Deep under the Nevada desert in the 1980s the US conducted secret nuclear weapons research.

Among the experiments was an effort to see if nuclear fusion, the reaction which powers the sun, could be sparked on earth in a controlled setting.

The experiments were classified, but it was widely known among physicists that the results had been promising.

That knowledge caught the attention of two young graduate students working at the Los Alamos National Laboratory in the late 2000s, Conner Galloway and Alexander Valys.

The Los Alamos lab was originally set up in 1943 as a top-secret site to develop the first nuclear weapons. Located near Santa Fe, New Mexico it is now a US government research and development facility.

“When Alex and I learned about those tests at Los Alamos, our reaction was like ‘wow, inertial fusion has already worked!’. Laboratory-scale pellets were ignited, the details were classified, but enough was made public that we knew that ignition was achieved,” says Mr Galloway.

Nuclear fusion is the process of fusing hydrogen nuclei together, which produces immense amounts of energy. The reaction creates helium and not the long-lived radioactive waste of the fission process which is used in existing nuclear power stations.

If fusion can be harnessed, then it promises abundant electricity, generated without producing CO2.

Those tests in the 1980s led to the US government building the National Ignition Facility (NIF) in California, a project to see if nuclear fuel pellets could be ignited using a powerful laser.

After more than a decade of work, in late 2022 researchers at NIF made a breakthrough. Scientists conducted the first controlled fusion experiment to produce more energy from the reaction than that supplied by the lasers which sparked it.

While physicists around the world marvelled at that breakthrough, it had taken the scientists at NIF much longer than expected.

“They were energy starved,” says Mr Galloway.

He doesn’t mean they needed more snacks, instead the NIF laser was only just powerful enough to ignite the fuel pellet.

Mr Galloway and Mr Valys think that more powerful lasers will make it possible to build a working fusion reaction that can supply electricity to the power grid. To do that they founded Xcimer, based in Denver.

NIF had to make do with a laser that could pump out two megajoules of energy. Mr Galloway and Mr Valys are planning to experiment with lasers that can supply up to 20 megajoules of energy.

“We think 10 to 12 [megajoules] is the sweet spot for a commercial power plant,” says Mr Galloway.

Such a laser beam would hit the fuel capsule with a powerful punch. It would be like taking the energy of a 40-tonne articulated lorry travelling at 60mph and focussing it on the centimetre-sized capsule for a few billionths of a second.

More powerful lasers will allow Xcimer to use larger and simpler fuel capsules than NIF, which found it difficult to perfect them.

Xcimer joins dozens of other organisations around the world trying to build a working fusion reactor.

There are two main approaches. Smashing a fuel pellet with lasers falls under the category of inertial confinement fusion.

The other way, known as magnetic confinement fusion, uses powerful magnets to trap a burning cloud of atoms called plasma.

Both approaches have daunting engineering challenges to overcome.

In particular, how do you extract the heat generated during fusion so you can do something useful with it, like drive a turbine to make electricity?

“I suppose my scepticism is, I haven’t yet even seen a persuasive conceptual diagram of how you manage the process of taking energy out while keeping the fusion reaction going,” says Prof Ian Lowe at Griffith University in Australia.

He has spent his long career working in energy research and policy. While Prof Lowe supports the development of fusion technology, he just argues that a working fusion reactor won’t come fast enough to help bring down CO2 emissions and tackle climate change.

“My concern is that even the most optimistic view is that we’d be lucky to have commercial fusion reactors by 2050. And long before then we need to have decarbonized the energy supply if we’re not going to melt the planet,” he says.

Another challenge is that the fusion reaction produces high energy particles that will degrade steel, or any other material that lines the reactor core.

Those in the fusion industry don’t deny the engineering challenges, but feel they can be overcome.

Xcimer plans to use a “waterfall” of molten salt flowing around the fusion reaction to absorb the heat.

The founders are confident that they can fire the lasers and replace the fuel capsules (one every two seconds) while keeping that flow going.

The flow of molten salt will also be thick enough to absorb high energy particles that could potentially damage the reactor.

“We just have two relatively small laser beams coming in from either side [of the fuel pellet]. So you only need a gap in the flow big enough for those beams, and so you don’t have to turn off and turn on the entire flow,” says Mr Valys.

But how quickly can them make such a system work?

Xcimer plans to experiment with the lasers for two years, before building a target chamber, where they can target the fuel pellets.

The final stage would be the working reactor, which they hope would be plugged into the electricity grid in the mid-2030s.

To fund the first phase of their work, Xcimer has raised $100m (£77m) . The money will be used to build a facility in Denver and the prototype laser system.

Hundreds of millions dollars more will be needed to build a working reactor.

But for the founders of Xcimer, and other fusion start-ups, the prospect of cheap, carbon-free electricity is irresistible.

“You know, it’ll change the trajectory of what’s possible for humanity’s progress,” says Mr Valys.