This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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." 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.
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