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  • 1
    Electronic Resource
    Electronic Resource
    Stationary symmetric magnetohydrodynamic flows (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 3544-3564 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The problem of translation symmetric stationary magnetohydrodynamic (MHD) flow is analyzed with respect to the different permitted flow regimes. A minimal set of equilibrium functions is constructed facilitating the complete explicit solution in the case of self-similar flow. It is found that limiting line and Alfvén singularities leads to a division in four distinct velocity regimes which cannot be connected by continuous flows. Instead, the three types of MHD shocks appear, enabling the system to pass from the high to the low speed regimes. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872251
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  • 2
    Electronic Resource
    Electronic Resource
    Calculation of resistive magnetohydrodynamic spectra in tokamaks (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1554-1561 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Resistive magnetohydrodynamic spectra of toroidal plasmas are calculated using the recently developed Jacobi–Davidson eigenvalue solver. Poloidal mode coupling in finite aspect ratio tokamaks yields gaps in the ideal Alfvén continuous spectrum. If resistivity is included, the ideal continua disappear and are replaced by damped global waves located on specific curves in the complex frequency plane. The end points of these curves join the tips of the ideal continua and the boundaries of the ideal spectral gap. The eigenfunctions of the waves on these resistive curves are shown to have definite parity in the poloidal harmonics. It is shown that for very small toroidicity the topology of the resistive spectrum is completely different from the cylindrical one. Independent of the size of the inverse aspect ratio the ideal gap remains visible in the resistive spectrum. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873409
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  • 3
    Electronic Resource
    Electronic Resource
    Once more: The continuous spectrum of ideal magnetohydrodynamics (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3143-3154 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A controversy on the existence of continuous spectra of ideal magnetohydrodynamics in addition to the well-known Alfvén and slow continua, dating back to a conjecture by Grad [Proc. Natl. Acad. Sci. USA 70, 3377 (1973)] and revived by Lashmore-Davies, Thyagaraja, and Cairns [Phys. Plasmas 4, 3243 (1997)], is once more resolved by demonstrating that the resolvent operator is bounded in the relevant domain: There are no additional continua. In addition, the solution of the initial value problem is constructed in terms of the three-dimensional Green's dyadic, which is free of apparent singularities and clearly exhibits the classical continua as δ functions on the diagonal. This construction provides the connection with the proper and improper normal modes and shows that the local dynamics on the magnetic surfaces is described by the classical continua. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873041
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  • 4
    Electronic Resource
    Electronic Resource
    Modeling the excitation of global Alfvén modes by an external antenna in the Joint European Torus (JET) (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 1605-1613 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The active excitation of global Alfvén modes using the saddle coils in the Joint European Torus (JET) [Plasma Physics and Controlled Nuclear Fusion Research 1984, Proceedings of the 10th International Conference, London (International Atomic Energy Agency, Vienna, 1985), Vol. 1, p. 11] as the external antenna, will provide information on the damping of global modes without the need to drive the modes unstable. For the modeling of the Alfvén mode excitation, the toroidal resistive magnetohydrodynamics (MHD) code CASTOR (Complex Alfvén Spectrum in TORoidal geometry) [18th EPS Conference On Controlled Fusion and Plasma Physics, Berlin, 1991, edited by P. Bachmann and D. C. Robinson (The European Physical Society, Petit-Lancy, 1991), Vol. 15, Part IV, p. 89] has been extended to calculate the response to an external antenna. The excitation of a high-performance, high beta JET discharge is studied numerically. In particular, the influence of a finite pressure is investigated. Weakly damped low-n global modes do exist in the gaps in the continuous spectrum at high beta. A pressure-driven global mode is found due to the interaction of Alfvén and slow modes. Its frequency scales solely with the plasma temperature, not like a pure Alfvén mode with a density and magnetic field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871310
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  • 5
    Electronic Resource
    Electronic Resource
    Low frequency Alfvén waves induced by toroidal flows (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4208-4222 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The magnetohydrodynamic (MHD) spectra of toroidally rotating, axisymmetric tokamaks are analyzed. The continuous spectrum equations for the coupled localized Alfvén and slow modes are derived. The effect of flow on this spectrum is threefold: Doppler shift, centrifugal, and Coriolis effects enter. By exploiting a low-β, large aspect ratio expansion, the newly found low frequency Δm=0 Alfvén gap [van der Holst et al., Phys. Rev. Lett. 84, 2865 (2000)] located on the rational mode surfaces is analyzed. This gap is created by the geodesic curvature of the field lines, together with finite pressure, and by centrifugal and Coriolis effects. It is determined by a three mode interaction involving a central Alfvén mode and two sideband slow modes. From the same scheme, another new purely flow-induced gap inside the Δm=0 gap is found. Also, toroidal flow-induced Alfvén eigenmodes (TFAE) are found in the Δm=0 gap. All of these waves may be useful for MHD spectroscopy. Since the gap structures as well as the global wave are in the low frequency regime, they will have important implications for stability. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1308084
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  • 6
    Electronic Resource
    Electronic Resource
    Comment on "Alfvén ‘resonance' reconsidered: Exact equations for wave propagation across a cold inhomogeneous plasma'' [Phys. Plasmas 1, 3523 (1994)] (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3550-3551 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In a recent paper Bellan [P. M. Bellan, Phys. Plasmas 1, 3523 (1994)] criticizes magnetohydrodynamics for not being an accurate model of plasma behavior at low frequencies and leading to the incorrect prediction of singularities associated with the Alfvén wave resonances. Grave errors of the paper having to do with misunderstanding of what is a physical model and with misrepresentation of the original contributions to the field are exposed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871471
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  • 7
    Electronic Resource
    Electronic Resource
    On the existence of a thermal continuum in nonadiabatic magnetohydrodynamic spectra (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 866-868 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The linear magnetohydrodynamic (MHD) spectrum of an infinite straight cylinder is investigated, taking nonadiabatic terms in the energy equation into account. It is shown that in nonideal MHD, a new continuum arises. Unlike the "classical'' Alfvén and slow continua, this thermal continuum consists of modes that are purely exponentially growing or decaying in time. Thus unstable continua can be encountered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859842
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  • 8
    Electronic Resource
    Electronic Resource
    Free boundary resistive modes in tokamaks (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1545-1558 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: There exist a number of observations of magnetohydrodynamic (MHD) activity that can be related to resistive MHD modes localized near the plasma boundary. To study the stability of these modes, a free boundary description of the plasma is essential. The resistive plasma–vacuum boundary conditions have been implemented in the fully toroidal resistive spectral code castor (Complex Alfvén Spectrum in Toroidal Geometry) [Proceedings of the 18th Conference on Controlled Fusion and Plasma Physics, Berlin, edited by P. Bachmann and D. C. Robinson (European Physical Society, Petit-Lancy, Switzerland, 1991), p. 89]. The influence of a free boundary, as compared to a fixed boundary on the stability of low-m tearing modes, is studied. It is found that the stabilizing (toroidal) effect of a finite pressure due the plasma compression is lost in the free boundary case for modes localized near the boundary. Since the stabilization due to the favorable average curvature in combination with a pressure gradient near the boundary is small, the influence of the pressure on the stability is much less important for free boundary modes than for fixed boundary modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860894
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  • 9
    Electronic Resource
    Electronic Resource
    rf stabilization of external kink modes in the presence of a resistive wall (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2366-2372 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The problem of controlling external kink instabilities that arise on the slow time scale characteristic of magnetic field diffusion through a conducting wall with finite resistivity is considered. An expression for the growth rate of these "resistive wall'' kink modes is derived in a low-beta cylindrical tokamak model assuming a thin wall and small resistivity, including the additional stabilizing effect of a spatially localized, externally applied surface force. It is shown that the marginal stability condition is identical to that of the ideal kink in the absence of a conducting wall. The model is applied to ponderomotive stabilization arising from the near field of an ion-Bernstein-wave (IBW) antenna system. It is found that radio-frequency (rf) fields below the marginal stability threshold can significantly reduce the growth rate of the m=2 instability. The required rf fields can be attained using present IBW antennas with Ωi(approximately-less-than)ω〈2Ωi in tokamaks with relatively low magnetic fields and may therefore be useful for high-beta and second-stability studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859502
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  • 10
    Electronic Resource
    Electronic Resource
    Quenching of external kink modes by localized radio-frequency fields (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 804-814 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The quenching of external kink modes by a spatially localized, externally applied surface force is studied in a low-beta cylindrical tokamak model. It is shown that m=2 modes may be stabilized by the application of a radio frequency (rf) field in the ion cyclotron range of frequencies (ICRF) excited by either a toroidal or a poloidal antenna structure surrounding the plasma. Whereas linear mode coupling induced by toroidal localization is negligible, that caused by poloidal localization may degrade the stabilization and, thus, require increased power levels for stability. The model is directly applicable to the ponderomotive force exerted by an evanescent slow wave in the ion cyclotron range of frequencies. Quantitative estimates are made for ponderomotive stabilization of kink modes by means of ion-Bernstein-wave (IBW) antennas. This method of stabilization has the advantage that the antenna system does not have to match the spatial or temporal structure of the unstable modes. For typical IBW antenna systems, it is found that the required rf fields for kink stabilization are reasonable if the current profile is not too flat.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859005
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