The prospects for developing nuclear fusion as a feasible source of energy have significantly improved, say experts.
The UK government has recently announced an investment of £200m to deliver electricity from a fusion reactor by 2040.
Private companies and governments have told the BBC they aim to have demonstration models working within five years.
But huge hurdles remain, say critics.
With the price of wind and solar continuing to drop, experts say these existing renewables might offer a more economical and timely method of tackling climate change and generating energy than an unproven technology like fusion.
Nuclear fusion is an attempt to replicate the processes of the Sun on Earth. It differs significantly from nuclear fission, which has been our only way of getting electricity from atoms since the 1950s.
Fission has proven to be hugely expensive. It generates large amounts of radioactive waste and raises serious concerns about safety and the proliferation of weapons.
Fusion is the process that drives our Sun.
Every single second, millions of tonnes of hydrogen atoms crash together in the tremendous temperatures and pressures of our parent star. This forces them to break their atomic bonds and fuse to make the heavier element, helium.
For decades, researchers have been trying to replicate this process on Earth, or “build the Sun in a box” as one physicist dubbed it. The basic idea is to take a type of hydrogen gas, heat it to more than 100 million degrees until it forms a thin, fragile cloud called a plasma, and then control it
with powerful magnets until the atoms fuse and release energy.
Potentially, it can generate power that is low carbon, with much smaller amounts of waste. It also comes without the danger of explosions.
To deliver the fusion concept, countries have focused their energies on a major international co-operative effort called Iter.
The Iter project involves 35 countries and, right now, it is constructing a huge test reactor in southern France.
The plan is to have the first plasma generated in 2025. However, getting from this step to producing energy is extremely difficult.
Iter has also been beset by long delays and budget overspend which means it is unlikely to have a demonstration fusion power plant working even by 2050.
“One of the reasons that Iter is late is that it is really, really hard,” said Prof Ian Chapman, chief executive of the UK Atomic Energy Authority.
“What we are doing is fundamentally pushing the barriers of what’s known in the technology world. And of course you reach hurdles and you have to overcome them, which we do all the time and Iter will happen, I am completely convinced of it.”
Until Iter is up and running in 2025, the UK based Joint European Torus (Jet) remains the world’s largest fusion experiment.
It has secured EU funding until the end of 2020, but what happens after that, and the participation of the UK in Iter after Brexit remain unclear.
To give some sense of certainty, the UK government recently announced £220m for the conceptual design of a fusion power station by 2040.
Over the next four years, researchers based at Culham in Oxfordshire will develop designs for a fusion power plant called Step or Spherical Tokomak for Energy Production.
The most widely known approach to making fusion happen involves a doughnut shaped vacuum chamber called a Tokomak. Hydrogen gas is heated to 100 million degrees C at which point it become a plasma. Powerful magnets are used to confine and steer the plasma until fusion occurs.
In the UK, researchers have developed a different form of Tokamak, that more resembles an apple core than a doughnut. Called a Spherical Tokamak, it has the advantage of being more compact, potentially allowing future power plants to be located in towns and cities.
“If you look at some of the very big units, the big machines that we are looking at, just finding geographically somewhere to put them is difficult,” said Nanna Heiberg from the UK Atomic Energy Authority.
“What you really want to do is put them close to where the energy is required. And so if you can do them in a much smaller footprint, you can put them closer to the users and you can put more of them around the country for example.
While governments are wrestling with Iter, many are also driving ahead with their own national plans. China, India, Russia and the US among others are working on developing commercial reactors.
