Stellarator equilibrium axis-expansion to all orders in distance from the axis for arbitrary plasma beta

Abstract: A systematic theory of the asymptotic expansion of the magnetohydrodynamic (MHD) equilibrium in the distance from the magnetic axis is developed to include arbitrary smooth currents near the magnetic axis. Compared to the vacuum and the force-free system, an additional magnetic differential equation must be solved to obtain the pressure-driven currents. It is shown that there exist variables in which the rest of the MHD system closely mimics the vacuum system. Thus, a unified treatment of MHD fields is possible. The mathematical structure of the near-axis expansions to arbitrary order is examined carefully to show that the double-periodicity of physical quantities in a toroidal domain can be satisfied order by order. The essential role played by the normal form in solving the magnetic differential equations is highlighted. Several explicit examples of vacuum, force-free, and MHD equilibrium in different geometries are presented.