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 $\eta$ is small compared to an imposed evolution, $a/v$, timescale, which means $R_m\equiv\mu_0va/\eta>>1$, 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, $j_\eta\equiv vB/\eta$, 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 $a/v$. 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 $R_m>>1$. 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 $\psi_p(\psi,\theta,\varphi)$ and a vector $\vec{x}(\psi,\theta,\varphi)$ that gives the point in space associated with each point in $(\psi,\theta,\varphi)$ canonical coordinates. The non-resonant Fourier terms in $\psi_p$ 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