Fusion, no longer exclusive to the sun and stars, provides means for endless energy on earth.

Nuclear fusion, the process that powers the sun and stars, offers the dream of a safe, inexhaustible and environmentally friendly source of energy. Producing practical amounts of fusion energy on earth requires the confinement of a very hot, ionized fuel known as plasma, akin to a small sun. ITER (originally the International Thermonuclear Experimental Reactor), which is expected to go online in Cadarache, France, in 2018, will be the first facility capable of achieving sustained burning plasma, a necessary next step in developing the promise of fusion energy. Dr. Ned Sauthoff, who runs the U.S. ITER project office at the Oak Ridge National Laboratory (ORNL), in Oak Ridge, Tennessee, remarked that “ITER will give us greater assurance that fusion is capable of providing abundant, economical and environmentally benign energy in the future.” To succeed, ITER will require a significant parallel research effort to “enhance understanding of the operating characteristics of tokamaks (doughnut-shaped chambers used in fusion research in which a plasma is heated and confined by magnetic fields).

ITER will be 56 feet in diameter. Inside, nuclear fusion, in just half a gram of tritium-deuterium fuel, will produce 500 million watts of power, ten times greater than the external power required to heat the plasma. (Tritium and deuterium are heavy isotopes of hydrogen that fuse together to form helium).

ITER will begin to approach the power output and energy gain required for a practical power plant, but as a whole, it will not be a power generator. The United States has joined the European Union, Japan, China, Russia, India and South Korea in supporting the effort.

One area of potential concern for large-scale tokamaks is the dissipation of energy from the plasma. Understanding what causes heat to stay in such fuel, and what causes it to leak out, has been a long-standing problem. But now researchers at the Princeton Plasma Physics Laboratory (PPPL) in Princeton, New Jersey, have shown that tiny eddies generated in the plasma stream, like swirls in a fast-flowing river, may be one cause of heat loss from the plasma. “This is a breakthrough in the understanding of turbulence in these plasmas,” said PPPL director Rob Goldston.

Fusion reactions occur at high temperatures when two light nuclei, atoms stripped of their negatively-charged electrons, slam into each other and fuse to create a heavier nucleus. In the process matter is converted to energy, which in principle can be harnessed to operate a steam turbine and generate electricity. The challenge is to find the right conditions under which a fusion power plant can produce more energy than is needed to heat the roiling plasma.

PPPL broke the world record for fusion power generation in 1994 with its Tokamak Fusion Test Reactor (TFTR) by producing momentarily a whopping 10.7 million watts. If converted to electricity, this amount of power could satisfy the needs of about 3,000 average-size homes.

But the doughnut-shaped TFTR, like all conventional tokamaks, requires powerful and expensive magnets to produce the high pressures needed for perpetuating fusion reactions. In 1984, physicist Martin Peng in ORNL’s Fusion Energy Division realized that it might be possible to reduce the energy in those magnets with a novel design that confined plasmas to a ball with a hole in its center—more like a cored apple than a doughnut. This simpler, more efficient spherical torus design, which has more modest magnets, would also give scientists an unprecedented wide-angle view of the swirling of ions and electrons inside. And so with great enthusiasm, in February, 1999, physicists flipped the switch on PPPL’s 20-foot diameter National Spherical Torus Experiment (NSTX).

Typically in NSTX, the plasma core reaches about 20 million degrees Celsius. However, like other magnetic fusion devices, the NSTX plasma is much cooler at its edge where it comes in contact with the high-purity carbon tiles that surround it—or what Goldston calls “the physical world.” The plasma edge temperature in NSTX is only 1 million degrees Celsius. The tremendous temperature difference between the core and the edge can create turbulence in the plasma as energetic particles, both atomic nuclei and electrons, collide and swing around the magnetic fields that are supposed to keep them in line. Physicists had previously measured the centimeter-sized turbulence created by the nuclei, but none had confirmed the existence of the tiny, but long-theorized form of electron turbulence.

To tackle this problem, the PPPL team directed microwave beams at the plasma and measured how they scattered by the turbulence, a trick that was only possible due to NSTX’s very open design. “This is a totally cool technique,” Goldston said of the methodology. “We have extremely good spatial resolution, plus or minus 5 percent of the radius of the plasma.” These precise measurements allowed a team led by PPPL physicist Ernesto Mazzucato to confirm the existence of those pesky electron eddies, about 1 millimeter in diameter.

“We now have the suspect under the spotlight, but we don’t know for sure how strongly this turbulence is causing the heat to leak out,” Goldston said. By varying a number of parameters, including the drop in electron temperature and the variation of magnetic fields across the plasma, the physicists may even have learned how to eliminate this electron turbulence. Their results, in very good agreement with theoretical predictions, were published in 2008 in Physical Review Letters.