Fusion power promises a clean and almost inexhaustible source of energy. Physicists developing the tokamak type of reactor have made progress over the past few decades, but several challenges remain. One of them is replacing pulsed operation–creating the superhot plasma for only tens of seconds at a time–with continuous operation. Now a Swiss team reports in the 10 April PRL that they have successfully tested what may be the most promising technology for the steady-state mode. They aimed carefully tuned microwave beams into a tokamak and drove a controlled plasma current without help from the usual pulsed technique. Although this current lasted for only about two seconds, the time was limited only by the pulse length of the microwaves, which in principle could be very long.
For a tokamak to produce useful power, magnetic fields must confine the hydrogen plasma at a sufficient temperature and density and for a long enough period of time. Part of the stabilizing field is generated by a plasma current circulating through the doughnut-shaped vessel, and this current is normally maintained by induction: A time-varying current through a coil (the transformer) in the center of the doughnut creates a time-varying magnetic field that induces the plasma current. But the inductive technique requires pulsed operation because the transformer current must change continuously in time. (An oscillating transformer current would periodically destroy the plasma current.)
Pulsed operation is not only bad business for a power company, because of wasted time between pulses; it also creates repeated mechanical and thermal stresses on all of the systems involved. The most popular method for generating a steady current is to send electromagnetic waves into the plasma to drive the current. One version of this, called lower hybrid current drive, has been used since the early 1980’s, but it’s hard to get these waves into the plasma and control their effects, explains Olivier Sauter of the Swiss Federal Institute of Technology in Lausanne (EPFL).
The preferred approach, called electron cyclotron current drive (ECCD), uses microwaves at a higher frequency, but the technology to produce such high frequencies with high power is relatively new. In ECCD, the microwave beam is tuned close to the so-called cyclotron frequency at which electrons orbit about magnetic field lines as they zip around inside the tokamak along helical paths. The beam is aimed at an oblique angle to the torus and boosts electrons traveling around it in one direction over those traveling the other way.
Using three 83 GHz, 0.5 MW microwave beams, Sauter and his colleagues produced 2-second-long periods of continuous plasma current under conditions of high temperature and density and showed that the plasma remained stable throughout that time. Sauter explains that longer durations will be possible as improved microwave sources become available–technology that is already under development in labs worldwide.
“It’s the best demonstration so far” for ECCD, says Tony Taylor of General Atomics in San Diego. Fred Skiff of the University of Iowa calls the work “significant” because some skeptics doubted that ECCD could work under realistic reactor conditions. Both praise the advantages of the technique, including its ability to target specific locations within the plasma, which could allow for a feedback control of plasma instabilities in future tokamak designs.