Why it matters: A fusion power plant only makes sense if it can run for days on end, much like the coal and gas plants it would replace. Most tokamaks, however, still work in bursts, because the giant magnet coil driving their plasma current eventually has to stop and reset. Physicists in San Diego have now shown that radio waves can carry some of that load instead, using a trick that every experiment in the 1980s failed to pull off.

The coil in question, a central solenoid, works a lot like a transformer. It keeps ramping up its own current to induce one in the plasma, but it eventually runs out of room to ramp. Those stop-start cycles also wear on the machine over time, so engineers would much rather run a reactor in steady state.

The new method, called helicon current drive, gets around that by beaming 476 MHz radio waves into the plasma, where they nudge electrons along and keep current flowing without the coil's help. Bob Pinsker, a General Atomics physicist on the new paper, was among those who tried it back then. He told the American Nuclear Society that the theory predicted a very weak effect in '80s-era plasmas, so researchers were "kind of hoping for a miracle."

That miracle never showed up, so the idea sat on the shelf for decades. These days, though, DIII-D can produce plasmas that come fairly close to reactor conditions. On top of that, General Atomics built a "comb-line" antenna with 30 radiating elements, more than twice as many as earlier tokamak antennas carried, to launch the waves more efficiently.

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