Scientists working on the DIII-D tokamak experiment, a national facility located at General Atomics in San Diego, have announced a doubling of the fusion power density over what previous experiments and theory had identified as an upper limit to the allowable plasma pressure. The advance was made by using small control magnets to correct for small imperfections in the main magnetic field that confines the hot fusion plasma fuel. The advance should result in fusion power plant designs having lower cost of electricity than predicted by previous designs. Information related to the advance is posted at: http://fusion.gat.com/diii-d/releases/.
General Atomics issued a press release July 2 stating, "Researchers at the U. S. Department of Energy funded DIII-D National Fusion Facility at General Atomics, the largest fusion energy experiment in the United States, have nearly doubled the usual limits on pressure in a fusion energy device by spinning the hot, fusion fuel very rapidly. A significant scientific advance in understanding the pressure limit in fusion energy devices made these higher limits possible. These results are an important step towards controlled fusion power production that is feasible, economical, and attractive."
The release states, "High pressure in the fusion fuel is critical because the power released from fusion reactions increases very rapidly with increasing pressure. However, previous experiments and theory have identified an upper limit to the allowable pressure, called the free-boundary pressure limit. Beyond this pressure limit the hot fusion fuel becomes unstable, bulges outward, contacts the metal chamber wall, and cools rapidly."
In the early 1990's, theoretical and experimental work had suggested that the plasma pressure might be increased beyond the usual free-boundary pressure limit by rapidly spinning the fusion fuel. Current experimental plasmas are easily spun at extremely high rates (10 to 100 miles/second) like a spinning top. In the initial experiments on DIII-D that sought to raise the plasma pressure while spinning the fusion fuel, the spin rate would always slow down and the hot plasma would become unstable and be lost. "Scientists felt that the free-boundary pressure limit was unavoidable-we could not get beyond it. Sustaining the pressure beyond this limit is a significant scientific breakthrough," said Dr. Ronald D. Stambaugh, Program Director. "The observed slow-down of the spinning plasma was a big...