US researchers have achieved a world first in an ambitious experiment that aims to recreate the conditions at the heart of the sun and pave the way for nuclear fusion reactors.

The scientists generated more energy from fusion reactions than they put into the nuclear fuel, in a small but crucial step along the road to harnessing fusion power. The ultimate goal - to produce more energy than the whole experiment consumes - remains a long way off, but the feat has nonetheless raised hopes that after decades of setbacks, firm progress is finally being made.

Fusion energy has the potential to become a radical alternative power source, with zero carbon emissions during operation and minimal waste, but the technical difficulties in demonstrating fusion in the lab have so far proved overwhelming. While existing nuclear reactors generate energy by splitting atoms into lighter particles, fusion reactors combine light atomic nuclei into heavier particles.

In their experiments, researchers at the National Ignition Facility at the Lawrence Livermore National Laboratory in California used a bank of 192 powerful lasers to crush a minuscule amount of fuel so hard and fast that it becomes hotter than the sun.

The process is not straightforward. The lasers are fired into a gold capsule that holds a 2mm-wide spherical pellet. The fuel is coated on the inside of this plastic pellet in a layer as thin as a human hair.

When the laser light enters the gold capsule, it makes the walls of the gold container emit x-rays, which heat the pellet and make it implode with extraordinary ferocity. The fuel, a mixture of hydrogen isotopes called tritium and deuterium, partially fuses under the intense conditions.

The scientists have not generated more energy than the experiment uses in total. The lasers unleash nearly two megajoules of energy on their target, the equivalent, roughly, of two standard sticks of dynamite. But only a tiny fraction of this reaches the fuel. Writing in Nature, the scientists say fusion reactions in the fuel released at best 17 kilojoules of energy.

Though slight, the advance is welcome news for the NIF scientists. In 2012, the project was restructured and given more modest goals after six years of failure to generate more energy than the experiment consumes, known as “ignition”.

The dream of controlled fusion still remains a distant hope. Omar Hurricane, the lead author of the report, said it was too early to say whether it was even possible with the NIF facility. The researchers need to get a hundred times more energy from the fusion reactions before the process can run itself, and more for it to deliver an overall surplus of energy.

—By arrangement with The Guardian