Non-axisymmetric divertor topology*

The fundamental properties of divertors on toroidal plasmas are determined by the magnetic flux tubes that carry the plasma from the plasma edge to the divertor chamber.  In non-axisymmetric magnetic fields, they are defined by the flux that passes through holes that arise in what would otherwise be irrational magnetic surfaces outside of the confining surfaces.  The name for these holes is turnstiles and for the surfaces on which they lie is cantori.  The holes can be characterized by the probability per toroidal transit that a magnetic field line will escape confinement and strike the surrounding wall.  This probability increases as the separation from outermost confining magnetic surface becomes greater.  For a sufficiently large perturbation to an axisymmetric divertor, the escaping flux tubes go from being determined by the singularity in q to being determined by cantori/turnstiles as are stellarator divertors.  Non-axisymmetric divertors are subject to more control than tokamak.  The method of studying divertor topology and divertor control will be discussed.