The most complicated scientific instrument in human history has reached the halfway point in its construction, according to those behind the project.

The €18 billion International Thermonuclear Experimental Reactor (ITER) aims to prove that power generated by nuclear fusion can be produced on a commercial scale and is sustainable.

Fusion is the same energy source that causes the Sun to gives the Earth its light and warmth.

A global collaboration of scientists and engineers, including a number from Ireland, are currently building a hydrogen fusion machine with 10 million parts in the south of France to prove the process can be artificially replicated on our planet.

It will involve heating hydrogen plasma to 150 million degrees Celsius, ten times hotter than the core of the Sun.

This will happen inside a tokamak, a donut-shaped reactor surrounded by giant magnets that keep the plasma under control and circulating.

The magnets will operate at -269 degrees Celsius, the same temperature as interstellar space.

As well as getting the machine operating, the big question scientists will be trying to answer is whether or not they can get the reactor producing more energy than it takes to make it work.

But while the construction is half complete, there is a huge body of work left to do, with "first plasma" not due until December 2025. 

"The stakes are very high for ITER," said Bernard Bigot, Ph.D., Director-General of ITER. 

"When we prove that fusion is a viable energy source, it will eventually replace burning fossil fuels, which are non-renewable and non-sustainable."

"Fusion will be complementary with wind, solar, and other renewable energies."

Fusion is considered the holy grail of energy production because it doesn't generate carbon, is environmentally sustainable and is hugely powerful.

ITER scientists say just a pineapple sized amount of hydrogen should be able to create as much energy as 10,000 tons of coal.

Proponents also say that fusion is safe and there is no possibility of a meltdown, as the reactor will simply shut down without external help if the process is disrupted.

It also doesn't produce harmful radioactive waste like nuclear fission plants.

ITER is a partnership of 35 countries, with seven members - the European Union, China, India, Japan, Korea, Russia, and the United States-is fabricating a significant portion of the machine.

The EU is paying 45% of the cost with China, India, Japan, Korea, Russia, and the United States each contributing  9% equally. 

Much of the cost is going back to member state countries through manufacturing contracts.