Resumen de Free-boundary extension of the siesta code and its application to the wendelstein 7-x stellarator
Ideal magnetohydrodynamics (MHD) codes are of utmost importance to analyse equilibria of different experiments. The well known VMEC code (Variational Moments Equilibrium Code) does the three-dimensional ideal MHD analysis assuming nested magnetic surfaces. SIESTA (Scalable Iterative Equilibrium Solver for Toroidal Applications) is a code that takes a step further than VMEC, relying on VMEC's solution, it computes the ideal MHD equilibrium solution of a given problem, without the assumption of nested magnetic surfaces. This results in the possible development of magnetic islands and stochastic regions. SIESTA, as was originally conceived, has a limiting aspect: it would only solve the equilibrium inside of the last closed flux surface (LCFS) found by VMEC. This condition implies that the results obtained for equilibria where there are possible instabilities or perturbations close to the LCFS are not well computed since SIESTA leaves the LCFS untouched.
In this work a free-plasma-boundary version of SIESTA is developed in order to overcome this original limitation. The approach used consists of extending the analysis domain given by VMEC, in such a way that the vacuum region, or at least the most important part of it, is within the analysis volume of SIESTA. This requires the extension of the numerical analysis mesh guaranteeing the continuity of the metric elements on the mesh; a good approximation of the magnetic field solution in all the volume, and a pressure solution which couples with the magnetic field.
The new version of SIESTA is applied to the specific case of the Wendelstein 7-X stellarator, at the IPP Greifswald (Germany), making comparisons with previous studies of equilibria showing the development of neoclassical bootstrap currents which cause the divertor island chain to shift its position. The previous studies were carried out with the VMEC-EXTENDER code combination, which is the general tool for ideal MHD equilibrium studies used in IPP. While their method is not self consistent, in the sense that it is a combination of the results of two different codes, it has shown to be correct for the vacuum case and has been tested to be close to the experiment.
