The twisted origin of spheromaks
DOI: 10.1063/1.2409989
Driving a sufficiently large current along a plasma-filled seed magnetic field can result in the formation of a spheromak—a self-organized, stable, vortexlike, isolated toroid of plasma. Typically, researchers use a coaxial “gun” (at right in this image) through which magnetic forces shoot current-carrying plasma into a vacuum chamber. The seed field extends out along the gun axis and then folds back; the current flows along the seed field and produces a path of bright light. Spheromak formation is believed to involve a dynamo process, whereby the seed magnetic field is amplified at the expense of the field associated with the current. Now, Caltech physicists Scott Hsu and Paul Bellan have captured images of the plasma in their 1.5-m-diameter × 2-m-long vacuum chamber, while measuring the magnetic fields. Their results demonstrated not only the dynamo amplification process but also its origin. At a critical current threshold, a so-called kink instability develops, and as shown here, the initially straight current becomes helical. Acting as a solenoid, the helical current effectively amplifies the original seed field, and eventually the spheromak becomes detached from the gun and floats freely in its state of minimum magnetic energy. Spheromaks represent a possible plasma confinement configuration for nuclear fusion. Insight into their formation should help in the design of future magnetic confinement fusion experiments. Spheromak formation is also important for understanding plasma dynamics in the solar corona and in astrophysical jets. (
