  Magnetic helicity, and more broadly magnetic field line topology, imposes
constraints on the plasma dynamics. Helically interlocked magnetic rings are
harder to bring into a topologically non-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, tokamaks, the galactic and extragalactic
medium, with a special emphasis on extragalactic bubbles.
