The pursuit for clean energy has achieved a major leap on the last day of MIT’s 23-year old nuclear fusion reactor.
Nuclear fusion is the holy grail of energy sources, and for a good reason. The same process that is powering our sun would provide us with a nearly limitless clean, safe, and carbon-free energy resource that produces more power than it needs to keep itself running.
On Earth, 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. And though good things usually don’t come easily – nuclear fusion being no wild exception – any progress made in this field is great news.
This progress in particular is a straight up world record for plasma pressure, a key ingredient to producing energy from nuclear fusion.
MIT’s Plasma Science and Fusion Center have achieved over 2 atmospheres of pressure for the first time in the Institute’s Alcator C-Mod tokamak nuclear fusion reactor, with the temperature inside reaching over 35 million degrees Celsius, which is approximately twice as hot as the center of the sun.
But we still have ways to go. 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 that 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 the new record represents 2.05 atmospheres, a 15 percent improvement. 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.
Although the Alcator C-Mod reactor’s funding has been ceased due to budgetary pressures, it went out on a high note. “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.”
Alcator C-Mod is the world’s only compact, high-magnetic-field fusion reactor with advanced shaping in a design called a tokamak (a transliteration of a Russian word for “toroidal chamber”), which confines the superheated plasma in a donut-shaped chamber. 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. Its magnetic field is more than double what is typically used in other designs, which quadruples its ability to contain the plasma pressure.
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, a tokamak currently under construction in France, will be approximately 800 times larger in volume than Alcator C-Mod, but it will operate at a lower magnetic field. ITER is expected to reach 2.6 atmospheres when in full operation by 2032, according to a recent Department of Energy report.
To understand how Alcator C-Mod’s design principles could be applied to power generation, MIT’s fusion group is now working on adapting newly available high-field, high-temperature superconductors that will be capable of producing magnetic fields of even greater strength without consuming electricity or generating heat. These superconductors are a central ingredient of a conceptual pilot plant called the Affordable Robust Compact (ARC) reactor, which could generate up to 250 million watts of electricity.
