed multifunctional metal alloys that emit and absorb heat at the same time and change their size and volume under the influence of a magnetic field.
Physicists studied the influence of magnetic field on thin film structures
A team of scientists from Immanuel Kant Baltic Federal University together with their colleagues from Russia, Japan, and Australia studied the influence of inhomogeneity of magnetic field applied during the fabrication process of thin-film structures made from nickel-iron and iridium-manganese alloys, on their properties.
'Magnetic topological insulator' makes its own magnetic field
A team of U.S. and Korean physicists has found the first evidence of a two-dimensional material that can become a magnetic topological insulator even when it is not placed in a magnetic field.
Scientists develop a new way to remotely measure Earth's magnetic field
By zapping a layer of meteor residue in the atmosphere with ground-based lasers, scientists in the US, Canada and Europe get a new view of Earth's magnetic field.
Magnetic field milestone
Physicists from the Institute for Solid State Physics at the University of Tokyo have generated the strongest controllable magnetic field ever produced.
New world record magnetic field
Scientists at the University of Tokyo have recorded the largest magnetic field ever generated indoors -- a whopping 1,200 tesla, as measured in the standard units of magnetic field strength.
Researchers discover link between magnetic field strength and temperature
Researchers recently discovered that the strength of the magnetic field required to elicit a particular quantum mechanical process corresponds to the temperature of the material.
Astronomers observe the magnetic field of the remains of supernova 1987A
For the first time, astronomers have directly observed the magnetism in one of astronomy's most studied objects: the remains of Supernova 1987A (SN 1987A), a dying star that appeared in our skies over thirty years ago.
Watch: Insects also migrate using the Earth's magnetic field
A major international study led by researchers from Lund University in Sweden has proven for the first time that certain nocturnally migrating insects can explore and navigate using the Earth's magnetic field.
More Magnetic Field News and Magnetic Field Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Rethinking Anger
Anger is universal and complex: it can be quiet, festering, justified, vengeful, and destructive. This hour, TED speakers explore the many sides of anger, why we need it, and who's allowed to feel it. Guests include psychologists Ryan Martin and Russell Kolts, writer Soraya Chemaly, former talk radio host Lisa Fritsch, and business professor Dan Moshavi.
Now Playing: Science for the People

#538 Nobels and Astrophysics
This week we start with this year's physics Nobel Prize awarded to Jim Peebles, Michel Mayor, and Didier Queloz and finish with a discussion of the Nobel Prizes as a way to award and highlight important science. Are they still relevant? When science breakthroughs are built on the backs of hundreds -- and sometimes thousands -- of people's hard work, how do you pick just three to highlight? Join host Rachelle Saunders and astrophysicist, author, and science communicator Ethan Siegel for their chat about astrophysics and Nobel Prizes.