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Fusion: Giant leap in pursuit of clean energy

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Staff Writer | October 22, 2016
Fusion
Science   A new world record for plasma pressure

On Sept. 30, at 9:25 p.m. EDT, scientists and engineers at MIT's Plasma Science and Fusion Center made a leap forward in the pursuit of clean energy.

The team set a new world record for plasma pressure in the Institute's Alcator C-Mod tokamak nuclear fusion reactor. Plasma pressure is the key ingredient to producing energy from nuclear fusion, and MIT's new result achieves over 2 atmospheres of pressure for the first time.

Alcator leader and senior research scientist Earl Marmar will present the results at the International Atomic Energy Agency Fusion Energy Conference, in Kyoto, Japan, on Oct. 17.

Nuclear fusion has the potential to produce nearly unlimited supplies of clean, safe, carbon-free energy. Fusion is the same process that powers the sun, and it can be realized in reactors that simulate the conditions of ultrahot miniature "stars" of plasma - superheated gas - that are contained within a magnetic field.

For over 50 years it has been known that to make fusion viable on the Earth's surface, the plasma must be very hot (more than 50 million degrees), it must be stable under intense pressure, and it must be contained in a fixed volume.

Successful fusion also requires that the product of three factors - a plasma's particle density, its confinement time, and its temperature-- reaches a certain value. Above this value (the so-called "triple product"), the energy released in a reactor exceeds the energy required to keep the reaction going.

Pressure, which is the product of density and temperature, accounts for about two-thirds of the challenge. The amount of power produced increases with the square of the pressure - so doubling the pressure leads to a fourfold increase in energy production.

During the 23 years Alcator C-Mod has been in operation at MIT, it has repeatedly advanced the record for plasma pressure in a magnetic confinement device. The previous record of 1.77 atmospheres was set in 2005 (also at Alcator C-Mod).

While setting the new record of 2.05 atmospheres, a 15 percent improvement, the temperature inside Alcator C-Mod reached over 35 million degrees Celsius, or approximately twice as hot as the center of the sun. The plasma produced 300 trillion fusion reactions per second and had a central magnetic field strength of 5.7 tesla.

It carried 1.4 million amps of electrical current and was heated with over 4 million watts of power. The reaction occurred in a volume of approximately 1 cubic meter (not much larger than a coat closet) and the plasma lasted for two full seconds.

Other fusion experiments conducted in reactors similar to Alcator have reached these temperatures, but at pressures closer to 1 atmosphere; MIT's results exceeded the next highest pressure achieved in non-Alcator devices by approximately 70 percent.

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 cease funding to Alcator due to budget pressures from the construction of ITER. Following that decision, the U.S. Congress restored funding to Alcator C-Mod for a three-year period, which ended on Sept. 30.


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