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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 249-259 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The magnetohydrodynamic stability of force-free plasma–vacuum systems (curl B=μB in the plasma, with constant μ) is studied in circular cylinders with identified ends (topological torus). A necessary stability criterion is derived by considering large poloidal mode numbers. This takes a simple form (the magnetic rotation numbers at the axis and plasma–vacuum interface must have opposite signs) if the magnetic field lines in the interface are not closed. If they are closed, then violation of this simple condition does not imply instability unless the aspect ratio exceeds some value which depends on both the numerator and denominator of the rational magnetic rotation number at the interface. For aspect ratios greater than unity, combination with the criterion for stability to internal kinks implies that the inhomogeneity parameter ||μ|| must be above the threshold for reversal of the toroidal current density, but below that for reversal of the poloidal one. This condition is independent of the wall radius, in contrast to the well-known necessary and sufficient stability criterion in the limit of infinite aspect ratio, which remains sufficient for arbitrary aspect ratios, and which requires that ||μ|| be in a smaller interval that does depend on the wall radius.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 3652-3663 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Taylor's theory of relaxed toroidal plasmas (states of lowest energy with fixed total magnetic helicity) is extended to include a vacuum between the plasma and the wall. In the extended variational problem, one prescribes, in addition to the helicity and the magnetic fluxes whose conservation follows from the perfect conductivity of the wall, the fluxes whose conservation follows from the assumption that the plasma-vacuum interface is also perfectly conducting (if the wall is a magnetic surface, then one has the toroidal and the poloidal flux in the vacuum). Vanishing of the first energy variation implies a pressureless free-boundary magnetohydrostatic equilibrium with a Beltrami magnetic field in the plasma, and in general with a surface current in the interface. Positivity of the second variation implies that the equilibrium is stable according to ideal magnetohydrodynamics, that it is a relaxed state according to Taylor's theory if the interface is replaced by a wall, and that the surface current is nonzero (at least if there are no closed magnetic field lines in the interface). The plane slab, with suitable boundary conditions to simulate a genuine torus, is investigated in detail. The relaxed state has the same double symmetry as the vessel if, and only if, the prescribed helicity is in an interval that depends on the prescribed fluxes. This interval is determined in the limit of a thin slab. © 2001 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1255-1258 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The theory of resistive free boundary modes localized at the plasma–vacuum interface in a plane slab equilibrium is improved and extended. If one proceeds to sufficiently small wavelengths, then the stability criterion (the current density vector must not have a component along the magnetic field vector in the interface) remains unchanged, but the unstable eigenmodes become independent of resistivity, and their growth rates diverge like the inverse square root of the wavelength. © 1998 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 4494-4498 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Modes localized at a plasma–vacuum interface are studied in plane slab equilibria by using resistive magnetohydrodynamics. Such modes are unstable whenever the current density and the magnetic field are not perpendicular to each other in the interface. The perturbations have no radial nodes but large mode numbers in the other two directions. The instability occurs for a wide range of angles between the nodal lines and the magnetic field lines in the interface and does not depend on the presence of a mode resonant surface. © 1995 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 30 (1989), S. 307-312 
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: Nonlinear eigenvalue problems are considered which are defined by linear homogeneous systems of ordinary differential equations, subject to linear homogeneous boundary conditions, both depending analytically on a complex eigenvalue parameter ω, and on an additional small positive parameter ε such that all solutions have variation O(ε−1). There is, in general, a family of eigenvalues that, in the limit ε→0, become densely spaced and form a definite curve in the complex ω plane (exceptions arise only for some special boundary conditions). This eigenvalue curve is constructed for arbitrary nonsingular systems without turning points. It depends on no details of the boundary conditions other than their type (i.e., the numbers of boundary conditions involving only the left, only the right, or both end points of the interval). In the special cases of either only two-point boundary conditions (such as periodicity) or only one-point boundary conditions with equally many conditions at each end point, and of differential equations involving only even derivatives whose coefficients are constant to leading order, the curve is simply given by the roots of the local dispersion relation. The distance of the eigenvalues from the curve is O(ε) in general, but O(ε1/l log ε−1), with some positive integer l, in special cases.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 2901-2907 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The extension of the theory of relaxed plasmas to external magnetic fields whose field lines intersect the wall is concisely formulated and then applied to the Extrap experiment [J. R. Drake, Plasma Phys. Controlled Fusion 26, 387 (1984)]. It is found that the external octupole field, though not affecting the phenomenon of current saturation, inhibits field reversal at parts of the wall if it is sufficiently strong to generate magnetic x points within the plasma.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 682-683 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The "classical spheromak'' (current density proportional to the magnetic field, spherical plasma–vacuum interface, homogeneous magnetic field at infinity) is unstable to global magnetohydrodynamic tilting modes, even if an arbitrary, perfectly conducting, rigid wall bounds the vacuum.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 535-539 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ordinary plasma corners are studied in two-dimensional magnetohydrostatic equilibria with arbitrary axial current density profiles. The corner structure depends only on the nearby current density profile and the multiplicity of the magnetic stagnation point. A simple corner, for instance, is right-angled if the current density is bounded at the separatrix or diverges more slowly than any power of the poloidal magnetic flux, but is acute-angled or even cusped otherwise. As an application, the magnetohydrodynamic interchange stability criterion is shown to be violated near simple corners in Z-pinch equilibria.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 398-403 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The straight EXTRAP experiment [Phys. Scr. 16, 147 (1977); Nucl. Instrum. Methods 207, 223 (1983)] is modeled as a magnetohydrostatic equilibrium with cylindrical symmetry and a plane magnetic field. The plasma is surrounded by a vacuum that extends to infinity, and in which four axial rod currents are placed symmetrically. The plasma current density is either parallel or antiparallel to the rod currents, thus having one sign throughout. In the stability analysis, the configuration is assumed to have a finite axial length (the boundary conditions appropriate to a rigid perfect conductor are imposed at the two ends), and the rods are assumed to be permeable to the magnetic field. According to ideal magnetohydrodynamics, such an equilibrium is globally unstable to flute modes and/or slip modes unless the rod currents vanish (circular Z pinch). This is due to the increase away from the center of the octupole field of the rods.
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 1291-1292 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Stability of the thermodynamic equilibrium is put forward as a simple test of the validity of dynamic equations, and is applied to perpendicular gyroviscous magnetohydrodynamics (i.e., perpendicular magnetohydrodynamics with gyroviscosity added). This model turns out to be invalid because it predicts exponentially growing Alfvén waves in a spatially homogeneous static equilibrium with scalar pressure.
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