PORTLAND, Ore. MIT researchers said they have succeeded in using radio waves to mix the 50 million degree C plasma at its Alcator C-Mod fusion reactor.
While a host of obstacles remain for bringing commercial fusion reactors online, the ability to use radio waves to quell plasma turbulence has been a stumbling block to further progress.
"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 at MIT's Plasma Science and Fusion Center.
"We launched several million watts of radio waves into the plasma using a dipole antenna at frequencies ranging between 50 and 80 megahertz," Marmar added. "In the process, we are breaking up the turbulent eddies--making them smaller--which will ultimately determine how well the magnetic bottle is confining the plasma."
A doughnut-shaped magnetic bottle is required to confine plasma heated to temperatures that no solid container could withstand. The Alcator C-Mod, which has the highest plasma pressure of any facility, contains plasma using the highest magnetic field of any fusion reactor. Thus, MIT's demonstration of RF "mix and stir" also be used in other fusion reactors, such as International Thermonuclear Experimental Reactor now under construction in France.
Radio frequencies were first proposed to control plasma flow 20 years ago. MIT researchers claim their technique is the first to experimentally confirm that the technique works in a fusion reactor. The RF mix-and-stir method was perfected by MIT physicist Yijun Lin and principal researcher John Rice.
Next, the MIT researchers plan to optimize their process by adjusting the frequency of the radio waves, the strength of the magnetic field and the mix of ions in the plasma.
Nevertheless, several hurdles still must be overcome before fusion reactors can be commercialized, including engineering materials to capture and neutralize the high-energy neutron flux, a by-product of the fusion reaction. Neutrons are not radioactive, but current methods of capturing them in a blanket surrounding the reactor produce radioactive by-products.
Marmar noted that neutron flux is not part of the radioactive waste created when they are captured in a "blanket" surrounding the reactor.
Compared to fusion reactors, current commercial fission reactors produce many times more radioactive waste, with half-lives in the tens of thousands of years. Proponents of nuclear fusion reactors argue that radioactive by-products from the fusing of atoms has a half-life shorter than 100 years.
Fusion critics say confinement and storage of radioactive by-products must still be solved before fusion reactors can be commercialized.