Keeping the plasma in a tokamak stable is critical to nuclear fusion reactions. But occasionally, up-and-down ripples sometimes referred to as “sawtooth” instabilities occur in the temperature and density of the plasma that fuels the reaction inside the tokamak.
If the swings or “sawtooth” movements combine with other instabilities in the plasma they can halt the reactions. And why some plasmas are free of the sawtooth gyrations has puzzled scientists.
In searching for an answer to the puzzle, researchers at the DOE’s Princeton Plasma Physics Laboratory (PPPL) at Princeton University’s Forrestal Campus in New Jersey have tackled the mechanism, called “magnetic flux pumping,” that keeps the current from becoming strong enough to trigger the sawtooth instability.
Plasma is one of the four states of matter, and under normal conditions on Earth, it cannot exist as freely as solid, liquid or gas. In our sun and other stars, plasma is abundant. Here on Earth, we can generate this super-heated jelly of highly charged particles in fusion reactors.
Leading the research that uncovered the process was physicist Dr. Isabel Krebs, lead author of a Physics of Plasmas paper describing the mechanism. Krebs used the PPPL-developed M3D-C1 code to simulate the process on the high-performance computer cluster at PPPL.
PPPL researchers used a number of different simulations. In the 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 the hybrid scenarios, the current remains flat in the core of the plasma while the pressure of the plasma stays sufficiently high. This, in turn, creates what is called “a quasi-interchange mode” that acts like a mixer that stirs up the plasma while deforming the magnetic field.
This powerful mixing keeps the current flat and prevents the sawtooth instability from forming. It is this same mixing effect that maintains the magnetic field that protects the Earth from cosmic rays. In this case, it is the molten iron core of the planet that serves as the mixer.
The simulations also showed that the mixer can regulate itself. For example, if the flux pumping grows too strong, the current at the core of the plasma stays “just below the threshold for the sawtooth instability,” according to Krebs. And understanding this mechanism could lead to measures to correct the sawtooth instabilities.
“This mechanism may be of considerable interest for future large-scale fusion experiments such as ITER,” Krebs said. She believes that the creation of a hybrid scenario could produce flux pumping and deter sawtooth instabilities for the international fusion experiment under construction in France.
