Ground zero: A circular access port affords a glimpse into a 10-meter-diameter target chamber where, in the coming months, powerful lasers will be fired with the goal of setting off small thermonuclear explosions. (See more images.)

It’s late April and workers are assembling the last parts of the National Ignition Facility (NIF), a sprawling building covering the area of three football fields at Lawrence Livermore National Laboratory in Livermore, CA. Dressed in hard hats, hair nets, lab coats, and latex gloves, they have gathered at the target chamber, a sphere 10 meters in diameter and bristling with 48 burnished-aluminum ducts that together house 192 separate laser beams. Each beam on its own is one of the world’s most powerful, says Bruno Van Wonterghem, operations manager at NIF. Together they deliver 50 to 60 times the energy of any other laser.

The workers are preparing to install a key piece of equipment–the target-alignment sensor–at the end of a tapered boom that can be extended into the center of the chamber. Scientists will use the sensor to position a gold canister the size of a pencil eraser at the center of the sphere and align it with the laser beams. In a series of experiments over the coming months, if all goes according to plan, those lasers will strike the gold canister with a pulse 3 to 20 nanoseconds long, generating a bath of high-energy x-rays. These in turn will cause a two-­millimeter pellet containing hydrogen isotopes to implode. “All of that kinetic energy gets transformed into heat,” says Van ­Wonterghem. The hydrogen pellet will reach a temperature of 100 million °C and a density 100 times that of lead–enough to start a fusion reaction.

Fusion, in which atomic nuclei combine to form atoms of a new element, is the key reaction fueling nuclear bombs and the sun. (In the NIF experiments, hydrogen isotopes combine to form helium nuclei while releasing neutrons and x-rays.) It has also long been held up as a potential source of abundant energy, if only the reactions could be harnessed in a controlled setting. That’s challenging, because a plasma hot enough for hydrogen nuclei in it to fuse is so hot that it would destroy any containment material. Scientists have conceived two general solutions. The first and most mainstream is to confine the plasma in a powerful electromagnetic field. That is what’s supposed to happen at the multinational, $14 billion ITER project in France, which is expected to be operational by 2018.

NIF takes a fundamentally different tack. By using lasers to compress the hydrogen fuel, it will mimic the extreme heat and density inside a star. The resulting fusion reaction is controlled not by confining it electromagnetically but by limiting the amount of fuel. NIF will produce a tiny thermonuclear explosion, so small that it can be studied in a 10-meter chamber. In fact, NIF’s primary mission is to shed light on high-­temperature and high-density physics, including the reactions in nuclear weapons, by re-creating conditions inside stars and bombs.

Researchers debate which approach will be the most useful for generating electricity; so far it’s too early to be sure. But it looks likely that NIF will be the first facility to reach a significant milestone in the quest for laser-based fusion power: the ignition of a self-sustaining reaction that produces more energy than was put in by the laser. Previous experiments and computer simulations suggest that the 192 lasers at NIF are powerful and precise enough to set off such a chain reaction–one that will continue to burn until the hydrogen fuel runs out.

There are still huge challenges to be met before fusion can be harnessed to generate electricity. But achieving controlled fusion burn “will be an incredible event,” says Edward Moses, a principal associate director at Livermore who’s in charge of NIF. “We think we’re coming to a new era.”