Calculation of Boozer magnetic coordinates for multiple plasma regions (with either closed or open flux surfaces) connected by magnetic separatrices
- Associazione EURATOM-ENEA sulla Fusione, CR Frascati, C.P. 65-00044, Frascati, Rome (Italy)
Magnetic coordinates ({psi}{sub T}=radial label of flux surfaces, {theta}=poloidal, and {phi}=toroidal angle) are introduced in toroidal magnetoplasma equilibria in order to straighten the field lines [described by: {theta}-{iota}({psi}{sub T}){phi}=constant on any flux surface, {iota}/({psi}{sub T}) being the rotational transform]. The simplest method for analyzing the ideal magnetohydrodynamic (MHD) stability expands the perturbed plasma displacement {xi}-vector in magnetic coordinates and solves the normal mode equation through one-dimensional (1D) radial finite elements. This paper extends the calculation of (Boozer) magnetic coordinates to simply connected equilibria that embed a magnetic separatrix, with regular X-points (B-vector{ne}0), and reach the symmetry axis, with singular magnetic X-points (B-vector=0). These configurations include multiple plasma regions, whose outermost one (surrounding plasma) is not composed by toroidal surfaces closed around a single magnetic axis. Two examples are chosen: (i) flux-core-spheromak (FCS) configurations, where the surrounding plasma is a screw pinch, with open flux surfaces; (ii) Chandrasekhar-Kendall-Furth (CKF) configurations, where it is a toroidal shell, carved by multiple toroidal plasma regions. This paper shows that a proper ordering of the radial coordinate {psi}{sub T}, the requirement of continuity for {theta} and {phi} and an {iota} matching condition (between neighboring mesh points on opposite sides of the connecting separatrix) resolve the ambiguities in the definition of magnetic coordinates in both CKF and FCS cases. However, a few metric coefficients diverge at the separatrices; therefore, often numerical MHD stability codes do not use magnetic coordinates there, but adopt local two-dimensional (2D) finite elements. This paper instead investigates all the divergences, in order to allow for the asymptotic analysis of {xi}-vector near the separatrices, with the purpose of maintaining the magnetic coordinate method and the 1D radial finite elements in the ideal MHD stability analysis.
- OSTI ID:
- 20782343
- Journal Information:
- Physics of Plasmas, Vol. 12, Issue 11; Other Information: DOI: 10.1063/1.2122487; (c) 2005 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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