Physicists in the U.S. used radio waves to drive a fusion plasma current in a nuclear experiment. Led by Jeff Lestz at General Atomics, the team showed these waves can help reactors run continuously. Bob Pinsker said, "The pulse length of the tokamak is limited by the available flux-swing."
For the first time, a team of physicists in the U.S. has clearly demonstrated a "helicon current drive" in a nuclear fusion experiment. Led by Jeff Lestz at General Atomics, San Diego, the team took advantage of the latest advances in tokamak technology to show that radio waves steered to travel around the donut-shaped device can be absorbed by its confined plasma, driving an electric current inside it.
Published in Physical Review Letters, the results could help future fusion reactors to operate continuously.
Driving steady currents
Physicists are now closer than ever to harnessing the practically limitless energy offered by nuclear fusion. To date, the most intensively studied approach is the tokamak: a reactor shaped like a hollow donut, where plasma is held in place by powerful magnetic fields. For decades, the device has been the basis of world-leading projects including the ITER reactor, currently under construction in France, and General Atomics' own DIII-D research tokamak.
Inside a tokamak, plasma is confined by a magnetic field generated largely by a strong electric current flowing "toroidally" through the plasma itself, around the circumference of the donut. In conventional designs, this current is induced by a central solenoid placed in the "hole" of the donut. Acting like the primary coil of a transformer, it induces a current in the plasma as its own current changes.
Ideally, this plasma current would flow continuously, but a transformer can only induce current while the solenoid's magnetic field is changing. To keep the plasma current flowing, the solenoid must steadily sweep through the full range of currents it can carry.
The total change in magnetic flux it can provide in this way is called its "flux swing." Once it is used up, the plasma current can no longer be sustained, and the plasma must be shut down while the solenoid is reset, ready for a new pulse of operation.
"The drawback of this arrangement is that the pulse length of the tokamak is limited by the available flux-swing in that primary solenoid," explains Bob Pinsker at General Atomics, who has studied tokamak fusion since the 1980s. "The pulse length in the DIII-D tokamak is limited to about 10 seconds, while the very much larger ITER device will be limited to less than about 1,000 seconds in its nominal operation."
The helicon current drive
One way around this problem is to drive the plasma current without relying on the solenoid. In the most efficient technique demonstrated so far, radio waves are beamed into the plasma that push electrons in one direction around the torus. However, in larger, hotter reactor plasmas, these waves would likely be absorbed too close to the plasma's edge, before reaching the region where extra current is needed.
In the 1980s, ideas emerged for using a different type of radio wave in the same frequency range, called the "helicon." Since helicons are absorbed less readily by electrons, they could penetrate deeper into a hot reactor plasma. They can be launched using a specialized antenna named a "comb-line": a row of radiating elements laid out along the tokamak's toroidal direction, facing into the plasma.
Power fed into one end of the comb-line passes from element to element, so that each oscillates slightly later than its neighbor. This creates a rippling pattern that launches radio waves into the plasma, while steering them preferentially in one direction around the torus. Electrons traveling along the magnetic field at roughly the same speed as the waves can then "surf" along them, sustaining the current without any help from the solenoid.
Efficient absorption
In 2014, Lestz, Pinsker and their colleagues built on this idea to consider whether a helicon current drive could finally be tested on DIII-D. "In one sense, our approach of using the fast wave in the lower hybrid range of frequencies for non-inductive current drive in a tokamak plasma was quite old," Pinsker describes. "But the confluence of three developments at DIII-D was new, and they made the experiment practical."
By then, comb-line antennas could launch helicon waves efficiently in a single direction, DIII-D could produce plasmas hot enough to absorb them directly, and improved measurement techniques could detect tiny amounts of current deep inside the plasma.
In their latest study, Lestz's team has now demonstrated this idea using a megawatt-level helicon system, showing that the waves deposited their energy in the plasma's core. Compared with similar experiments where the helicons were replaced by a comparable amount of microwave heating, the structure of the plasma's magnetic field also changed noticeably faster—a clear signature of extra current being driven by the helicons.
"Despite the long history of attempts to do closely related experiments dating back to the mid-1980s, our result represented the first clear success in this effort," Pinsker says. "Technologically, our success with the comb-line antenna is also a first, in terms of a very clear and obvious electron heating being observed using this as the wave-launching structure. It will also encourage other efforts to use similar wave-launchers."
In turn, the results could bring one of the most hotly anticipated advances in power generation a step closer to reality.