A fusion reactor at MIT that has repeatedly set world records over its 23-year lifespan broke one more the day before it was shuttered due to defunding by the federal government.
MIT’s Plasma Science and Fusion Center (PSFC) announced it achieved 2.05 atmospheres of pressure in its tokamak-type (doughnut-shaped) nuclear fusion device — the Alcator C-Mod, or ARC. Pressure, which contains and concentrates superheated plasma, is key to creating a fusion device that is self-sustaining.
With this result, the Alcator C-Mod broke its own record of 1.77 atmospheres, which was set in 2005.
“This is a remarkable achievement that highlights the highly successful Alcator C-Mod program at MIT,” Dale Meade, former deputy director at the Princeton Plasma Physics Laboratory, who was not directly involved in the experiments, told MIT News. “The record plasma pressure validates the high-magnetic-field approach as an attractive path to practical fusion energy.”
What MIT alone has done is create the world’s strongest magnetic containment field for a reactor of its size. The higher the magnetic field, the greater the fusion reaction and the greater the power that’s produced.
MIT’s fusion reactor is the smallest tokamak-type device. Computerworld toured the facility and spoke with PSFC Director Dennis Whyte, who explained how the experiments were paving a “viable pathway forward” in plasma containment to make net fusion energy.
The C-Mod’s high-intensity magnetic field — up to 8 Tesla, or 160,000 times the Earth’s magnetic field — allows the device to create the dense, hot plasmas and keep them stable at more than 80 million degrees, according to MIT. Its magnetic field is more than double what is typically used in other designs, which quadruples its ability to contain the plasma pressure.
The goal of the Alcator C-Mod, also known as ARC (for affordable, robust, compact reactor), was to pave the way toward producing 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.
Making a smaller reactor would also have made it less expensive to build. Additionally, the ARC would be modular, allowing its many parts to be removed for repairs to upgrades, a step not previously achieved.
Currently, the main hurdle to fusion energy is sustainability. The fusion reaction is only momentary and requires more energy to create than it generates.
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 set MIT’s reactor apart was its superconductor technology, which would enable it to create 50 times the power it actually draws.
Unless a new device is announced and constructed, the pressure record just set in the C-Mod will likely stand for the next 15 years. The International Experimental Reactor (ITER), a tokamak under construction in France, will be approximately 800 times larger in volume than the Alcator C-Mod and will operate at a lower magnetic field. The ITER is expected to reach 2.6 atmospheres when in full operation by 2032, according to a recent U.S. Department of Energy report.
Fusion reactors work by superheating hydrogen gas in a vacuum, essentially fusing hydrogen atoms to 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.
Fusion reactors would have several advantages over today’s fission nuclear reactors. For one, fusion reactors would produce little radioactive waste. Instead, 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 compared with thousands of years from fission waste products, according to Brandon Sorbom, an MIT Ph.D candidate and member of the PSFC team.
The reactors would also use less energy to operate than fission reactors.
Three factors are required to successfully create fusion: a plasma’s particle density, its confinement time and its temperature.
Pressure, which is the product of density and temperature, accounts for about two-thirds of the challenge, MIT stated.
“The amount of power produced increases with the square of the pressure — so doubling the pressure leads to a fourfold increase in energy production,” the institute stated.
The latest experiments were planned by the MIT team and collaborators from other laboratories in the U.S., including the Princeton Plasma Physics Laboratory, the Oak Ridge National Laboratory and General Atomics, and conducted on the Alcator C-Mod’s last day of operation.
While Alcator C-Mod’s contributions to the advancement of fusion energy have been significant, it is a science research facility. In 2012, the DOE decided to end funding for the Alcator and use that money toward the U.S.’s share for the construction of the ITER Tokamak in France. Following that decision, Congress restored funding to the Alcator C-Mod for a three-year period, which ended on Sept. 30.
Seven nations, including the U.S., have collaborated to build the ITER Tokamak in southern France. The ITER fusion chamber has a fusion radius of 6.5 meters, and its superconducting magnets would produce 11.8 Teslas of force.
The ITER reactor is about twice the size of the ARC and weighs 3,400 tons, making it 16 times as heavy as any previously manufactured fusion vessel. The reactor will be between 11 meters and 17 meters in size and will 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 $14 billion to $20 billion. Whyte, however, said the 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.
MIT’s ARC reactor was projected to cost $4 billion to $5 billion and would have been completed in four to five years, according to Sorbom.
When completed, the 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.
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