Ideal MHD and two-fluid axisymmetric free-boundary equilibrium

The codes FLOW[1] and FLOW2[2] are ideal MHD and two-fluid axisymmetric equilibrium codes that allow for arbitrary rotation in the plasma. In their published implementation they can only run as fixed-boundary equilibrium codes. In this work we present an extension to free-boundary mode for both codes. The FREE-FIX[3] code is used to provide boundary conditions for the two equilibrium codes based on a desired plasma shape and on assigned coil geometry. Since magnetic surfaces (tangential to the magnetic field) and flow surfaces (tangential to the plasma rotation) are different in the two-fluid model, different positions for X-points and separatrix are calculated for each set of surfaces. Noting that the proper plasma edge definition has to be given in terms of flow surfaces, we investigate the shift in X-point, separatrix and strike points on the divertor between the two sets of surfaces. Magnetic surfaces calculated by FLOW are used as a reference for comparison with the ideal MHD model. We investigate the issue by running both codes in free-boundary mode for realistic plasma and coil geometries and comparing the magnetic surfaces obtained by FLOW and FLOW2 and the flow surfaces calculated by FLOW2.

[1] L Guazzotto et al. Phys. of Plasmas 11 604

[2] L Guazzotto et al. Phys. of Plasmas 22 092503

[3] L Guazzotto Plasma Phys. Control. Fusion 59 122001