The U.S. government lab that made a long-awaited breakthrough in fusion energy late last year has run five similar experiments since then without being able to replicate the results.

The milestone came in December as the the Lawrence Livermore National Laboratory near San Francisco reported achieving the first fusion reaction that produced more energy than it took to create — a threshold known as ignition. The achievement opened the possibility of power plants one day running on nuclear fusion, the same energy source as the sun.

Since then, the lab has run five similar experiments, shooting the world’s most powerful laser at small diamond capsules filled with hydrogen, said Lawrence Livermore Director Kim Budil. But so far, ignition hasn’t been achieved again.

The quality of the carefully made capsules wasn’t quite as high as the one used in December, and three of the five experiments used less laser power than before, she said. Although, "the hope is always there,” Budil said, the lack of ignition wasn’t a surprise.

"We’ve learned a lot through those experiments, and we’re very confident we’ll get back above that threshold,” Budil told reporters at an event at the lab Monday celebrating December’s breakthrough. "But it’s still very much an R&D project at this point.”

Speaking at the event, U.S. Energy Secretary Jennifer Granholm said the federal government would spend $45 million over the next four years to create a series of research hubs pursuing the kind of fusion achieved at Lawrence Livermore, known as inertial confinement.