A major step toward the long-sought goal of producing electricity from atomic fusion occurred late Wednesday night when the most powerful fusion reactor ever built was successfully fired at Sandia National Laboratory in New Mexico.

The device contained no nuclear fuel, but the test proved that the newly designed and built machine can perform as intended. About two years of further testing and tuning will be needed before scientists put nuclear fuel into the machine -- the Particle Beam Fusion Accelerator II -- and try to ignite the first controlled fusion reaction in the laboratory.

The Sandia machine represents one of two main experimental approaches to achieving controlled fusion, called inertial confinement. A rival method, called magnetic confinement and using a device called a tokamak, is under development at Princeton University.

Progress in magnetic confinement has come more slowly than expected while work on Sandia's inertial confinement method has moved faster, overtaking the older magnetic approach.

The theoretical promise of fusion power has long been attractive because, if it works, it could produce electricity using abundant forms of hydrogen as a fuel and create almost no radioactive wastes.

Unlike existing atomic power plants, which rely on the more or less spontaneous splitting, or fission, of radioactive atoms to release heat, fusion reactors would produce energy only after delivering huge pulses of power to pea-sized pellets of hydrogen fuel.

The effect of the pulse is to compress the pellet to a density perhaps a thousand times greater than normal, raising its temperature to about 100 million degrees Celsius. The result is a miniature thermonuclear explosion in which the atoms of hydrogen fuse and convert some of their subatomic particles into energy. A similar reaction occurs in a hydrogen bomb, when the pulse delivered by an atom, or fission, bomb causes the hydrogen explosion.

If a practical fusion reactor can be made, it would have to produce enough power to deliver the next pulse of energy to the next pellet and have enough left over to sell. It would explode a series of perhaps 10 pellets a second to sustain the heat output. As with most electrical power plants, the heat would drive steam through turbines connected to generators.

The Sandia machine is a 108-foot diameter wheel in which 36 particle accelerators, situated like spokes, deliver simultaneous pulses that converge on the wheel's hub. The pulses consist of brief bursts of electrically charged lithium atoms that slam against anything at the hub. In Wednesday's test the hub was occupied not by a fuel pellet but by instruments that measured the characteristics of the pulses.

When it is fully operational, the device is expected to deliver 100 trillion watts of power at the hub for a pulse lasting about 50 billionths of a second.

While firing, the pulse will equal 50 times the world's continuous electrical generating capacity. The first test was made at about 70 percent of capacity. If the Sandia reactor meets goals, the next major hurdle will require new versions of reactors that can achieve "break-even," demonstrating that controlled fusion can produce as much energy as it consumes. After that would come reactors that produce surplus energy.

Most experts say it will be decades before fusion power becomes practical. Some, recalling that similar forecasts were made decades ago, say it may never happen