Carefully manipulating the outer skin of plasma can create cascades of effects that help create the stability needed to sustain fusion reactions, scientists have found. The research, led by physicist Dylan Brennan of the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL), could provide insight into the physics required to stabilize plasma in doughnut-shaped fusion facilities known as tokamaks. These include ITER, the multinational facility being built in France to demonstrate the practicality of fusion power.
Fusion combines light elements in the form of plasma - the hot, charged state of matter composed of free electrons and atomic nuclei - and in the process generates massive amounts of energy in the sun and stars. Scientists are seeking to replicate fusion in devices on Earth for a virtually inexhaustible supply of safe and clean power to generate electricity.
Researchers used computing power unavailable even a few years ago to reveal that controlling the electric current at the edge of the plasma in tokamaks was key to maintaining the stability of fusion plasmas. Such control can affect areas of the plasma closer to the core. "We have shown that all the layers matter when you try to stabilize the plasma," said Brennan, lead author of a paper reporting the results in Nuclear Fusion. "It turns out that the plasma edge strongly couples to the inner regions."
The finding, reached in collaboration with physicists at General Fusion, a private Canadian company exploring the development of fusion energy in Burnaby, British Columbia, points out that the outer layers play an especially important role in the overall stability of the plasma.
The findings grew from analysis of what happens when plasma is compressed, a technique sometimes used to heat plasma to the super-high temperatures needed to sustain fusion reactions. They observed that the electric current that courses through the outer layer of the plasma can either prevent instabilities from interfering with the fusion process or amplify them.
Without stability, the plasma within a tokamak would become disorganized and the fusion reactions would cease. "These findings are important because they help us get the calculations right," said Brennan. "Computer algorithms using such data can steer the plasma toward stability in real time in fusion experiments, not only in those planned at General Fusion but in U.S. national labs and at ITER," Brennan said.
"A plasma is like a flickering candle," he said. "In both fusion plasma and candle flames, there's a heat source at one end and some wiggling at the other. In tokamaks, however, the heat source is so strong that the plasma can blow itself out. We are trying to stop that from happening."
As the tokamak operates, the current in the plasma's outer layer causes the magnetic field closest to the edge to vibrate like the head of a drum. These vibrations echo through the plasma, exciting other magnetic fields. Depending on the conditions, the vibrations can either weaken the instabilities or strengthen them.
The research shows that to determine the stability of the system one must assume that all the layers matter, because the interactions between layers can either stabilize or destabilize the plasma. "The conditions at one local point in the plasma are not sufficient for you to characterize the total situation," Brennan said. "You have to consider the whole system."
The scientists at General Fusion were pleased with the results, which signaled that the compression heating method had promise. "The experiment showed that yes, there are ways to set up the plasma so it maintains stability under pressure," said Michael Delage, chief technology officer at General Fusion and a paper coauthor.
Brennan believes that scientists in the future should assume that all layers of the plasma matter when analyzing the behavior of all fusion machines, even those that do not use compression to heat the plasma, to increase the efficiency of the devices. "This exercise of showing a physics result for a complicated mechanism of coupling between surfaces suggests that we can now do this kind of work routinely on any kind of device," said Brennan. "That's exciting news."
The scientists next plan to explore whether the current-caused stabilization is affected by the plasma's rotation.
This current collaboration between PPPL and General Fusion was funded by General Fusion. Brennan said the research demonstrates the fruitful possibilities inherent in the partnership between a DOE national laboratory like PPPL and private industry, where alternative approaches to fusion are being pursued.
