Stability of plasmas through magnetic helicity

Abstract

Magnetic helicity, and more broadly magnetic field line topology, impose constraints on the plasma dynamics. Helically interlocked magnetic rings are in a non-trivial topological state. It is harder to bring them into a topologically trivial state than two rings that are not linked. This particular restriction has the consequence that helical plasmas exhibit increased stability in laboratory devices, in the Sun and in the intergalactic medium. Here we discuss how a magnetic field is stabilizing the plasma and preventing it from disruption by the presence of magnetic helicity. We present observational results, numerical experiments and analytical results that illustrate how helical magnetic fields strongly contribute to the long-term stability of some plasmas. We discuss several cases, such as that of solar corona, toroidal fusion devices, the galactic and extragalactic medium, with a special emphasis on extragalactic bubbles.