American scientists have taken a big step in trying to fully master a process that could potentially lead to nearly inexhaustible energy supplies.
Nuclear fusion does not rely on fossil fuels and does not produce harmful greenhouse gases, so it can help fight climate change.
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What is nuclear fusion?
Nuclear fusion is the process that gives energy to the Sun.
Scientists from more than 50 countries have been trying to recreate it on Earth since the 1960s.
They hope that this could eventually provide vast amounts of clean energy for the entire world.
In nuclear fusion, pairs of tiny particles called atoms are heated and forced together to make one heavier particle.
This is the opposite of nuclear fission, in which heavy atoms are split apart.
Nuclear power plants currently use nuclear fission to produce electricity.
Why is nuclear fusion important?
Nuclear fission produces a lot of 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 faster.
Nuclear fusion does not require fossil fuels such as oil or gas.
It also does not create the effect of greenhouse gases, which trap the sun's heat and cause climate change.
Most fusion experiments use hydrogen, which can be extracted cheaply from seawater and lithium, meaning that fuel supplies can last for millions of years.
It has been described as the "Holy Grail" of energy production.
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How nuclear worksa fusion?
When two atoms of a light element such as hydrogen are heated and combined to form one heavier element such as helium, the nuclear reaction produces vast amounts of energy that can be captured.
But it is very difficult to combine two identical elements.
Since they have the same positive charge, they naturally repel each other.
It takes a lot of energy to overcome this resistance.
On the Sun, this happens thanks to extremely high temperatures of about ten million degrees Celsius and significant pressure - more than 100 billion times greater than the Earth's atmosphere.
On Earth, scientists have used various techniques to try to create these conditions.
But it turned out to be very difficult to maintain the required high temperature and pressure long enough.
The US National Ignition Facility (NIF) announced that it had successfully used a 192-beam laser to convert a small amount of hydrogen into enough energy to power about 15-20 boilers.
This means that for the first time scientists have managed to generate more energy than the lasers invested in the experiment.
When will nuclear fusion be possible?
Despite significant progress in recent years, we are still far from large-scale nuclear fusion.
In February, European scientists at the JET laboratory in Great Britain broke their own world record for the amount of energy produced in five seconds.
But even the successful NIF experiment in the US did not produce more energy than was needed to make the lasers work at all, and the research program to get to this point cost billions of dollars.
Although physicists welcomed the results from the US and described them as a big step, they point out that much more work is needed before nuclear fusion can be used to power homes or businesses.
Scientists will now focus on faster and cheaper fusion reproduction.
How safe is nuclear fusion?
The International Atomic Energy Agency (IAEA) has called nuclear fusion "intrinsically safe".
The conditions required to initiate and sustain fusion are so extreme that it is impossible for it to get out of control.
"Fusion is a self-limiting process: if you can't control the reaction, the machine turns itself off," explained Sehila Gonzalez de Vicente of the IAEA.
The radioactive waste produced by this process compared to nuclear fission is also much easier to handle and store.
Can nuclear fusion help fight global warming?
Nuclear fusion does not rely on fossil fuels such as oil or gas and does not produce any of the greenhouse gases.
Unlike solar energy or wind energy, nuclear fusion does not depend on favorable weather conditions.
It uses two relatively abundant materials found on Earth: lithium and hydrogen.
Widespread use of nuclear fusion could help countries meet targets for zero-emission production by 2050.
However, it will be many years before recent experimental successes can be significantly scaled up.
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