A three-dimensional magnetohydrodynamic equilibrium in an axial coordinate with a constant curvature

Motivated by the superior confinement observed in the relaxed three dimensional (3D) states in the reversed field pinch, 3D plasma equilibria in coordinate systems based on space curves with a constant curvature as the axial coordinates are studied by using the method of metric perturbation. Constancy of the curvature allows the development of magnetohydrodynamic equilibrium with asymptotic good 2D flux surfaces near the coordinate axis. The perturbation parameter is the product of the torsion variation along the coordinate axis and the distance from it. The lowest order equilibrium with good 2D flux surfaces is symmetric with respect to translation along the space curve. It embodies the approximate toroidal-helical symmetry and is determined by a generalized Grad–Shafranov equation which includes information of the constant curvature and the average torsion of the space curve. Based on this fundamental equilibrium, a formal scheme is developed that allows us to find the ideal MHD equilibrium taking into account the full metric variation of the torsion along the spatial axis. In this limit, the flux surfaces are shown to exist for the full plasma. Numerical examples are given for the lowest order equilibria. It is suggested that equilibria based on this type of coordinates can allow easier evaluation of plasma shapes and magnetic boundary conditions that are more compatible with the relaxed central core. This could be the needed requirement for strongly self-organized equilibria with a large good confinement region.