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[en] We studied, by micromagnetic numerical calculations, asymmetric vortex-core reversals driven by the m = −1 and m = +1 azimuthal spin-wave modes' excitations in soft magnetic circular nano-disks. We addressed the similarities and differences between the asymmetric core reversals in terms of the temporal evolutions of the correlated core-motion speed, locally concentrated perpendicular gyrofield, and magnetization dip near the original vortex core. The criterion for the core reversals was found to be the magnetization dip that must reach the out-of-plane magnetization component, mz = −p, with the initial polarization p, where p = +1 (−1) for the upward (downward) core magnetization. The core-motion speed and the associated perpendicular gyrofield, variable and controllable with static perpendicular field, Hz, applied perpendicularly to the disk plane, must reach their threshold values to meet the ultimate core-reversal criterion. Also, we determined the Hz strength and direction dependence of the core-switching time and threshold exciting field strength required for the core reversals, whose parameters are essential in the application aspect. This work offers deeper insights into the azimuthal spin-wave-driven core-reversal dynamics as well as an efficient means of controlling the azimuthal-modes-driven core reversals
[en] The power-law series for the poloidal magnetic flux function, up to the third-order terms, is presented for the case where two nulls of the poloidal magnetic field are separated by a small distance, as in a snowflake divertor. Distinct from the earlier results, no assumptions about the field symmetry are made. Conditions for the realization of an exact snowflake are expressed in terms of the coefficients of the power series. It is shown that, by a proper choice of the coordinate frame in the poloidal plane, one can obtain efficient similarity solutions for the separatrices and flux surfaces in the divertor region: the whole variety of flux surface shapes can be characterized by a single dimensionless parameter. Transition from a snowflake-minus to a snowflake-plus configuration in the case of no particular symmetry is described. The effect of the finite toroidal current density in the divertor region is assessed for the case of no particular symmetry.
[en] The selection of an optimal parametric angle theta describing a closed magnetic flux surface is considered with regard to accelerating the convergence rate of the Fourier series for the Cartesian coordinates x(theta,phi) identical with R - R0 and y(theta,phi) identical with Z - Z0. Geometric criteria are developed based on the Hamiltonian invariants of Keplerian orbits. These criteria relate the rate of curve traversal (tangential speed) to the curvature (normal acceleration) so as to provide increased angular resolution in regions of largest curvature. They are, however, limited to either convex or starlike domains and do not provide rapid convergence for complex domains with alternating convex and concave regions. A generally applicable constraint criterion, based directly on minimizing the width of the x and y Fourier spectra, is also derived. A variational principle is given for implementing these constraints numerically. Application to the representation of three-dimensional magnetic flux surfaces is discussed
[en] The effects on magnetic-field-line structure of adding various static transverse magnetic fields to a Solovev-equilibrium field-reversed configuration is examined. It is shown that adding fields that are anti-symmetric about the axial mid-plane maintains the closed field-line structure, while adding fields with planar or helical symmetry opens the field structure. Anti-symmetric modes also introduce pronounced shear
[en] At present, the striations observed on the Hα or Hβ emission signal of ablated pellets are not well understood. An attractive explanation of these striations would be a limitation of the ablation process due to the smaller amount of energy available on resonant magnetic surfaces. To evaluate the ability of the latter to produce the observed features, a model has been developed which takes into account the Maxwellian distribution of electrons, the proportion of trapped particles, the geometric effects linked with the shear and the finite lifetime of the filaments. In tokamaks, it is found that this model is unable to reproduce the observed striations for smooth q-profiles but that the general shape of the Hα (or Hβ) signal can be fitted by introducing local flattenings (e.g. small magnetic islands) around resonant surfaces. In low shear stellarators, striations (rather shallow) due to rational q-values can exist in the absence of magnetic islands, but only if a suprathermal electron tail is present. (author). 20 refs, 6 figs, 1 tab
[en] The complete set of universal local relationships between geometrical (the curvature and torsion of the force lines of the magnetic field and the field complementary to it) and magnetic (|B|, |∇Φ|, b · (∇ x b), and the local shear s) quantities in currentless magnetic configurations comprising a system of equilibrium nested magnetic surfaces, including those with several magnetic axes, is derived. Possible applications of these relationships are discussed.
[en] The FACETS (Framework Application for Core-Edge Transport Simulations) project began in January 2007 with the goal of providing core to wall transport modeling of a tokamak fusion reactor. This involves coupling previously separate computations for the core, edge, and wall regions. Such a coupling is primarily through connection regions of lower dimensionality. The project has started developing a component-based coupling framework to bring together models for each of these regions. In the first year, the core model will be a 1 dimensional model (1D transport across flux surfaces coupled to a 2D equilibrium) with fixed equilibrium. The initial edge model will be the fluid model, UEDGE, but inclusion of kinetic models is planned for the out years. The project also has an embedded Scientific Application Partnership that is examining embedding a full-scale turbulence model for obtaining the crosssurface fluxes into a core transport code.