Researchers have found a way to keep plasma in nuclear fusion reactors stable and prevent temperature and density levels from careening up and down.

The new findings constitute a significant development in the quest for nuclear fusion energy, which many believe will provide unlimited, green energy once engineers learn how to harness this power source.

Stabilizing Fusion Plasma  

A team of physicists at the U.S. Department of Energy's Princeton Plasma Physics Laboratory at Princeton University's Forrestal Campus in New Jersey has developed simulations of the mechanism that keeps fusion plasma stable.

Plasma is one of the four states of matter. However, under normal conditions on Earth, it cannot exist as freely as solid, liquid, or gas. In the stars, plasma is naturally abundant, but on Earth this super-heated jelly of highly charged particles is generated in fusion reactors, such as stellarators and doughnut-shaped tokamaks.

Sometimes, plasma found in fusion reactors vacillate back and forth in terms of temperature and density. The unstable zigzagging, combined with other events inside the reactor, causes reactions to stop and halts entire operations.

However, some plasmas have been found to be very stable. They do not exhibit the sawtooth swings that create turbulence in other plasma. Physicists have long been searching for this baffling mechanism that causes some plasmas to remain stable.

Fusion Energy  

Inside the heart of nuclear fusion reactors, scientists attempt to replicate the same process that powers the stars and hydrogen bombs.

The process works when super-heated hydrogen atoms suspended in plasma crash into one another, splitting into highly charged ions and electrons that fuse to form helium. As fusion occurs, the atoms produce enormous amounts of heat and energy that can potentially be used to generate electricity.

A small amount of liquid hydrogen can generate as much electricity as 28 tons of coal, but without the radioactive waste that comes with nuclear fission reactors such as Chernobyl and Fukushima.

A nuclear fusion reactor comprises a magnetic field that confines the plasma, where fusion reactions take place. A mechanism called magnetic flux pumping keeps the current at the center of the plasma. This is what keeps some plasmas stable enough to keep reactions going.

The researchers at PPPL, led by post-doctoral research associate Isabel Krebs, have successfully created simulations of this mechanism using the M3D-C1 code on PPPL's computers. Details of the new research are published in the journal Physics of Plasmas.

Magnetic Flux Pumping

According to the simulation, magnetic flux pumping can happen in hybrid scenarios of the standard regimes. These are high-confinement mode (H-mode), where the plasma is stable and better confined, and low-confinement mode (L-mode), where turbulence causes the plasma to leak some of its energy. The mechanism can also happen in advanced scenarios, where the plasma operates at a steady state.

In a hybrid scenario, the plasma's current remains flat at the center of the soup of highly charged gas, while pressure remains high. This combination creates what is called a quasi-interchange mode.

This mode serves as a mixer. It shakes up the plasma and distorts the magnetic field, which then generates a powerful effect that keeps the current of the plasma flat in its core. This is the very same effect that keeps plasma temperatures and density from seesawing back and forth.

It is also the same mechanism by which the Earth generates a magnetic field that protects it from highly charged particles streaming from the sun. The liquid metal at the center of the planet is the mixer.

Magnetic flux pumping is a self-regulating mechanism, as shown by the simulations. If the mixer becomes too strong, the plasma's current stays just below a threshold that keeps it from going haywire.

Future Applications

"This mechanism may be of considerable interest for future large-scale fusion experiments such as ITER," says Krebs.

The researchers suggest that operators of ITER, the most ambitious nuclear fusion project in construction in Provence, France, may develop magnetic flux pumping by experimenting with the timing of the neutral beam power that heats up the plasma.