Improved magnetic coordinate representation for ideal ballooning stability calculations with the COBRA code Original Research Article

The overall efficiency of the recently developed ideal ballooning code COBRA is greatly improved by using a magnetic equilibrium description based on the same magnetic coordinates used to compute it by the VMEC 3D equilibrium code. VMEC determines this set of coordinates by requiring a spectrally condensed Fourier representation of the solution, releasing in the process the constraint of straight-field-line coordinates. This increases the analytic complexity of the ideal ballooning stability problem, since another equation must be now solved to follow the magnetic field line when integrating the ideal ballooning equation. However, from a numerical point of view, the much smaller number of Fourier harmonics needed together with the elimination of an equilibrium mapping to straight-line coordinates saves an enormous amount of time. This enhancement might prove to be essential within the compact stellarator optimization environment in which the speed and accuracy of this code can play an important role.