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$24 billion nuclear fusion experiment that could lead to unlimited energy is now 50% complete

The International Thermonuclear Experimental Reactor, or ITER, is being built in southern France
It has been dogged by delays and a surge in costs to about 20 billion euros ($23.7 billion)
It is designed to show a fusion reactor can produce more energy than it consumes
The project is now on track to begin superheating hydrogen atoms in 2025 - known as 'first plasma'

A vast international experiment designed to demonstrate that nuclear fusion can be a viable source of energy is halfway toward completion, the organization behind the project said Wednesday.

Construction of the International Thermonuclear Experimental Reactor, or ITER, in southern France has been dogged by delays and a surge in costs to about 20 billion euros ($23.7 billion).

ITER's director-general, Bernard Bigot, said the project is on track to begin superheating hydrogen atoms in 2025, a milestone known as 'first plasma.'

In the middle of the rising Tokamak Building a well is preserved for the ITER machine. While ITER won't generate electricity, scientists hope it will demonstrate that such a fusion reactor can produce more energy than it consumes. Assembly activities will proceed in a bottom-up fashion, beginning with captive components down in the basement levels, then the base of the cryostat, vacuum vessel sectors, magnets and an estimated one million components (ten million individual parts) will be integrated into the world's largest tokamak.

FUSION POWER EXPLAINED

Fusion involves placing hydrogen atoms under high heat and pressure until they fuse into helium atoms.

When deuterium and tritium nuclei - which can be found in hydrogen - fuse, they form a helium nucleus, a neutron and a lot of energy.

This is down by heating the fuel to temperatures in excess of 150 million°C, forming a hot plasma.

Strong magnetic fields are used to keep the plasma away from the walls so that it doesn't cool down and lose it energy potential.

These are produced by superconducting coils surrounding the vessel, and by an electrical current driven through the plasma.

For energy production. plasma has to be confined for a sufficiently long period for fusion to occur.'We have no contingency plan,' he told The Associated Press in a phone interview from Paris.

ITER is the most complex science project in human history.

The hydrogen plasma will be heated to 150 million degrees Celsius, ten times hotter than the core of the Sun, to enable the fusion reaction.

The process happens in a donut-shaped reactor, called a tokamak,1 which is surrounded by giant magnets that confine and circulate the superheated, ionized plasma, away from the metal walls.

The superconducting magnets must be cooled to minus 269°C, as cold as interstellar space

Scientists have long sought to mimic the process of nuclear fusion that occurs inside the sun, arguing that it could provide an almost limitless source of cheap, safe and clean electricity.

Unlike in existing fission reactors, which split plutonium or uranium atoms, there's no risk of an uncontrolled chain reaction with fusion and it doesn't produce long-lived radioactive waste.

A joint project to explore the technology was first proposed at a summit between U.S. President Ronald Reagan and Soviet leader Mikhail Gorbachev in 1985, with the aim of 'utilizing controlled thermonuclear fusion for peaceful purposes ... for the benefit for all mankind.'

It took more than two decades for work to begin at the site in Saint-Paul-les-Durance, about 50 kilometers (30 miles) northeast of Marseille.

The project's members - China, the European Union, India, Japan, South Korea, Russia and the United States - settled on a design that uses a doughnut-shaped device called a tokamak to trap hydrogen that's been heated to 150 million degrees Celsius (270 million Fahrenheit) for long enough to allow atoms to fuse together.

Cranes stand at the construction site of the ITER ( the International Thermonuclear Experimental Reactor) in Cadarache, southern France.

Only two levels remain to be poured before the bioshield is complete. Each opening in the circular wall (corresponding to a similar opening in the cryostat and the vacuum vessel) will provide access for systems and equipment.

The process results in the release of large amounts of heat.

While ITER won't generate electricity, scientists hope it will demonstrate that such a fusion reactor can produce more energy than it consumes.

There are other fusion experiments, but ITER's design is widely considered the most advanced and practical. Scientists won't know until 2035, following a decade of testing and upgrades, whether the device actually works as intended.

INSIDE THE ITER: HOW IT CREATES ENERGY

ITER uses a strong electric current to trap plasma inside a doughnut-shaped device long enough for fusion to take place.

The device, known as a tokamak, was conceived by Soviet physicists in the 1950s. But it's proving tough to build, and could be even tougher to operate.

Iter nuclear engineers have recruited rocket scientists to help create super-strong materials that can withstand temperatures hotter than the sun.

The Iter team claim a technique for building launcher and satellite components has turned out to be the best way for constructing rings to support the powerful magnetic coils inside the machine.

The Tokamak and its plant systems housed in their concrete home. An estimated one million parts will be assembled in the machine alone. Image format: 72dpi - 4500 px width.

Spanish company CASA Espacio is making the rings using a method they have perfected over two decades of building elements for the Ariane 5, Vega and Soyuz rockets.

'Forces inside ITER present similar challenges to space,' explains Jose Guillamon, Head of Commercial and Strategy.

'We can't use traditional materials like metal, which expand and contract with temperature and conduct electricity.

'We have to make a special composite material which is durable and lightweight, non-conductive and never changes shape.'

The magnets themselves are massive. Engineering & Technology reports that the one currently being built is 45 feet long, 30 feet wide, and 3 feet deep.

Iter nuclear engineers have recruited rocket scientists to help create super-strong materials that can withstand temperatures hotter than the sun. With a diameter of 5 m and a solid cross-section of 30x30 cm, Iter's compression rings will hold the giant magnets in place.

The final design will use 18 of these magnets, each weigh between 113,400kg and 226,800kg (250,000 and 500,000lbs)—which is about the same as a Boeing 747 airplane.

CASA Espacio has been at the forefront of developing a technique for embedding carbon fibres in resin to create a strong, lightweight material to hold these magnets.

The composite is ideal for rocket parts because it retains its shape and offers the robust longevity needed to survive extreme launches and the harsh environment of space for over 15 years.Still, fusion experts said Wednesday's milestone was noteworthy.

'The glass is half full, rather than half empty,' said Tony Donne of EUROfusion, a consortium of European research organizations and universities that provide scientific advice for ITER.

Donne said the appointment of Bigot had helped the project overcome what he called a 'very difficult period' during which political considerations had hampered construction of what some consider the most complicated machine ever built.

Cost remains an issue, though, and Bigot was visiting Washington on Wednesday to drum up support from the United States, which contributes about 9 percent of the budget.

Much of the funding goes to suppliers in the member states - in the case of the U.S. that includes General Atomics, which is building the central solenoid, an 18-meter (59-foot) electromagnet that's powerful enough to lift an aircraft carrier.

FILE - In this Sept. 15, 2016 file photo, cranes stand at the construction site of the ITER ( the International Thermonuclear Experimental Reactor) in Cadarache, southern France. A vast international experiment designed to demonstrate that nuclear fusion can be a viable source of clean and cheap energy is halfway toward completion. The organization behind the ITER announced the milestone Wednesday Dec. 6, 2017 and confirmed it's aiming to conduct a first test run in 2025. (AP Photo/Claude Paris, File)

Now under construction, Iter's rings will each withstand 7,000 tonnes – the equivalent of the Eiffel Tower pressing against each one of the six rings. Carbon fibres are woven like fabric and embedded in a resin matrix to create a lightweight, durable and stable composite

Bigot said most other members, including the European Union which pays 45 percent of the budget, had pledged their financial support for years to come and he was hopeful the Trump administration would see the benefits of staying on board.

'All countries including the United States know that their energy supply is not sustainable beyond this century,' said Bigot, who was previously France's nuclear energy chief.

Should Washington cut its funding, the project won't collapse, he said. 'It's too important for the other members. But there would be some delay.'

Gerald Navratil, a professor of applied physics at Columbia University, said fusion could help solve the problem of how to reliably produce large amounts of electricity without emitting greenhouse gases, noting ITER's current cost is comparable to that of developing a large passenger aircraft.

'Energy is such an important part of our technological society that expenditure of 20 billion to develop a new energy source is really not out of line,' he s