Fusion: Stepping closer to reality

Enter ITER, which would represent a major step toward a commercial fusion reactor. Researchers have designed it to generate at least five times the amount of power it consumes in sustaining fusion reactions. It would use a reactor roughly shaped like a hollow doughnut, surrounded by magnets. The plasma forms and the reactions occur within the doughnut. The magnetic fields are designed to keep the plasma from hitting the reactor walls. If it did, it would cool sufficiently to snuff the reactions. "No one would get hurt, but if you were trying to sell electricity, you wouldn't be very happy," Goldston quips.

For years, researchers worried that at the energy levels ITER was aiming for, the plasma would fail to remain stable or that the magnetic fields would fail to keep the plasma bottled up.

But since the mid-'90s, technological advances have yielded fresh insights into the way such reactors can operate. They include improved test equipment, new ways to tweak the reactions from outside the reactor vessel, and more-powerful computers that model the conditions in the reactors. "Now we know what we're looking at," Goldston says.

For example, when the plasma grows turbulent, it forms eddies and the plasma cools. Researchers had a difficult time figuring out what determined the size of the eddies and how to control them. With the added computational horsepower and the new instruments, they determined the factors that controlled their size. Just as important, they found that they could apply more push to the flowing plasma than the system would generate on its own, shearing off the eddies almost before they got started.

"If you play that card right, you get these regions that are very quiet" and have distinctly higher temperatures than the regions surrounding them, Goldston says.

Another troublesome question revolved around how powerful a magnetic field the ITER reactor would need to contain the plasma.

"This is a key issue," he says. "Magnetic fields cost money and plasma makes fusion. If you can hold a lot of plasma in a little magnetic field, you can make money. If you can only hold a little bit of plasma in a big magnetic field, then you ought to find a different job."

Researchers are encouraged by the results they've gotten in this area so far.

Scientists are targeting other issues as well, the DOE's Dr. Davies says. A search is under way for materials that can line the reactor chamber without succumbing to the corrosive effects of the reactions. Scientists are also seeking new materials that will lose lethal levels of radioactivity faster than is currently the case. Today's fission reactors generate large amounts of long-lasting radioactive waste. Fusion reactors are expected to generate smaller amounts of highly radioactive waste. Scientists would like to use materials, such as silicone carbide, that don't become radioactive at all.

In addition, researchers are looking at alternative approaches to designing the reactor core itself.

"The fusion energy program has risen to a new level of scientific understanding," Davies says. "We're now measuring and controlling plasmas consistent with computer simulations. This represents an enormous step forward."

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