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.
The team has now published the results in Physical Review Letters. In each test, the antenna coupled roughly 370 kW into the plasma for a full second. That was enough to drive around 20 kA of current near the core. Comparison shots using microwave heating of similar strength showed no such effect, so heat alone doesn't explain it.
Even so, the plasmas in these tests carried around 1 MA of current in total, so the helicons supplied only about 2% of it. Realistically, then, helicons won't let anyone retire the coil anytime soon. Instead, they could become one of several current drive tools a future reactor uses at once. That matters because every watt a plant spends keeping its current going is a watt it can't sell to the grid.
Meanwhile, China's EAST, which held a plasma for a record 1,066 seconds last year, and France's WEST already sidestep the coil on long runs using lower hybrid waves. But those waves struggle in dense plasma, so in a power plant they'd mostly be useful near the edge. Helicon waves aren't expected to hit that density wall at all.
