US government scientists have achieved net energy gain in a fusion reaction for the second time, a result that is set to fuel optimism that progress is being made toward the dream of limitless, zero-carbon power.

Physicists have since the 1950s sought to harness the fusion reaction that powers the Sun, but until December no group had been able to produce more energy from the reaction than it consumes—a condition also known as ignition.

Researchers at the federal Lawrence Livermore National Laboratory in California, who achieved ignition for the first time last year, repeated the breakthrough in an experiment on July 30 that produced a higher energy output than in December, according to three people with knowledge of the preliminary results.