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MIT team proposes ARC fusion reactor: affordable, robust, compact

Advances in magnet technology have enabled researchers at MIT to propose a new design for a practical compact tokamak fusion reactor that might be realized in as little as a decade: the ARC (affordable, robust, compact) reactor. The stronger magnetic field makes it possible to produce the required magnetic confinement of the superhot plasma—the working material of a fusion reaction—but in a much smaller device than those previously envisioned.

The reduction in size, in turn, makes the whole system less expensive and faster to build, and also allows for some ingenious new features in the power plant design. The proposed tokamak (donut-shaped) reactor is designed to have 500 MW fusion power at 3.3 m major radius and is described in a paper in the journal Fusion Engineering and Design.

The compact, simplified design allowed by high magnetic fields and jointed magnets, enabled by the use of high temperature superconductors. A liquid immersion blanket and jointed magnets greatly simplify the tokamak reactor design.

The ARC reactor, as described in Sorbom et al.

ARC is a ∼200–250 MWe tokamak reactor with a major radius of 3.3 m, a minor radius of 1.1 m, and an on-axis magnetic field of 9.2 T [tesla, a measure of magnetic field strength]. ARC has rare earth barium copper oxide (REBCO) superconducting toroidal field coils, which have joints to enable disassembly. This allows the vacuum vessel to be replaced quickly, mitigating first wall survivability concerns, and permits a single device to test many vacuum vessel designs and divertor materials. The design point has a plasma fusion gain of Qp ≈ 13.6, yet is fully non-inductive, with a modest bootstrap fraction of only ∼63%. Thus ARC offers a high power gain with relatively large external control of the current profile.

32 T superconductor magnet
Researchers at the National High Magnetic Field Laboratory are designing a 32 T superconducting magnet, due for completion in 2016. At 32 tesla, it will be ~8.5 tesla stronger than the current record.
In June 2015, a test for the 32 tesla magnet set a new world record of 27 tesla for an all-superconducting magnet.
Begun in 2009, the project represents a breakthrough in superconducting magnet technology on many fronts. Among other innovations, it combines low-temperature superconductors commonly used in today’s superconducting magnets—niobium tin and niobium titanium—with “YBCO,” a superconducting ceramic composed of yttrium, barium, copper and oxygen.
The finished, 2.3-ton magnet system will feature about 6 miles of YBCO tape, formed into 112 disc-shaped pancakes. Two inner coils of YBCO, fabricated at the MagLab will be surrounded by a commercial outsert consisting of three coils of niobium-tin and two coils of niobium-titanium.

This highly attractive combination is enabled by the ∼23 T peak field on coil achievable with newly available REBCO superconductor technology. External current drive is provided by two innovative inboard RF launchers using 25 MW of lower hybrid and 13.6 MW of ion cyclotron fast wave power. The resulting efficient current drive provides a robust, steady state core plasma far from disruptive limits. ARC uses an all-liquid blanket, consisting of low pressure, slowly flowing fluorine lithium beryllium (FLiBe) molten salt. The liquid blanket is low-risk technology and provides effective neutron moderation and shielding, excellent heat removal, and a tritium breeding ratio ≥ 1.1. The large temperature range over which FLiBe is liquid permits an output blanket temperature of 900 K, single phase fluid cooling, and a high efficiency helium Brayton cycle, which allows for net electricity generation when operating ARC as a Pilot power plant.

—Sorbom et al.

The new reactor is designed for basic research on fusion and also as a potential prototype power plant that could produce significant power. The basic reactor concept and its associated elements are based on well-tested and proven principles developed over decades of research at MIT and around the world, the team says.

Fusion, the nuclear reaction that powers the sun, involves fusing pairs of hydrogen atoms together to form helium, accompanied by enormous releases of energy. The hard part has been confining the superhot plasma while heating it to temperatures hotter than the cores of stars. The magnetic fields effectively trap the heat and particles in the hot center of the device.

While most characteristics of a system tend to vary in proportion to changes in dimensions, the effect of changes in the magnetic field on fusion reactions is much more extreme: The achievable fusion power increases according to the fourth power of the increase in the magnetic field. Thus, doubling the field would produce a 16-fold increase in the fusion power.

While the new REBCO superconductors do not produce quite a doubling of the field strength, they are strong enough to increase fusion power by about a factor of 10 compared to standard superconducting technology, said PhD candidate Brandon Sorbom, the lead author of the paper. This significant improvement leads to a cascade of potential improvements in reactor design.

ITER, the world’s most powerful planned fusion reactor currently under construction in France, is expected to cost around $40 billion. Sorbom and the MIT team estimate that the new design, about half the diameter of ITER (which was designed before the new superconductors became available), would produce about the same power at a fraction of the cost and in a shorter construction time.

But despite the difference in size and magnetic field strength, the proposed reactor, called ARC, is based on the same physics as ITER, said Dennis Whyte, a professor of Nuclear Science and Engineering and director of MIT’s Plasma Science and Fusion Center.

Another key advance in the new design is a method for removing the the fusion power core from the donut-shaped reactor without having to dismantle the entire device. That makes it especially well-suited for research aimed at further improving the system by using different materials or designs to fine-tune the performance.

In addition, as with ITER, the new superconducting magnets would enable the reactor to operate in a sustained way, producing a steady power output, unlike today’s experimental reactors that can only operate for a few seconds at a time without overheating of copper coils.

Another key advantage is that most of the solid blanket materials used to surround the fusion chamber in such reactors are replaced by a liquid material that can easily be circulated and replaced, eliminating the need for costly replacement procedures as the materials degrade over time.

As currently designed, the reactor should be capable of producing about three times as much electricity as is needed to keep it running, but the design could probably be improved to increase that proportion to about five or six times, Sorbom said. So far, no fusion reactor has produced as much energy as it consumes, so this kind of net energy production would be a major breakthrough in fusion technology, the team said.

The research was supported by the US Department of Energy and the National Science Foundation.


  • B.N. Sorbom, J. Ball, T.R. Palmer, F.J. Mangiarotti, J.M. Sierchio, P. Bonoli, C. Kasten, D.A. Sutherland, H.S. Barnard, C.B. Haakonsen, J. Goh, C. Sung, D.G. Whyte (2015) “ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets,” Fusion Engineering and Design doi: 10.1016/j.fusengdes.2015.07.008



ARC Reactor? Somebody is a fan of Iron Man...

Bob Wallace

We're making progress. Fusion has gone from "twenty years away and will always be twenty years away" to "ten years away and ?".


If small transportable units can be mass produced (by x 1,000 in factory) and installed in every town and city sector, most high voltage transmission lines could be elimnated.

Eventually, current large CPPs, NGPPs, NPPs could be replaced with fusion energy amplifiers.

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Controlling fusion reaction is the holy grail of all power generation. We need it. We need it to propagate mankind to other planets than planet earth and we need it to power space stations and spaceships far away from our sun. Glad to read that progress is made in this area although I will not stay up late at night waiting for this to become commercialized anytime soon. It is as complicated as it gets and we probably need 50 years rather than 10 years. Hell making a new car design from an existing design takes 6 years so this is not 10 years from commercialization.


How do you get 500 MW out of a 3.3 m wide reactor ?

This might give us electricity too cheap to meter.
Now where have I heard that before....

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It is fusion reactor! It is a controlled continuous hydrogen bomb detonation. Its circular magnetically controlled core burns hydrogen at over 100 million degrees Celsius! You do not need a huge reactor to make 500mW for that obviously. When we can make even stronger magnetic fields. Say magnetic fields at 50 tesla instead of 32 the reactor can be made even smaller and still do 500 MW. For spaceships we need really small reactors that could power seriously powerful ion engines for propulsion just like the solar observatory satellite that Space X launched a few month back and that now is sending breathtaking pictures of the earth back apart from monitoring solar activity of cause.


I don't think you need 500MW for a solar monitoring satellite, and it already has a thermonuclear reactor for power, the sun.

Deep space travel makes sense.


I accept that the Reactor can generate 500MW in a very small volume. My question is how do you get the energy out of that small space and turn it into electricity that can go onto the grid.

Steve Reynolds

"...how do you get the energy out of that small space and turn it into electricity that can go onto the grid."
Liquid Li2BeF4 salt at 900K (with heat capacity similar to water) carries the heat to where it is used to heat He gas.
The He gas drives a conventional gas turbine.



I believe that it uses a liquid salt bath (fluorine lithium beryllium (FLiBe)) to absorb and carry away the heat. The hot salt will then be used to heat helium to run a Brayton cycle (closed cycle gas turbine) power plant. There is a little detail in the caption for the drawing.

Anyway, I hope they get the funding to keep working on this project as this is the most hopeful thing that I have seen on Fusion. I would also like to see more work on traveling wave fission reactors that have the possibility of consuming the current nuclear waste and burning mostly U238.


So no CO2. Great. But are we going to stop heating the planet by brining 1000 tiny suns to the surface? I don't get it. Now will will have to use our meager water supplies to cool the liquid salts. I am no scientist so maybe I am missing something, but this approach raises questions for which I have not seen answers.


Most of you have no appreciation for what is happening in controlled Fusion research. Ever since the ITER project was given a multinational budget and marching orders, progress slowly accelerated. ITER has been surpassed in terms of scientific research and new ideas in technology have come from the engineers.

No longer is Fusion research merely small groups of Physicists happy to explore some new plasma instability. As the several orders or magnitude improvements in containment occurred each revealing a maddening new instability to be encountered, understood and controlled, we have finally emerged from that as ALL the instabilities were understood and countered.

ITER often described as the last Fusion Experiment and the First Engineering effort to construct a controlled Fusion Reactor. Ity is now forcing the Physicists to move over as the Engineers take over and have to design heretofore unfaced construction and operations considerations on the gargantuan ITER reactor.

Methods of producing miles of superconducting cables have forced manufacturers to multiply world wide production several times over.

Demand for both new materiels such as these ERBE superconductors, and non-activated materials and steels produced in quantity has forced manufacturers to develop factories to produce them.

Control electronics and their processing speeds have been pushed to ever faster speeds to control the plasma instabilities.

Software to drive the ITER controllers had to be designed built and debugged.

All this, plus big budgets, forced the experimental Scientists aside, forever willing to "study" a problem to death and practical Engineers seeking to construct a useable device have been pushed forward. ITER was and is much like the Apollo project forcing the technology.

Even though ITER won't see "First Plasma" until 1919, and will still be a testbed a decade later, every cognizant scientist and engineer in the field knows that ITER is obsolete.

A follow on design labled DEMO and begun as early as 2017 could undertake to design and build the first DEMOnstration, commercial Fusion Power station to add electricity to the Grid. It is no longer 50 years off.

While you all have been talking of redesigning 16th century wind mills, the solution to the Energy problems of Mankind is virtually here, now.

This advance in Superconducting materials would not have happened as quickly without ITER even though ITER won't have a foot of these new superconducting cables. The "non-inductive" energy inputs herein discussed come as a result of ITER work to make the ITER reactor near steady state operational. As do the design for these rugged massive antennas to beam energy into the Plasma.

In a much shorter time than the so-called "Energy Crisis" began in the 70s, all Mankind will have prodigious amounts of cleanly and inexhaustively generated electricity for its factories, cars and homes.


Holy Bat-Crap, Batman! I really thought we were 50-100 years away. If it really is down to 10-20 years, that's incredible.


The research to create fusion is like the cure for cancer. It keeps the researchers employed and there's a whole lot of hope and promises; but, not a lot of success.

When will the vapor turn into something useful?

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You do not need to use freshwater to cool a power plant. However, freshwater is widely used instead of air cooling because it is much cheaper and more compact and you save electricity for pumping cooling liquids. You can cool a power plant by air but the cooling unit will need to be a massive structure with cooling rips to do the job for say a 500Mw reactor.

Fusion energy also does not produce a lot of highly radioactive waste (like all fission reactors) that is impossible to get rid of or store safely for the tens of thousands of years it needs to be contained and protected (so no terrorist get to it either).

The solar monitoring satellite in mind uses less than 500 W to operate its ion engine at max capacity. The electricity is made by the satellite's solar panels. I was thinking of deep space travel and travel in our own solar system. Say transporting raw materials to earth and other giant space stations where people can live and work. We need fusion energy to colonize our solar system and beyond. And electrical ion rocket engines are going to replace chemical rocket engines as they only need a fraction of the fuel used in the chemical rocket engine.

Don't get too excited. It is still more than 50 years off before fusion energy will have any effect on electricity production on earth. It is not enough to make a working reactor. It needs to compete with making electricity by solar power and wind turbines before it matters. So you need decades of improvements before it makes sense. I think we need magnetic fields at 100 tesla and beyond.


J Martin:

The amount of heat required to run our civilisation is trivial compared to that from the sun, an hours worth of solar for a year's supply, or whatever the exact figure is.

So the output of reactors is commensurately small.

It is when greenhouse gases are trapping heat that things get sticky.

This would be hundreds or thousands of time less warming than present use of fossil fuel.


Thank you for the clarification, Henrik and Davemart. I can now get on board more enthusiastically.

Bob Wallace

"If small transportable units can be mass produced (by x 1,000 in factory) and installed in every town and city sector, most high voltage transmission lines could be elimnated."

The problem with the economics of small modular reactors is that in order to reach economies of scale you have to have enough customers who will pay up front to create manufacturing volume to bring the price down from day one.

If you don't have those pre-purchasers then you have to start building and bring the cost down over time. And where do you find a few hundred customers willing to pay very high prices for the early production? Where are the people who bought the $2,000 cell phones and even more expensive first laptops?

Plus, few cities will tolerate a reactor of any size close to them. There's no market for 'thousands'.


Those small transportable 1/10 size reactors would have to be built by the (10X) 1,000s to replace current CPPs, NGPPs and large older NPPs.

An international effort could share building cost. Components could be manufactures in low labour cost countries with final assembly close to end users.

Many towns/cities may prefer them to huge smelly garbage piles/mountains. It could use the same land area once you have destroyed the awful garbage.


Saying the reactor is 3.3M across is a bit like saying the cylinder in the car engine displaces 1.7 Liters. It's true, but the total system is a much bigger. Heat exchangers, turbines, and lots of other system elements still make this large, not portable, and plenty pricey.

I've been waiting for this breakthrough most of my life, and getting net energy out is really, really hard. Getting cost-competitive energy is much, much harder. Lockheed has a compact fusion project (http://www.lockheedmartin.com/us/products/compact-fusion.html).
EMC2 has done a number of contracts for the Navy on compact fusion (http://www.emc2fusion.org/). Everybody has a story for why this time it's different. I support almost every one of these avenues being pursued. However, I do not get my hopes up because, you know, it's really hard.


No tritium based reactor like this one is feasible at those power densities.

The neutron loading in the center is way too high for this to ever consequently be affordable.

Never ever going to happen. Period.


Yeah, I know. But it's really fun to paraphrase Robin and you don't get many chances like this LOL


What about the worst case scenario, i.e. what does it look like when such a reactor goes up in smoke?
Not much is needed for it to happen, you have over 10 million degrees Celsius inside, if some cooling system fails (mechanically or electrically), you get almost immediately a huge explosion.

1. How much radiation would it spread around?
2. What is the power of the blast (in TNT tonnes equivalent)

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Good you brought this topic up because it shows another advantage of fusion reaction over fission reaction (the conventional nuclear power plants that burns (splits) uranium or plutonium) and that produces a large amount of extremely radioactive waste.

The fusion reaction is kept alive by injecting microscopic frozen pellets of ultra heavy hydrogen atoms into its circular plasma core that subsequently ignites etc. If this injection process is interrupted or stopped the reactor stops. In other words, a fusion reactor can be turned on and off like a light ball. You start the reactor by hitting the first hydrogen pellet injected by multiple high energy laser beams so that it reaches its critical 10 to 100 million degrees Celsius temperature and starts burning by itself converting the hydrogen into helium. You stop it by stop feeding the plasma core with new hydrogen pellets. Each pellet can keep the reactor operating for a few microseconds or seconds (or something in that order). The only explosion I can think of happening in a fusion reactor is if too much hydrogen is added to the core at a given time span so that the plasma core expands and melts the inner walls of the reactor. That might expose the cooling liquids that vaporizes and causes an explosion. In that case the reactor stops or is stopped and it is ruined. It does not explode in a big way. If the plasma hit the inner walls of the reactor it will cool down below is burning temperature which is 10 to 100 million degrees Celsius and then the fusion process stops 100%. There is very little plasma by weight probably less than 1/1000 of a gram. So even though it is incredibly hot in a tiny string inside the reactor core it does not contain enough energy for a massive explosion. The only radioactive material in the fusion reactor will be the material the inner walls are made of as they are exposed to radiation from the burning plasma core all the time and will therefore also become more radioactive over time. That radiation is only a tiny fraction of the radioactive material and its radiation level that an old school fission reactor produces. I am not an expert but I believe it is millions of times less radioactive than a typical fission reactor. The only time when that radioactive material needs to be removed is for decommission of the fusion reactor or perhaps also for maintenance once every say 10 years.

Fusion reaction is the holy grail of energy production and if you ask me we should spend many times more on developing it. Make it a global research project with a 25 billion USD annual budget and run it for as many years as it takes to demonstrate a working reactor that makes electricity at 3 cents per kwh. And then make that technology available for any nation to exploit. Until that happens solar and wind power is a practical and affordable replacement for fossil fuel, bio fuels and old school nuclear fission power.


I'm surprised they didn't call it the Mr Fusion® reactor.

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