Magnetic-field properties in non-axisymmetric divertors:

In toroidal fusion plasmas, the particles that cross the plasma edge must be diverted into pumping chambers. The properties of the magnetic field between the plasma edge and the surrounding chamber walls are the basis for divertor design. Studies of what types of divertors are possible and how they can be controlled is greatly simplified by the use of a magnetic field line Hamiltonian that is a function of the toroidal flux plus a Fourier series in the poloidal and toroidal angles in which each term is somewhere resonant with the rotational transform defined by angle-independent part of the Hamiltonian. A procedure is given in the arXiv version of this abstract for determining this Hamiltonian for any given magnetic configuration. Variations in the magnitudes of the resonant terms determine what divertor features are potentially possible: connection length, width and locations of the interceptions on the chamber walls, and plasma shielding by a broad chaotic region. Variations in the rotational transform explore the robustness of solution to changes in the plasma. Stellarators require non-axisymmetric divertors, but they could also be used on tokamaks by enforcing quasi-axisymmetry on the core plasma.