Three-dimensional equilibria and Mercier stability calculations
Abstract
It is well known that an equilibrium to be used for stability calculations must be extremely accurate. These high accuracy requirements, in a fixed boundary calculation, are translated into high accuracy in the representation of the boundary. These requirements are even stricter for stellarator configurations, for which all the information about the magnetic configuration is given externally through the boundary. Many Fourier components are required to accurately represent the boundary input from a realistic coil system. For torsatron-type configurations, as many as 50 components can be needed to describe the last closed magnetic surface for the vacuum field. For a heliac configuration, the number of components can go up to 200. For 3-D calculations, there is another question of accuracy that does not apply to stability calculations for axisymmetric systems. This is the role of resonant components in the calculation of the geodesic curvature or the Pfirsch-Schlueter current. As Boozer argues, local flattening of the pressure profile eliminates the singularities generated by the resonant components. However, to implement it in a numerical calculation and to eliminate the resonant components, it is necessary to work in a coordinate system with straight magnetic field lines. This creates another problem, since the equilibrium representation in a straight magnetic field lines coordinate system requires many more components than the optimal equilibrium representation developed by Hirshman and co-workers over the last decade and implemented in the VMEC equilibrium code. The VMEC equilibrium code is used and the results to the straight magnetic field line coordinate system are transformed to calculate the input for the stability analysis. The accuracy of the transformation and the convergence of the equilibrium in the new coordinate system are the major points discussed.
- Publication:
-
Presented at the 13th Conference on the Numerical Simulation of Plasmas
- Pub Date:
- July 1989
- Bibcode:
- 1989nsp..confR..17L
- Keywords:
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- Magnetic Fields;
- Magnetohydrodynamic Stability;
- Plasma Equilibrium;
- Computerized Simulation;
- Stellarators;
- Three Dimensional Models;
- Plasma Physics