An elusive goal in the global race to create clean, green nuclear energy has been achieved, US scientists say.
It's being hailed as a "breakthrough", but how significant is the announcement?
For decades, scientists have tried to harness fusion energy, which is the process of making energy using the same phenomenon that powers the Sun.
Among other issues, they've been unable to do so in a way that generated more energy than was expended.
Now, scientists from the Lawrence Livermore National Laboratory near San Francisco say they've achieved "net energy gain" for the first time.
So how did they do it, and are we much closer to fusion energy being a viable source of abundant, clean and reliable power?
There are two types of nuclear energy: fission and fusion.
The process of fission has been commercially viable for decades and is used in conventional nuclear power plants to generate about 10 per cent of the world's power.
Energy is created by splitting a big, heavy atom like uranium, which also produces radioactive waste.
Fusion fuses lighter elements such as hydrogen together to produce a heavier element.
The process releases millions of times more energy than burning coal, oil or gas, and about four times as much as nuclear fission reactions (at equal mass of fuel).
On top of generating all this energy, it doesn't produce long-lived radioactive waste.
Sounds great, right? But there's a catch: smooshing particles together requires extreme temperatures and pressures.
Recent nuclear fusion experiments, for instance, have reached temperatures of 100 million degrees Celsius, or seven times hotter than the Sun.
Apart from the problem of sustaining and controlling this reaction, generating sufficient heat and pressure requires a lot of energy.
Through decades of attempts, scientists have been unable to achieve what is known as "net energy gain" – or getting more energy back than was put into the reaction.
The US scientists essentially created mini-explosions in the lab.
They used lasers to blast a small cylinder containing deuterium and tritium, which are heavier isotopes of hydrogen.
Heat and pressure generated from the lasers compresses the isotopes until they overcome their mutual electrical repulsion by stripping off electrons.
Nuclei from deuterium and tritium in the plasma soup then fuse together, giving off relatively huge amounts of energy.
"You fire the lasers, they heat up the target, the target explodes and it is gone," said Matthew Hole, a physicist at the Australian National University (ANU) who is working on other fusion experiments.
Using pressure to ignite an explosion — a technique known as inertial confinement— isn't new; the same process occurs in hydrogen bombs, he said.
Earlier this year, the US team reported they'd been able to create 170 kilojoules of energy — the equivalent energy of 9-volt batteries – and reach a point where it self heated, kicking off more fusion reactions.
The latest experiments ramped up the energy output to 3.15 megajoules, which is more than the 2.05 megajoules of energy delivered by the lasers.
"Assuming the entire energy yield can be harnessed, then it would produce 875 W/hr, just shy of 1kW hr," Professor Hole said.
That's roughly equivalent to boiling a jug for 20 minutes.
Yes! The laser technique the US team used is one of two major methods to produce fusion.
The other technique confines deuterium and tritium under extreme heat – hundreds of millions of degrees – in a magnetic field in a donut-shaped vessel called a tokamak.
Unlike the laser method, which essentially creates a series of explosions, a tokamak keeps the plasma under continual pressure so the reaction is sustained.
This method, known as toroidal magnetic confinement, was used by teams in the UK and Europe last year to produce 52 megajoules of energy for five seconds – a record for the amount of energy extracted from nuclear fusion, although the result fell short of net energy gain.
It's definitely a breakthrough for science, but it's still a long way off being a viable source of renewable energy.
"It is an impressive technological feat," Professor Hole said.
"There are a lot of steps needed to translate that from a lab experiment to a power plant."
Professor Hole says it may be difficult to generate large-scale continuous power from the inertial confinement method used in the experiment, as it relies on pulses of energy, or creating a series of explosions.
To create useful amounts of energy, a fusion power plant that uses lasers would need to reproduce the reaction many times per second.
With the quest for continuous energy, there's also a need for materials capable of withstanding these high pressures and temperatures for prolonged periods of time.
Finally, the size of the net energy gain needs to be much larger. That's because although the reaction itself generated an excess of energy, significantly more energy was required to power the lasers that delivered energy to the reaction, said Ken Baldwin, a professor of physics at ANU.
This is known as the "wall-plug efficiency" — or the amount of energy drawn from the grid that is deposited on the fusion fuel, versus the amount generated.
Even though it achieved net energy gain, the recent experiment had a wall-plug efficiency of about 0.5 per cent.
"In coming decades, we'll have to get the wall-plug efficiency above the break-even point," Professor Baldwin said.
Yes, Australia is working with an international coalition of countries and institutions researching fusion energy.
In 2016, Australia's nuclear agency ANTSO signed a cooperation agreement with the ITER international nuclear fusion energy organisation that's building what has been described as the world's biggest experiment.
Thirty-five nations are collaborating to build and operate the ITER Tokamak experimental fusion reactor in southern France.
Unlike the Livermore National Laboratory's experiment, the ITER reactor uses the toroidal technique.
"ITER has a goal of demonstrating break-even in the early 2030s," Professor Baldwin said.
Other Australian participants include the ANU, University of Sydney, Curtin University, University of Newcastle, University of Wollongong and Macquarie University.
Meanwhile, China is working on its own fusion project known as the Experimental Advanced Superconducting Tokamak (EAST). Earlier this year, it sustained a nuclear fusion reaction for more than 17 minutes, breaking the record for the longest reaction.