Session HP1 - Poster Session V.
POSTER session, Tuesday afternoon, October 24
Exhibit Hall AB, Qu\'{e}bec City Convention Centre
Flux coordinates with straight magnetic field lines have been widely used in stellarator research, since they facilitate calculations of transport and stability. On the contrary, MHD equilibrium calculations routinely use the VMEC 3D equilibrium coordinates, in which magnetic field lines are not straight but which provide a very compact Fourier representation of the solution. Therefore a mapping to straight coordinates must be carried out, whenever any assessment of the stability and transport properties of the equilibrium is to be computed. This mapping usually requires a much larger number of Fourier modes than the VMEC representation, bringing in issues of numerical convergence and accuracy, as well a computational speed. To avoid this, we have implemented the VMEC coordinates within the COBRA ballooning stability code [1]. The stellarator optimization procedure using COBRA benefits greatly from this, since it avoids repetitive application of the lengthy mapping procedure. The new code has been applied to the design of a quasi-omnigeneous compact stellarator (QOS). The speed of the calculation is enhanced more than 20 times relative to the previous code based on straight flux coordinates, and convergence issues that had previously retarded the optimization procedure have been eliminated. [1] SANCHEZ, R., HIRSHMAN, S.P., WHITSON, J.C. and WARE, A.S., Journ. Comp. Phys. 161 (2000) 547.