For the past 20 years, MIT's Plasma Science and Fusion Center (PSFC) has been experimenting with nuclear fusion through the world's smallest tokamak-type (doughnut-shaped) nuclear fusion device -- the Alcator C-Mod.
The goal? To produce the world's smallest fusion reactor -- one that crushes a doughnut-shaped fusion reaction into a 3.3 meter radius -- three of which could power a city the size of Boston.
And MIT researchers are getting close to their goal, despite a recent cut in federal funding that could slow their progress.
The lessons already learned from MIT's smaller Alcator C-Mod fusion device have enabled researchers, including MIT Ph.D candidate Brandon Sorbom and PSFC Director Dennis Whyte, to develop the conceptual ARC (affordable, robust, compact) reactor.
"We wanted to produce something that could produce power, but be as small as possible," Sorbom said.
A working ARC fusion reactor would use 50 megawatts (MW) of power to produce 500MW of fusion power, 200MW of which could be delivered to the grid. That's enough to provide 200,000 people with electricity.
While three other fusion devices roughly the same size as the ARC have been built over the past 35 years, they didn't produce anywhere near its power. What sets MIT's reactor apart is its superconductor technology, which would enable it to create 50 times the power it actually draws. (MIT's PSFC last year published a paper on the prototype ARC reactor in the peer reviewed journal ScienceDirect.)
The ARC reactor's powerful magnets are modular, meaning they can be easily removed and the central vacuum vessel in which the fusion reaction occurs can be replaced quickly; besides allowing upgrades, a removable vessel means a single device could be used to test many vacuum vessel designs.
Fusion reactors work by super heating hydrogen gas in a vacuum, the fusing of hydrogen atoms form helium. Just as with splitting atoms in today's fission nuclear reactors, fusion releases energy. The challenge with fusion has been confining the plasma (electrically charged gas) while heating it with microwaves to temperatures hotter than the Sun.
The result of successfully building an ARC reactor would be a plentiful source of clean and reliable power, because the needed fuel -- hydrogen isotopes -- is in unlimited supply on Earth.
"What we've done is establish the scientific basis...for, in fact, showing there's a viable pathway forward in the science of the containment of this plasma to make net fusion energy -- eventually," Whyte said.
Fusion research today is at the threshold of exploring "burning plasma," through which the heat from the fusion reaction is confined within the plasma efficiently enough for the reaction to be sustained for long periods of time.
Normally, gas such as hydrogen is made up of neutral molecules bouncing around. When you superheat a gas, however, the electrons separate from the nuclei creating a soup of charged particles rattling around at high speeds. A magnetic field can then press those charged particles into a condensed shape, forcing them to fused together.
The 40-year conundrum of fusion power is that no one has been able to create a fusion reactor that puts out more power than is required to operate it. In other words, more power is required to keep the plasma hot and generating fusion power than the fusion power it produces.
Europe's working tokamak reactor named JET, holds the world's record for power creation; it generates 16MW of fusion power but requires 24MW of electricity to operate.
MIT's researchers, however, believe they have the answer to the net power problem and it'll be available in a relatively tiny package compared to today's nuclear fission power plants. By making the reactor smaller, it also makes it less expensive to build. Additionally, the ARC would be modular, allowing its many parts to be removed for repairs to upgrades, something not previously achieved.
What sets MIT's fusion device apart
What MIT alone has done is create the world's strongest magnetic containment field for a reactor its size. The higher the magnetic field, the greater the fusion reaction and the greater the power produced.
"We're highly confident that we will be able to show this medium can make more fusion power than it takes to keep it hot," Whyte said.
Fusion reactors would have several advantages over today's fission nuclear reactors. For one, fusion reactors would produce little radioactive waste. Fusion reactors produce what are called "activation products" with the fusion neutrons.
The small amount of radioactive isotopes produced are short lived, with a half life lasting tens of years vs. thousands of years from fission waste products, Sorbom said.
The reactors would also use less energy to operate than fission reactors.
While MIT's current Alcator C-Mod produces no electricity, it demonstrates the effects of a magnetic containment field on super-heated plasma, and by hot we're talking about 100 million degrees Fahrenheit. By comparison, our Sun is a chilly 27 million degrees Fahrenheit.
Far from being dangerous, the 100-million-degree plasma instantly cools and resumes a gaseous state when it touches the inner sides of the reactor. That's why a powerful magnetic containment field is needed.
Just like a fission nuclear reactor, a fusion reactor would essentially be a steam engine. The heat from the controlled fusion reaction is used to turn a steam turbine that, in turn, drives electrical generators.
MIT's current C-Mod fusion device uses plentiful deuterium as its plasma fuel. Deuterium is a hydrogen isotope that is not radioactive and can be extracted from seawater.
In order to create a conceptual ARC reactor, however, a second hydrogen isotope is needed: tritium. That's because the rate at which deuterium-deuterium isotopes fuse is about 200 times less than the rate at which deuterium-tritium isotopes fuse.
Tritium, while radioactive, only has a half-life of about 10 years. Although tritium does not occur naturally, it can be created by bombarding lithium with neutrons. As a result, it can be easily produced as a sustainable source of fuel.
With fusion reactors, smaller is better
While MIT's reactor might not fit conveniently into Tony Stark's chest (that is a movie after all), it would be the smallest fusion reactor with the most powerful magnetic containment chamber on earth. It would produce the power of eight Teslas or about two MRI machines.
By comparison, in southern France, seven nations (including the U.S.) have collaborated to build the world's largest fusion reactor, the International Thermonuclear Experimental Reactor (ITER) Tokamak. The ITER fusion chamber has a fusion radius of 6.5 meters and its superconducting magnets would produce 11.8 Teslas of force.
However, the ITER reactor is about twice the size of ARC and weighs 3,400 tons -- 16 times as heavy as any previously manufactured fusion vessel. The D-shaped reactor will be between 11 meters and 17 meters in size and have a tokamak plasma radius of 6.2 meters, almost twice the ARC's 3.3-meter-radius.
The concept for the ITER project began in 1985, and construction began in 2013. It has an estimated price tag of between $14 billion and $20 billion. Whyte, however, believes ITER will end up being vastly more expensive, $40 billion to $50 billion, based on "the fact that the U.S. contribution" is $4 billion to $5 billion, "and we are 9% partners."
Additionally, ITER's timetable for completion is 2020, with full deuterium-tritium fusion experiments starting in 2027.
When completed, ITER is expected to be the first fusion reactor to generate net power, but that power will not produce electricity; it will simply prepare the way for a reactor that can.
MIT's ARC reactor is projected to cost $4 billion to $5 billion dollars and could be completed in a four to five years, Sorbom said.
The reason ARC could be completed sooner and at one-tenth the cost of ITER is due to its size and the use of the new high-field superconductors that operate at higher temperatures than typical superconductors.
Typically, fusion reactors use low-temperature super conductors as magnetic coils. The coils must cooled to about 4 degrees Kelvin, or minus 452 degrees Fahrenheit, to function. MIT's tokamak fusion device uses a "high-temperature" rare-earth barium copper oxide (REBCO) superconducting tape for its magnetic coils, which is far less expensive and efficient. Of course, "high temperature" is relative: the REBCO coils operate at 100 degrees Kelvin, or about minus 280 degrees Fahrenheit, but that's warm enough to use abundant liquid nitrogen as a cooling agent.
"The enabling technology to be able to shrink the fusion device size is this new superconducting technology," Sorbom said. "While the [REBCO] superconductors have been around since the late 1980s in labs, in the last five years or so companies have been commercializing this stuff into tapes for large scale projects like this."
In addition to size and cost, REBCO tape is also able to increase fusion power 10-fold compared to standard superconducting technology.
Before MIT's ARC can be built, however, researchers must first prove they can sustain a fusion reaction. Currently, MIT's C-Mod reactor runs only a few seconds each time it's fired up. In fact, it requires so much power, that MIT must use a buffer transformer in order store enough electricity to run it without browning out the city of Cambridge. And, with a plasma radius of just 0.68 meter, C-Mod has is far smaller than even the ARC reactor would
So before it builds the ARC reactor, MIT's next fusion device -- the Advanced Divertor and RF tokamak eXperiment (ADX) -- will test various means to effectively handle the Sun-like temperatures without degrading the plasma performance.
After achieving sustainable performance, the ARC will determine whether net power generation is possible. The last hurdle before fusion reactors can supply power to the grid is transferring the heat to a generator.
Feds cut funding
MIT's C-Mod tokamak reactor is one of the three major fusion research facilities in the U.S., along with DIII-D at General Atomics and the National Spherical Torus Experiment Upgrade (NSTX-U) at the Princeton Plasma Physics Laboratory.
Throwing a wrench into its efforts, MIT learned earlier this year that funding for its fusion reactor under the Department of Energy (DOE) is coming to an end. The decision to shut down Alcator C-Mod was driven by budget constraints, according to Edmund Synakowski, associate director of science for Fusion Energy Sciences (FES) at the DOE.
In the current budget, Congress has provided $18 million for MIT's C-Mod, which will support at least five weeks of operations in its final year and cover the costs associated with the shutdown of the facility, Synakowski said in an email reply to Computerworld. (Researchers hope to find other funding sources to make up for the loss.)
The PSFC has about 50 Ph.D students working to develop fusion energy. Past students have left MIT to start their own companies or take develop academic projects outside of MIT.
Making sure that scientists and students at MIT can transition into collaborations at other DOE-funded fusion energy research facilities in the U.S. -- especially the two primary facilities: DIII-D at General Atomics in San Diego, and NSTX-U at Princeton Plasma Physics Laboratory -- has been "one of the major concerns," Synakowski said.
Over the past fiscal year, FES worked with MIT to establish a new five-year cooperative agreement, beginning on Sept. 1, 2015, to enable its scientists to transition to FES-funded collaborations.
Whyte, however, believes the promise of fusion energy is too important for research to wind down.
"Fusion is too important to have only one pathway to it," Whyte said. "My motto is smaller and sooner. If we can [create] the technology that allows us to access smaller devices and build a variety of them..., then this allows us to get to a place where we've got more options on the table to develop fusion on a faster timescale."
And, Whyte said, the scientific basis for small fusion reactors has been established at MIT.
"We did that despite the fact that we have the smallest of the major experiments around the world. We actually have the record for achieving pressure of this plasma. Pressure is one of the fundamental bars you have to get over," Whyte said. "We're very excited about this."