59 research outputs found
Magnetic field evolution and reconnection in low resistivity plasmas
The mathematics and physics of each of the three aspects of magnetic field
evolution -- topology, energy, and helicity -- is remarkably simple and clear.
When the resistivity is small compared to an imposed evolution, ,
timescale, which means , magnetic field line chaos
dominates the evolution of field-line topology in three-dimensional systems.
Chaos has no direct role in the dissipation of energy. A large current density,
, is required for energy dissipation to be on a
comparable time scale to the topological evolution. Nevertheless, chaos plus
Alfv\'en wave damping explain why both timescales tend to be approximately an
order of magnitude longer than the evolution timescale . Magnetic helicity
is injected onto tubes of field lines when boundary flows have vorticity. Chaos
can spread but not destroy magnetic helicity. Resistivity has a negligible
effect on helicity accumulation when . Helicity accumulates within a
tube of field lines until the tube erupts and moves far from its original
location.Comment: arXiv admin note: text overlap with arXiv:2009.08779 by other author
Magnetic-field properties in non-axisymmetric divertors
The design of any large stellarator requires a plan for the removal of the
particles and the heat that are exhausted across the plasma edge. The particle
exhaust must be diverted into pumping chambers. Although the physics of
diverted plasmas has many subtleties, the magnetic field configuration between
the plasma edge and the surrounding chamber walls is the foundation upon which
divertor design is based. A stellarator magnetic field can be separated into a
magnetic field line Hamiltonian and a vector
that gives the point in space associated with
each point in canonical coordinates. The non-resonant
Fourier terms in can be removed by a canonical transformation; the
resonant Fourier terms determine the field line properties in the plasma edge
and divertor. These terms can be varied to determine what types of field
structures can be produced and how they can be controlled by external magnetic
fields
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