Charging their way toward fusion.

Zap! The Particle Beam Fusion Accelerator II (PBFA-II) is billed by its owner, Sandia National Laboratories of Albuquerque, N.M., as the world's most powerful accelerator. It is designed to yield power in pulses, and when it reaches its design power, its pulses will surpass by several times the instantaneous power of all the world's electric generating plants combined. Finished about a month ahead of schedule, PBFA-II fired its first pulse on Dec. 11, at 70 percent of its design energy of 100 trillion watts, at 8:09 p.m., mountain standard time.

PBFA-II's completion opens a new stage on the long journey toward controlled thermonuclear fusion. As Pace VanDevender, director of pulsed power sciences for Sandia, puts it, "The world now has the best light-ion accelerator for inertial confinment fusion that can be built."

PBFA-II stores up electrical energy, 3.5 million joules of it, and concentrates if into pulses 50 billionths of a second long. The pulses energize a diode that generates a beam of lithium ions. Ultimately the ions will strike a target of thermonuclear fuel. The effect of the impact will implode the target, and the implosion is expected to compress and heat the target to the point where nuclear fusions begin. At design energy, PBFA-II centimeter onto the surface of the target, a pellet of a few millimeters' radius.

For the next two years the experimenters expect to be busy with the means of gneerating and focusing the beam of lithium ions, which will come from the walls of a cylindrical diode and impinge on the target (hung in the center of the diode) from all sides. Production and focus of such an ion beam present severe technical challenges, although the experimenters were much encouraged earlier this year by unexpected successes in that endeavor with PBFA-II's predecessor, PBFA-II should be ready to begin experiments aimed at igniting fusions in fuel pellets, with the ultimate hope of reactions that produce more energy from fusion that it takes to get the fusion going, which would be a net energy gain and what everybody in the controlled fusion business is looking for. PBFA-I is now being converted into the SATURN X-ray machine, which will simulate effects of nuclear weapons on various material samples.

In appearance, PBFA-II, 108 feet in diameter and 20 feet deep, looks like a lot of electrical equipment submerged in a swimming pool. Only in part of the pool is the liquid water, however; the other part is filled with oil. The liquids serve as dielectrics separating the plates of huge capacitors. PBFA-II consists of 36 power-compressing modules spaced in four layers around the pool. Each module is a series of capacitors and switches that compress the energy step by step into shorter and shorter time intervals until it gets doen to 50 billionths of a second. All 36 modules feed their output simultaneously to the diode in the center of the pool.

The installation was built within its budget of $48 million. The builders managed its design so that improvements in technology could be incorporated as construction went on, and so its capabilities are up to date as of the completion. One improvement not yet incorporated is a new kind of switch that could double the power output to 200 trillion watts.