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  • 1
    Electronic Resource
    Electronic Resource
    Threshold for neoclassical magnetic islands in a low collision frequency tokamak (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 248-265 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A kinetic theory for magnetic islands in a low collision frequency tokamak plasma is presented. Self-consistent equations for the islands' width, w, and propagation frequency, ω, are derived. These include contributions from the perturbed bootstrap current and the toroidally enhanced ion polarization drift. The bootstrap current is independent of the island propagation frequency and provides a drive for the island in tokamak plasmas when the pressure decreases with an increasing safety factor. The polarization drift is frequency dependent, and therefore its effect on the island stability cannot be deduced unless ω is known. This frequency is determined by the dominant dissipation mechanism, which for low effective collision frequency, νeff=ν/ε〈ω, is governed by the electrons close to the trapped/passing boundary. The islands are found to propagate in the electron diamagnetic direction in which case the polarization drift is stabilizing and results in a threshold width for island growth, which is of the order of the ion banana width. At larger island widths the polarization current term becomes small and the island evolution is determined by the bootstrap current drive and Δ′ alone, where Δ′ is a measure of the magnetic free energy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871830
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  • 2
    Electronic Resource
    Electronic Resource
    Theory of isolated, small-scale magnetic islands in a high temperature tokamak plasma (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 4575-4585 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A theory for the existence of noninteracting small-scale, "drift'' magnetic islands in a high temperature tokamak plasma is presented. This situation contrasts with that discussed by Rebut and Hugon [Plasma Phys. Controlled Fusion 33, 1085 (1991)] which involves a background "sea'' of magnetic turbulence caused by island overlap. The islands are driven by the effect of finite ion Larmor radius on the particle drifts and they propagate with a velocity comparable to the diamagnetic velocity. In contrast with the work of Smolyakov [Plasma Phys. Controlled Fusion 35, 657 (1993)] collisions are assumed to be rare. Although the saturated island size is independent of the collision frequency in the model discussed here, collisions play a crucial role in determining the frequency of the magnetic islands. An estimate is made of the anomalous heat transport which results from the fluctuations in the electrostatic potential associated with these magnetic islands. The predicted thermal diffusivity has several, but not all, of the characteristics of the Rebut–Lallia–Watkins transport model. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871016
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  • 3
    Electronic Resource
    Electronic Resource
    First physics results from the MAST Mega-Amp Spherical Tokamak : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2101-2106 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: First physics results are presented from MAST (Mega-Amp Spherical Tokamak), one of the new generation of purpose built spherical tokamaks (STs) now commencing operation. Some of these results demonstrate, for the first time, the novel effects of low aspect ratio, for example, the enhancement of resistivity due to neo-classical effects. H-mode is achieved and the transition to H-mode is accompanied by a tenfold steepening of the edge density gradient which may enable the successful application of electron Bernstein wave heating in STs. Studies of halo currents show that these less than expected from conventional tokamak results, and measurements of divertor power loading confirm that most of the power flows to the outer strike points, easing the power handling on the inner points (a critical issue for STs). © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1352595
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  • 4
    Electronic Resource
    Electronic Resource
    Access to second stability region for coupled peeling-ballooning modes in tokamaks (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 873-876 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The peeling mode restricts access to the second stability region of the ideal ballooning mode at the tokamak plasma edge. Using a two-dimensional, high toroidal mode number eigenmode code employing a model tokamak equilibrium, it is shown that a window to second stability exists for a sufficiently deep magnetic well. The different mode structures of the various eigenmode branches are studied. In particular, when access to second ballooning stability exists, a ballooning mode perturbation at the first stability boundary can extend deep into the plasma core, and then instability is likely to result in large scale loss of plasma energy. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873326
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetohydrodynamic stability of tokamak edge plasmas (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2687-2700 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new formalism for analyzing the magnetohydrodynamic stability of a limiter tokamak edge plasma is developed. Two radially localized, high toroidal mode number n instabilities are studied in detail: a peeling mode and an edge ballooning mode. The peeling mode, driven by edge current density and stabilized by edge pressure gradient, has features which are consistent with several properties of tokamak behavior in the high confinement "H"-mode of operation, and edge localized modes (or ELMs) in particular. The edge ballooning mode, driven by the pressure gradient, is identified; this penetrates ∼n1/3 rational surfaces into the plasma (rather than ∼n1/2, expected from conventional ballooning mode theory). Furthermore, there exists a coupling between these two modes and this coupling provides a picture of the ELM cycle.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872956
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  • 6
    Electronic Resource
    Electronic Resource
    Numerical studies of edge localized instabilities in tokamaks (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 1277-1286 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new computational tool, edge localized instabilities in tokamaks equilibria (ELITE), has been developed to help our understanding of short wavelength instabilities close to the edge of tokamak plasmas. Such instabilities may be responsible for the edge localized modes observed in high confinement H-mode regimes, which are a serious concern for next step tokamaks because of the high transient power loads which they can impose on divertor target plates. ELITE uses physical insight gained from analytic studies of peeling and ballooning modes to provide an efficient way of calculating the edge ideal magnetohydrodynamic stability properties of tokamaks. This paper describes the theoretical formalism which forms the basis for the code.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1459058
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  • 7
    Electronic Resource
    Electronic Resource
    Resistive wall modes and nonuniform wall rotation (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4062-4072 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The resistive wall mode (RWM) poses a threat to many plasma confinement devices. The continuous rotation of the wall relative to the plasma makes it appear perfectly conducting, because of the skin effect, but this is ineffective if the perturbation locks to the wall. This raises the question of whether a nonuniformly rotating wall is more effective. In this paper we discuss the effect of such nonuniform wall rotation, in both the toroidal and poloidal directions, on resonant and nonresonant RWMs. In the case of toroidal rotation it is shown that at large wall velocity both the resonant and nonresonant RWMs are stabilized, even though the nonresonant mode rotates with the maximum wall velocity. In the case of poloidal rotation RWMs do not lock to the wall and have a complicated behavior at intermediate velocities. However they are again stabilized by large wall velocity. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1388035
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  • 8
    Electronic Resource
    Electronic Resource
    Toroidal coupling of ideal magnetohydrodynamic instabilities in tokamak plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 584-592 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A theoretical framework is developed to describe the ideal magnetohydrodynamic (MHD) stability properties of axisymmetric toroidal plasmas. The mode structure is described by a set of poloidal harmonics in configuration space. The energy functional, δW, is then determined by a set of matrix elements that are computed from the interaction integrals between these harmonics. In particular, the formalism may be used to study the stability of finite-n ballooning modes. Using for illustration the s-α equilibrium, salient features of the n(large-closed-square)∞ stability boundary can be deduced from an appropriate choice of test function for these harmonics. The analysis can be extended to include the toroidal coupling of a free-boundary kink eigenfunction to the finite-n ideal ballooning mode. A unified stability condition is derived that describes the external kink mode, a finite-n ballooning mode, and their interaction. The interaction term plays a destabilizing role that lowers the instability threshold of the toroidally coupled mode. These modes may play a role in understanding plasma edge phenomena, L–H physics and edge localized modes (ELMs). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871886
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  • 9
    Electronic Resource
    Electronic Resource
    Ideal magnetohydrodynamic stability of the tokamak high-confinement-mode edge region : The 40th annual meeting of the division of plasma physics of the american physical society (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1925-1934 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The ideal magnetohydrodynamic (MHD) stability of the tokamak edge is analyzed, with particular emphasis on radially localized instabilities; it is proposed that these are responsible for edge pressure gradient limits and edge localized modes (ELMS). Data and stability calculations from DIII-D [to appear in Proceedings of the 16th International Conference on Fusion Energy, Yokohama (International Atomic Energy Agency, Vienna, 1998), Paper No. IAEA-F1-CN-69/EX8/1] tokamak equilibria indicate that two types of instability are important: the ballooning mode (driven by pressure gradient) and the peeling mode (driven by current density). The characteristics of these instabilities, and their coupling, are described based on a circular cross-section, large aspect ratio model of the tokamak equilibrium. In addition, preliminary results are presented from an edge MHD stability code which is being developed to analyze general geometry tokamak equilibria; an interpretation of the density threshold to access the high-confinement-mode (H-mode), observed on COMPASS-D [Plasma Phys. Controlled Fusion 38, 1091 (1996)] is provided by these results. Experiments on DIII-D and the stability calculations indicate how to control ELMs by plasma shaping.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873492
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  • 10
    Electronic Resource
    Electronic Resource
    The mode structure of high-n resistive ballooning modes (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 56-63 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The higher-order corrections (in an n−1/2 expansion) to resistive ballooning theory are analyzed in order to gain information about the radial structure of the Δ'-driven modes. This higher-order theory also predicts that the ballooning phase angle θ0 (which is undetermined in the leading-order theory) must be chosen so as to maximize the value of Δ'. The importance of applying this maximization is illustrated by an analytical calculation of Δ' as a function of θ0 for the s-α model in the limit of small α. It is demonstrated that for this case, one should choose θ0=90° and that the resulting value of Δ' can be very different from that obtained by setting θ0=0, as is frequently imposed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860403
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