Injecting tritium into magnetic fusion.

Researchers have for the first time achieved the fusion of deuterium and tritium nuclei in a magnetically confined plasma. The resulting nuclear reactions generated roughly 1.7 million watts of power in a burst lasting nearly one second, scientists a the Joint European Torus (JET) laboratory in Culham, England, reported last week.

"This is the first time that a significant amount of power has been obtained from controlled nuclear fusion reactions," says JET Director Paul-Henri Rebut. "It is clearly a major step forward in the development of fusion as a new source of energy."

Placing tritium in a magnetically confined plasma marks a significant break with previous practice. Because of the special care required for handling tritium, a radioactive hydrogen isotope, and the risk of contaminating the reactor itself, researchers had in the past concentrated on studying the behavior of magnetically confined plasmas consisting mainly of nonradioactive hydrogen isotopes -- either hydrogen itself or its heavier form, deuterium. But because the fusion of tritum and deuterium occurs at a lower temperature

and more rapidly than the reaction between two deuterium nuclei, they expected eventually to switch to a mixture of tritium and deuterium.

As a first step, scientists at the JET fusion reactor injected a small amount of tritium into a magnetically confined, intensely heated deuterium plasma. Under these conditions, tritium and deuterium nuclei fused to produce helium nuclei (alpha particles) and highly energetic neutrons.

Advancing to tritium "was the next logical thing to do," says Richard D. Petrasso of the Massachusetts Institute of Technology. "No one had ever before put tritium inside any sort of magnetic confinement device."

Data from the JET experiment should provide crucial information about how well instruments designed to measure such quantities as temperature and impurity levels inside a reactor function when bombarded by high-speed neutrons, Petrasso says. Researchers will also learn how long it takes to clean out leftover tritium embedded in the reactor's walls.

Although the experiment produced a significant amount of power, it required nearly 10 times more power to heat the tritium-deuterium plasma to a temperature high enough to initiate fusion. "It was an important step in trying to decide whether fusion will eventually be an [energy] option," Petrasso says. "But we've still got a long way to go."