Nuclear mattersFusion power nears

MIT's Alcator C-mod fusion reactor // Source: greensolutionsmag.com

The long-sought goal of a practical fusion-power reactor has inched closer to reality with new experiments from MIT’s experimental Alcator C-Mod reactor, the highest-performance university-based fusion device in the world.

The new experiments have revealed a set of operating parameters for the reactor — a so-called “mode” of operation — that may provide a solution to a longstanding operational problem: How to keep heat tightly confined within the hot charged gas (called plasma) inside the reactor, while allowing contaminating particles, which can interfere with the fusion reaction, to escape and be removed from the chamber.

Most of the world’s experimental fusion reactors, like the one at MIT’s Plasma Science and Fusion Center, are of a type called tokamaks, in which powerful magnetic fields are used to trap the hot plasma inside a doughnut-shaped (or toroidal) chamber. Typically, depending on how the strength and shape of the magnetic field are set, both heat and particles can constantly leak out of the plasma (in a setup called L-mode, for low-confinement) or can be held more tightly in place (called H-mode, for high-confinement).

Now, after some thirty years of tests using the Alcator series of reactors (which have evolved over the years), the MIT researchers have found another mode of operation, which they call I-mode (for improved), in which the heat stays tightly confined, but the particles, including contaminants, can leak away. This should prevent these contaminants from “poisoning” the fusion reaction. “This is very exciting,” says Dennis Whyte, professor in the MIT Department of Nuclear Science and Engineering and coauthor of some recent papers that describe more than 100 experiments testing the new mode.

Whyte presented the results in October at the International Atomic Energy Agency International Fusion Conference in South Korea. “It really looks distinct” from the previously known modes, he says.

While in previous experiments in tokamaks the degree of confinement of heat and particles always changed in unison, “we’ve at last proved that they don’t have to go together,” says Amanda Hubbard, a principal research scientist at MIT’s Plasma Science and Fusion Center and coauthor of the reports. Hubbard presented the latest results in an invited talk at the November meeting of the American Physical Society’s Division of Plasma Physics, and says the findings “attracted a lot of attention.” But, she added, “we’re still trying to figure out why” the new mode works as it does. The work is funded by the U.S. Department