Fusion Avalanches

Providing more than child's play, sandpile avalanches are offering insights into achieving optimal conditions in tokamaks, donut-shaped devices designed to produce energy from nuclear fusion. The first priority in a tokamak is to maintain a high-density, high-temperature plasma, especially an optimal configuration called the H-mode. But it is not completely understood how to get fusion plasmas into this state.

Sandpiles may provide some clues. Researchers (Sandra Chapman, University of Warwick, Sandra.Chapman@astro.warwick.ac.uk , 011-44-2476-523390) simulate avalanches in a sandpile by devising a computer model for redistributing sand and observing the ensuing avalanches. The occurrence of avalanches at all size scales is a hallmark of self-organized criticality (SOC). Since the mid-1990s, a sizeable body of research has found evidence of SOC in tokamak plasmas, where particles and energy are rapidly redistributed, at all size scales, to other areas of the plasma For some reason this "transport"seems to occur faster than ordinary diffusion allows.

But there's a twist: SOC behavior, by itself, would tend to produce a plasma with poor confinement of its particles. The new modeling suggests that the desirable H-mode occurs when some, but not all, of the conditions of SOC are met. The researchers have hopes that the H mode can be achieved by manipulating the plasma so that the "fast transport" only takes place at the short length scales. (Chapman, Dendy, and Hnat, Physical Review Letters, 26 March 2001.)