A group led by Jeff LeStz of General Atomics (San Diego) has for the first time clearly demonstrated in a fusion experiment the so-called helicon current drive, a way of creating an electric current in plasma using radio waves. The results were published in the journal Physical Review Letters.
A tokamak is a reactor shaped like a hollow doughnut, where plasma is confined by powerful magnetic fields. A strong electric current flows inside it, creating most of the confining field. Usually this is induced by a central solenoid, a coil in the hole of the doughnut that works like the primary winding of a transformer. But a transformer induces current only while its magnetic field is changing. When the supply of these changes is exhausted, there is nothing to sustain the current, and the discharge has to be stopped.
The limitation can be circumvented by creating current without a solenoid. The most effective of the methods shown so far is to direct radio waves into the plasma that push electrons in one direction. However, in large and hot reactor plasmas such waves are likely to be absorbed too close to the edge, without reaching the required region.
Back in the last century an idea emerged to use waves of the same frequency range but of a different type, helicons. They are absorbed less strongly by electrons and therefore penetrate deeper into hot plasma. They can be emitted by a special comb antenna: a row of radiating elements lined up along the torus and facing the plasma. Power fed to one end is passed from element to element, and each oscillates slightly later than its neighbor. A traveling wave arises, directed predominantly in one direction around the torus. Electrons moving along the magnetic field at roughly the same speed as the wave are picked up by it and sustain the current without the help of a solenoid.
In the new work, LeStz's team tested a megawatt helicon system and showed that the waves deposit energy in the central part of the plasma. Compared with similar experiments in which helicons were replaced by microwave heating of comparable power, the structure of the plasma's magnetic field changed noticeably faster, a clear sign of additional current created by the helicons.
The authors emphasize that this is an experimental demonstration on a research facility, not a ready-made solution for industrial reactors. The limitation is that the result was obtained on a single facility and requires verification in the hotter plasma of future reactors. Nevertheless, the work could bring closer one of the most anticipated developments in energy, a fusion reactor capable of operating continuously.