Nonlinear ideal interchanges in shaped 3D magnetic geometry*

A 3D nonlinear ideal MHD code is developed to study interchange modes in 3D shaped magnetic geometry.  Heat sources are introduced to allow possible non-stationary convection depending on the MHD stability properties.  The initial code development is done using UMHD (Guzdar et al, PF, 1993).   As a first example, unstable interchanges are shown in 2D Mirror geometry, from equilibrium to linear growth to nonlinear collapse.   An axisymmetric cylindrical mirror is considered next.   The code is Cartesian and thus has Cartesian hard-wall conducting boundaries.  An initial equilibrium, under heating, is shown to transit to a convecting quasi-equilibrium, with collapse of the density annulus.   At the next level, a 3D cylindrical helical stellarator geometry with single period will be considered.  Here, the rotational transform being weak, static equilibrium may not exist and a convecting state is expected.   Finally, stellarator geometry with multiple periods and more robust rotational transform will be considered.   As in an earlier NIMROD simulation (Schlutt et al., PoP, 2013), B.n is held constant on the boundary.  Perfectly conducting hard-walls are assumed.