Methods for Quantifying Magnetic Field Topology

Simon Candelaresi

Wednesday 1st May 15:00-16:00 Online

Abstract

For the dynamics of a magnetised plasma, the magnetic field line topology is an important factor. Tangled, linked or knotted fields are not easily broken apart without any violent reconnection events, while topologically trivial fields freely and quickly relax to a force-free state. Magnetic helicity is a long established quantifier of the field line topology. It can be easily used for turbulent and non-turbulent systems. Its presence has been shown to restrict the energy conversion rate from magnetic to kinetic energy. The field-line magnetic helicity is related to the magnetic helicity and can be applied to cases where there is a dominant magnetic field, while being of less use in fully developed turbulence. This is particularly useful for fusion plasmas. The two quadratic helicities are somewhat of an underappreciated pair of helicity quantifiers. While they are ideal invariants, in practical cases they can vanish quickly during reconnection events. Lastly, I am going to show how we can use knot invariants to put numbers on the topology of the field lines.

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