Ever since physicists at Britain's Cavendish Laboratory first glimpsed it in 1932, the possibility of mimicking the nuclear fusion that powers the sun has been a tantalizing vision. The process would leave no hazardous nuclear waste. The principal fuel - deuterium (doubly heavy hydrogen) - is as plentiful as water, from which engineers can easily extract it. The other fuel component - tritium (triply heavy hydrogen) - would be made in a blanket of lithium as this absorbs neutrons coming from the fusion reactor. Lithium is also fairly abundant. `Research is flourishing'

But for fusion to work on Earth, reactors must heat their fuel to at least 100 million degrees C. - some seven times hotter than the center of the sun. They must keep that searingly hot gas together long enough for significant amounts of fusion energy to flow.

Whenever research teams have thought they had that goal in sight, new difficulties have obscured it. But within the past year, experiments in Europe, Japan, and the United States appear to have put researchers close to their first major objective - energy break-even. Sometime this year or next, teams in each of these countries expect to show that their experimental reactors can, potentially, produce at least as much energy as they consume. They expect to actually demonstrate this feat within another few years. A daunting task for physicists

US Energy Secretary John S. Herrington has called the present laboratory success ``a major milestone in progress toward the development of fusion energy.'' R. S. Pease of Britain's Atomic Energy Authority - speaking with the understatement of an expert whose hopes have been dashed before - observes that the ``research is clearly flourishing.'' And summarizing what, for experts, is the most important point, Dale M. Meade of the Princeton Plasma Physics Laboratory explained that ``what these experiments show is that physics is not the restriction,'' according to the journal Science.

Even the most intrepid experimenters find the physics of controlled thermonuclear fusion daunting. Two positively charged atomic nuclei, whose natural inclination is to flee from each other, must be brought intimately together. Once their electrical repulsion is overcome, they may react or fuse together to produce a helium atom. They release energy - some of which is carried away by a neutron - as they merge. Grabbing nuclei with magnets

In the sun's center, a density 12 times that of lead encourages togetherness. But in typical experiments, the reacting gas, which physicists call a plasma, is roughly 100,000 times as rare as sea-level air. Experimenters must heat the plasma to high temperature so that the deuterium and tritium nuclei move fast enough - 2 to 3 million miles an hour - so they run into each other often enough to produce useful power.

Engineers also have to keep this mass of speedsters intact long enough for significant fusion to occur - about 1 to 2 seconds. Because the particles are electrically charged, magnetic fields can grab on to them. Fusion workers use some of the world's strongest magnets to bottle up their plasmas, which exert pressures of a few atmospheres. Tokamac reactors

The currently favored design confines the plasma in a large doughnut-shaped vessel. The doughnuts that have made the recent progress are called tokamacs. That's a Russian acronym for ``toroidal magnetic chamber.'' The European Community's Joint European Torus in England, Japan's JT-60, and the Tokamac Fusion Test Reactor at Princeton, N.J., are giant versions of this Soviet invention. (The Soviets have not yet gotten their latest big tokamac to operate.) These vessels are large enough for workers to stand inside.

Over the past year, deuterium plasma in one or another of these machines has reached temperatures as high as 230 million degrees C. - 15 times the temperature at the sun's core. The combination of plasma density and confinement time has come close to that needed for ignition. The next step is to try for self-sustaining fusion by introducing tritium. Deuterium alone won't ignite under these conditions. Miniature hydrogen bombs

There are other approaches. Laboratories in several countries use laser beams to compress fuel pellets to high densities. They, in effect, explode miniature hydrogen bombs. Many laboratories are also studying alternative tokamac designs and other types of magnetic bottles.

But for the moment, the main attention is on following up the major tokamac experiments with advanced machines that could explore actual reactor operating conditions.

Fusion experts don't expect to have a practical power station until the second quarter of the 21st century. But they do feel momentum building toward that goal.

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