Study of Helicity Conservation during Magnetic Reconnection in a Laboratory Plasma Work performed during NUF program supported by DOE : R. Crane, T. Carter, S. Hsu, H. Ji, M. Yamada ; Princeton Plasma Physics Laboratory
Magnetic helicity refers to the twistedness,
knottedness, and connectedness
of a topological magnetic configuration. The idea of helicity can be
applied to plasmas by defining the global quantity in terms of the
magnetic field, 
, and the magnetic vector potential, 
, dV, in a specified boundary.
Qualitatively,
this
quantity can be
seen as representing how much the B-field curls around itself. The notion
that this quantity is relatively conserved compared to the magnetic field
energy is not a new idea. In 1958
Woltjer introduced the idea of helicity conservation in force free fields,
and in
1974 Taylor used that idea in explaining the stability of a plasma in a
reverse field pinch by showing that it represented the state of lowest
magnetic energy. It is also thought that helicity is nearly conserved
during magnetic
reconnection, a process in which oppositely directed field
lines merge together and annihilate each other. This recombination allows
the plasma to rearrange and relax to a lower energy state. If
reconnection preserves most of the helicity of the plasma, then the
magnetic energy lost in the relaxation process should be much larger than
the loss of helicity. Using B-field profiles along with plasma current
measurements, we hope to measure the loss of the helicity contained in a
plasma relative to the loss of the magnetic field energy of that plasma.