MIT finds new mode for fusion reactions

MIT’s taken a step closer to practical fusion power with a technique that removes the contaminants that slow fusion reactions.

The team working on the Alcator C-Mod reactor – the highest-performance university-based fusion device in the world – has discovered a new set of operating parameters. These allow heat to be 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.

Like most of the world’s experimental fusion reactors, MIT’s is a ‘tokamak’ reactor, in which powerful magnetic fields are used to trap the hot plasma inside a doughnut-shaped chamber.

But 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, 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 them from ‘poisoning’ the fusion reaction.

Dennis Whyte, professor in the MIT Department of Nuclear Science and Engineering, says there have been various suggestions for removing the contaminants at intervals after they accumulate. Now, he says, “We seem to have discovered a completely different flushing mechanism … so they don’t build up in the first place.”

The findings could be extremely useful in enabling the next step forward in fusion energy, where fusion reactions and power are sustained mostly by ‘self-heating’ and don’t need large amounts of outside power.

Researchers expect to achieve this milestone, known as ‘fusion burn’, in the ITER reactor  currently being built in France. The findings from MIT “almost certainly could be applied” here, Whyte says.