Magnetic structure at the edge of a compact stellarator (NCSX)

The magnetic field between the plasma surface and wall of the National Compact Stellarator (NCSX), is mapped via field line tracing in finite beta (4%) configurations with part (20%) of the rotational transform generated by the bootstrap current. The edge plasma magnetics are more stellarator-like, with a complex 3D structure, and less like the ordered 2D symmetric structure of a tokamak. The field lines make a transition from ergodically covering a surface to ergodically covering a volume, as the distance from the last closed magnetic surface is increased. The first few toroidal transits from the starting points are highly structured, with significant flux expansion at the bean shaped cross-section. The later toroidal transits become moderately stochastic, but with Kolmogorov lengths that can be made smaller than the connection if the separation between wall and plasma is sufficiently large.