A researcher works inside the Wendelstein 7-X (W7-X), an experimental nuclear fusion reactor built in Greifswald, Germany, by the Max Planck Institute of Plasma Physics (IPP). The reactor, completed in October 2015, is the largest to date.
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.
A view around the inside corner of MIT's C-Mod experimental fusion reactor.
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.
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