Recent experiments in Germany and the United States have shown that fusion reaction rates and other properties in magnetically confined plasmas can be significantly improved by a relatively small amount of microwave power, applied at precisely the right location in the plasma. Tokamak plasmas and indeed most magnetically trapped plasmas are subject to the growth of "magnetic islands." These islands break up the smooth magnetic field surfaces that confine the plasma, leading to more rapid loss of heat from the plasma and making it more difficult to reach the high temperatures and pressures needed for nuclear fusion. Experiments first carried out in the ASDEX Upgrade tokamak (Max- Planck Institute, Garching, Germany) and, more recently, in the DIII-D tokamak (General Atomics, San Diego, CA) have confirmed theoretical predictions that islands due to high plasma pressure can be eliminated by adding a small amount of added electrical current at the island location. A narrow beam of microwaves can drive the desired current, with surgical precision, by interacting with electrons at the appropriate location. In experiments a magnetic island degraded the plasma pressure by about 20%. Adding one megawatt of microwave power, about one-tenth of the total power needed to heat the plasma, drove enough current to suppress the island. This allowed the plasma pressure to recover, resulting in a 35% increase in the fusion reaction rate at DIII-D. These pioneering experiments show the feasibility of improving the performance of fusion plasmas by small, precisely controlled modifications of their internal structure. Similar experiments have been carried out in tokamaks in the UK and Japan.

For further information, contact Hartmut Zohm, Max Planck Institut fur Plasmaphysik 011-49-89-3299-1925, haz@ipp.mpg.de or Robert La Haye, General Atomics, (858) 455-3134, lahaye@fusion.gat.com