Nuclear fusion breakthrough – what is it and how does it work?

US scientists have reached a major milestone in their attempts to perfect a process which could potentially deliver almost limitless supplies of energy.

Nuclear fusion does not rely on fossil fuels or produce harmful greenhouse gases, so could also help tackle climate change.

Nuclear fusion is the process which gives the Sun its energy.

Scientists from more than 50 countries have been trying to recreate it on Earth since the 1960s.

They hope it could eventually provide huge quantities of clean energy for the world.

In nuclear fusion, pairs of tiny particles called atoms are heated and forced together to make one heavier one.

It is the opposite of nuclear fission, in which heavy atoms are split apart. Nuclear power stations currently use nuclear fission to generate electricity.

Nuclear fission produces radioactive waste, which can be dangerous and must be stored safely - potentially for hundreds of years.

The waste produced by nuclear fusion is less radioactive and decays much more quickly.

Nuclear fusion doesn't need fossil fuels like oil or gas. It also doesn't generate greenhouse gases, which trap the Sun's heat and are responsible for climate change.

Most fusion experiments use hydrogen, which can be extracted cheaply from seawater and lithium, meaning fuel supplies could last for millions of years.

It has been described as the "holy grail" of energy production.

When two atoms of a light element such as hydrogen are heated and combine to form a single heavier element such as helium, the nuclear reaction produces massive amounts of energy which can be captured.

But getting two identical elements to combine is actually very hard.

Because they have the same positive charge they naturally repel each other.

A lot of energy is needed to overcome this resistance.

In the Sun, this happens thanks to extremely high temperatures of around ten million degrees Celsius, and significant pressure - more than 100 billion times that of the Earth's atmosphere.

On Earth, scientists have used various different techniques to attempt to recreate these conditions.

But it has proved very difficult to maintain the high temperature and pressure needed for long enough.

The US's National Ignition Facility (NIF) has announced it successfully used a 192-beam laser to turn a tiny amount of hydrogen into enough energy to power about 15 - 20 kettles.

This means that - for the first time - scientists were able to generate more power than the lasers put in to the experiment.

Despite a series of promising breakthroughs in the last few years, large-scale nuclear fusion is still several years away.

In February, European scientists at the UK-based JET laboratory smashed their own world record for the amount of energy produced over five seconds.

But even the successful NIF experiment in the US did not produce more energy than was needed to make the lasers work in the first place - and the research programme to get to this point has cost billions of dollars.

Although physicists have welcomed the US results - and described them as a true breakthrough moment - they point out that much more work is needed before nuclear fusion could be used to power homes or businesses.

Scientists will now focus on reproducing fusion more quickly and cheaply.

The International Atomic Energy Agency (IAEA) has called nuclear fusion "intrinsically safe."

The conditions required to start and maintain a fusion reaction are so extreme that it is impossible for it to run out of control.

"Fusion is a self-limiting process: if you cannot control the reaction, the machine switches itself off," explained the IAEA's Sehila González de Vicente.

The lower level of radioative waste produced by the process compared with nuclear fission is also much easier to handle and store.

Nuclear fusion does not rely on fossil fuels like oil or gas, and produces none of the greenhouse gases which drive global warming.

Unlike solar or wind energy it is not dependent on beneficial weather conditions.

It uses two relatively abundant materials found on Earth: lithium and hydrogen.

Widescale use of nuclear fusion could help countries meet their targets to produce "net zero" emissions by 2050.

However, it will be many years before recent experimental successes can be meaningfully scaled up.