Fusion power can seem a bit like the last bus at night; it's always coming, but never arrives. MIT is working to change that with a new compact tokamak fusion reactor design based on the latest commercially available magnetic superconductor technology. The ARC (affordable, robust, compact) reactor design promises smaller, cheaper reactors that could make fusion power practical within 10 years.

A commercially viable fusion reactor has been the Holy Grail of engineering since the 1950s, with the potential to turn almost all other major electricity sources into an historical footnote overnight. If perfected, it would essentially be an inexhaustible source of power, impacting on almost every aspect of life, from the environment to global politics. The trick is making it practical.

Put simply, fusion involves placing hydrogen atoms under very high heat and pressure until they fuse into helium atoms, which releases tremendous amounts of energy. The Sun does this as a matter of course, but reproducing those conditions on Earth outside of a hydrogen bomb has proven difficult.

There are a number of fusion reactor designs, but one of the most promising is the tokamak reactor, which is a hollow metal chamber shaped like a donut twisted into a figure eight. Inside the chamber is a vacuum into which hydrogen isotopes deuterium and tritium are introduced. These are superheated to the temperature of the Sun's interior forming a plasma that is contained and compressed by powerful magnetic fields. The magnetic coils responsible for producing these magnetic fields are key to the whole process and the biggest bottleneck to progress.

An international consortium, including scientists from the European Union, India, Japan, China, Russia, South Korea, and the United States, is planing to build the world's most powerful fusion reactor based on a tokamak. Work began on the International Thermonuclear Experimental Reactor (ITER) in 1985, and at an estimated cost of US$40 billion, it isn't slated to start full operations until 2027. Even then, it will be on a purely experimental basis.

MIT PhD candidate Brandon Sorbom holds REBCO superconducting tapes (left), which are the enabling technology behind the ARC reactor(Credit: MIT ARC team)

MIT's ARC reactor is an example of how a single change can completely alter the design of a system. It uses new commercially available superconductors made of rare-earth barium copper oxide (REBCO) superconducting tapes that are capable of producing high-magnetic field coils. The stronger magnetic fields generated by these coils do a better job of confining superhot plasma, so the reactor can be smaller, cheaper and take less time to build.

Intended for basic fusion power research, the ARC reactor is based on the same physics as ITER, though the team also describes it as a potential prototype plant that could generate significant amounts of power. According to MIT, the equations governing reactor design show that power output increases to the fourth power of the increase in the magnetic field. In other words, double the strength of the field and the power goes up 16 fold. The new superconductors being used by MIT should increase fusion power by a factor of 10 over standard superconducting technology, with knock-on effects for reactor design.

With a major radius of 3.3 m (10.8 ft) and a minor radius of 1.1 m (3.6 ft), the ARC is a 500 MW reactor that is half the diameter of ITER, but will boast a similar power output. Also, the new superconducting magnets will allow for a steady power output, while today's experimental reactors can only operate for a few seconds at a time before their copper coils overheat.

MIT has also designed the ARC reactor so that the fusion power core can be removed without needing to dismantle the reactor, which is a big plus for a research reactor. In addition, the solid cladding normally wrapped around the fusion chamber has been replaced with a circulating liquid. This eliminates the need to replace the cladding as it degrades, since the liquid can simply be replaced.

The researchers say the current design could generate three times more energy than is fed into it to keep it running, but they are hopeful of boosting this to five or six times in the future. Since no current fusion reactor can maintain even a sustained break even point, this would be a major breakthrough. The team says reactors like the ARC have been built in about five years, and that their design could generate electricity for about 100,000 people.

The team's results were published in Fusion Engineering and Design.

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