Reduction of radial losses in tandem mirrors

The condition for having strictly omnigenous magnetic fields (zero geodesic curvature and neoclassical step size) is generalized to determine the fields which give the smallest mean square neoclassical step size consistent with given boundary conditions and constraints. Because this transport minimization produces less restrictive field configurations than omnigenity, a wider class of practical applications is possible. An explicit set of ordinary differential equations is obtained for the transport-minimizing vacuum fields in long-thin tandem mirror geometry. Configurations with reduced transport are found at little cost in line averaged interchange stability. Additionally, for these configurations the constraint that no parallel current flow into the center cell (due to the Stupakov effect) may be imposed in a natural way