29th October 2016

New world record for fusion reactor

Massachusetts Institute of Technology has announced a new record for plasma pressure in an Alcator C-Mod tokamak nuclear fusion reactor – achieving over two atmospheres of pressure for the first time.



Credit: Bob Mumgaard/Plasma Science and Fusion Center

Scientists and engineers from the Plasma Science and Fusion Center at the Massachusetts Institute of Technology (MIT) have made a leap forward in the pursuit of clean energy. They report a new record for plasma pressure in the Institute's Alcator C-Mod tokamak nuclear fusion reactor, pictured above. Plasma pressure is the key ingredient to producing energy from nuclear fusion, and MIT's new result achieves over two atmospheres of pressure for the first time. Senior researcher, Earl Marmar, presented the results at the IAEA Fusion Energy Conference, in Kyoto, Japan, which ran from 17–22 October.

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 realised in reactors that simulate the conditions of ultrahot miniature "stars" of plasma – superheated gas – that are contained within a magnetic field.

For over half a century it has been known that to make fusion viable on Earth's surface, the plasma must be very hot (more than 50 million degrees), must be stable under intense pressure, and contained in a fixed volume. Successful fusion also requires that the product of three factors – a plasma's particle density, confinement time, and temperature – reaches a certain value. Above this value (the so-called "triple product"), the energy released from 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, it has repeatedly advanced the record for plasma pressure in a magnetic confinement device. The previous record of 1.77 atmospheres was in 2005 (also at Alcator C-Mod). While setting the new record of 2.05 atmospheres, a 15% improvement, the temperature inside Alcator C-Mod reached over 35 million degrees Celsius, or twice as hot as the centre 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 metre (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 before, but at pressures closer to 1 atmosphere; MIT's result exceeded the next highest pressure achieved in non-Alcator devices by 70 percent.

"This is a remarkable achievement that highlights the highly successful Alcator C-Mod program at MIT," says Dale Meade, former deputy director at the Princeton Plasma Physics Laboratory, who was not directly involved in the experiments. "The record plasma pressure validates the high-magnetic-field approach as an attractive path to practical fusion energy."

"This result confirms that the high pressures required for burning plasma can be best achieved with high-magnetic-field tokamaks such as Alcator C-Mod," says Riccardo Betti, Professor of Mechanical Engineering and Physics and Astronomy at the University of Rochester.

Alcator C-Mod is the world's only compact, high-magnetic-field fusion reactor with advanced shaping in a design called a tokamak, which confines the superheated plasma in a doughnut-shaped chamber. Its high-intensity magnetic field – up to eight tesla, or 160,000 times the Earth's magnetic field – allows the device to create the dense, hot plasmas and keep them stable at such incredibly high temperatures. Its magnetic field is more than double what is typically used in other reactor designs, which quadruples its ability to contain plasma pressure.

Unfortunately, while Alcator C-Mod's contributions to the advancement of fusion energy have been significant, the facility has now been officially closed following this latest experiment. In 2012, the Department of Energy (DOE) decided to cease funding, due to budget pressures from the construction of the ITER project, which is due to be switched on in 2022. Following that decision, Congress restored funding for a few more years – but that funding has now ended.

C-Mod was third in the line of high-magnetic-field tokamaks built and operated at MIT. Unless a new device is announced and constructed, the pressure record just set in C-Mod will likely stand for the next 15 years. ITER will be approximately 800 times larger in volume than Alcator C-Mod, but will operate at a lower magnetic field. ITER is expected to reach 2.6 atmospheres when it reaches full operation by 2032.



The Alcator C-Mod team celebrates the record setting plasma discharge on its last day of operation.

Credit: Jim Irby/Plasma Science and Fusion Center

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