Jeffrey P. Freidberg, professor of nuclear science and engineering, has described how fusion energy may one day turn a pickup-truck-full of seawater into enough electricity to power Boston for a year. Exploiting fusion’s enormous potential, however, is still more than three decades away.
MIT researchers moved that day a little closer to reality when they announced recently that they have succeeded in using radio waves to mix the 50-million degree C plasma at their Alcator C-Mod fusion reactor, a tokamak (magnetically confined, doughnut-shaped nuclear fusion device) at the MIT Plasma Science and Fusion Center. It is the tokamak with the highest magnetic field and highest plasma pressure in the world.
Many obstacles remain for bringing commercial fusion reactors online, but these experiments shows signs of solving one of the most vexing challenges. The heat of a fusion reaction is so extreme that it is impossible to contain within any kind of physical container. What’s more, the plasma generated in the reaction is unstable and prone to turbulence. Using a technique proposed more than 20 years ago at MIT (but had never been successfully tested), Institute researchers Yijun Lin and John Rice confined the plasma’s turbulence within a magnetic field “bottle” using something called the “mix-and-stir method.”
“Ours is the first definitive result showing that high-power radio waves can significantly affect the flow of the plasma,” said physicist Earl Marmar, division head of the Alcator Project. The researchers launched several million watts of radio waves into the plasma using a dipole antenna at frequencies ranging between 50 and 80 megahertz. “In the process, we are breaking up the turbulent eddies – making them smaller,” Marmat adds. “This will ultimately determine how well the magnetic bottle is confining the plasma.” – Deborah Halber