Improved magnetic coordinate representation for ideal ballooning stabilitycalculations with the COBRA code

The overall efficiency of the recently developed ideal ballooning code COBR A 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 co ndensed Fourier representation of the solution, releasing in the process th e constraint of straight-field-line coordinates. This increases the analyti c complexity of the ideal ballooning stability problem, since another equat ion must be now solved to follow the magnetic field line when integrating t he ideal ballooning equation. However, from a numerical point of view, the much smaller number of Fourier harmonics needed together with the eliminati on of an equilibrium mapping to straight-line coordinates saves an enormous amount of time. This enhancement might prove to be essential within the co mpact stellarator optimization environment in which the speed and accuracy of this code can play an important role. (C) 2001 Elsevier Science B.V. All rights reserved.