The Twisted Origin of Spheromaks

Researchers at the California Institute of Technology have made important progress in solving a long-standing mystery concerning the formation of spheromaks, self-organizing toroidal plasma configurations that are superficially reminiscent of smoke rings. It is well known that current-carrying plasmas embedded in an initial seed magnetic field can form spheromaks. The formation process is believed to involve some kind of dynamo process, whereby the internal magnetic fields become re-arranged or even amplified so as to achieve a stable minimum energy state for the internal magnetic forces. (Similar minimal-energy state arguments help explain why soap bubbles, for example, tend to be spheres rather than cubes or other shapes.) But until now, no one has definitively demonstrated just how a plasma transforms from an unstable, high internal energy configuration into a spheromak. The new experiment sheds light on the phenomenon by capturing images of plasmas as spheromaks form. The images show that plasma currents initially flow in straight lines along a confining magnetic field. Owing to an effect known as the kink instability, the plasma currents develop bends that twist into a helix (see image). The helix acts like a coiled current element, or solenoid, which amplifies the original, straight magnetic field. Above a certain threshold in the initial magnetic field, detached plasma spheromaks are formed. The researchers (contact: Paul Bellan, 626-395-4827) confirmed the theory behind the effect by measuring the rapid amplification of the magnetic field inside developing plasma solenoids. Spheromaks are potentially promising routes to plasma-based nuclear fusion, and insight into their formation will help in the design of future experiments-and possibly even a clean, safe energy source. In addition, spheromak formation is important for explaining the behavior of plasma in the solar corona, as well as understanding the physics of jets that sprout from black holes, galactic nuclei, and other astrophysical objects. (S. C. Hsu and P. M. Bellan, Physical Review Letters, 30 May 2003)