After years of disappointing results and missed deadlines, a $5 billion laser complex has now achieved a step that revives optimism that thermonuclear fusion, the process that powers the sun, can one day be harnessed for almost limitless energy.

At the National Ignition Facility at the Lawrence Livermore National Laboratory in California, 192 enormous lasers in a structure the size of a football stadium fire at a small gold cylinder, vaporizing it. That generates an onslaught of X-rays rushing inward toward a fuel pellet smaller than a peppercorn, crushing the hydrogen inside into helium, and releasing a burst of energy — effectively, a miniature hydrogen bomb.

That, at least, was the concept.

But for four years, since the facility began operations in 2009, the last step — the fusion of hydrogen atoms into helium — did not happen, not in significant quantity.

Then, last September, it did.

Writing in Thursday’s issue of the journal Nature, scientists working on the project report on the September shot as well as one in November. In both, the hydrogen fusion generated more energy than had been deposited into the hydrogen. However, laser-driven fusion remains far from practical, because only about 1 percent of the initial laser energy reached the hydrogen.