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  • 1
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A proof of principle magnetic feedback stabilization experiment has been carried out to suppress the resistive wall mode (RWM), a branch of the ideal magnetohydrodynamic (MHD) kink mode under the influence of a stabilizing resistive wall, on the DIII-D tokamak device [Plasma Phys. Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159; Phys. Plasmas 1, 1415 (1994)]. The RWM was successfully suppressed and the high beta duration above the no-wall limit was extended to more than 50 times the resistive wall flux diffusion time. It was observed that the mode structure was well preserved during the time of the feedback application. Several lumped parameter formulations were used to study the feedback process. The observed feedback characteristics are in good qualitative agreement with the analysis. These results provide encouragement to future efforts towards optimizing the RWM feedback methodology in parallel to what has been successfully developed for the n=0 vertical positional control. Newly developed MHD codes have been extremely useful in guiding the experiments and in providing possible paths for the next step. © 2001 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 192-195 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An "Intelligent Shell" [C. M. Bishop, Plasma Phys. Controlled Fusion 31, 1179 (1985)] is a resistive wall equipped with feedback loops intended to make it appear ideally conducting to the plasma. Two problems associated with intelligent shells are addressed, using a simple model solved numerically, namely, the influence on the stabilization efficiency of the number of feedback loops employed and of gaps between loops of the feedback system. Without gaps between the sensor loops it is found that a modest number of loops per period (such as eight) is almost as stabilizing as infinitely many loops. It is also found that the stabilizing effect is decreasing rapidly by increasing gaps between loops. © 1998 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1524-1529 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The issues concerning the response of a plasma, at or near a singular surface, to a magnetic perturbation with a phase velocity different from the plasma flow velocity, are important for a number of phenomena. Among these are ideal and nonideal magnetohydrodynamic stability of plasmas with shear flow or a flow relative to a resistive wall, sensitivity of rotating plasma to field errors, and the "locked mode'' phenomenon. Models for the singular surface response have been tested against results from "magnetic braking'' experiments in DIII-D [R. J. La Haye et al., Nucl. Fusion 32, 2119 (1992)]. Previous models are found unable to account for all of the experimental observations. A new heuristic nonlinear model presented in the paper may account for the observations. A key element in the model is turbulence developed at the singular surface; the turbulence is assumed driven by the singular layer dissipation and is assumed to impede the singular current through an anomalous resistivity. When the perturbation amplitude is sufficiently large, a positive feedback mechanism exists, since in the regime of interest, dissipation increases with decreasing singular current. For small perturbation amplitudes this mechanism is not operative so that previous models for the response may be valid. © 1996 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. 1976-1981 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The problem of plasma flow relative to a modulated magnetic field has been the subject of several studies. One motivation for studying this problem is the possibility of using a deliberately imposed surface of magnetic islands as a means of velocity profile control. This subject is also of importance for the study of stability against ideal and resistive magnetohydrodynamic (MHD) modes and the topic of locked modes. A two-dimensional (2-D) MHD simulation code is used to examine the behavior of a plasma flowing, in steady state, past a modulated magnetic field in "slab geometry.'' It is shown that at "low'' velocities the stress is dominated by the Maxwell and the viscosity terms and that forces are exchanged between the plasma and the magnetic field in a narrow boundary surrounding the island. It is found that the island is suppressed when the viscous force at the separatrix exceeds the maximum force that can be supported by an island. For "high'' velocities (velocities beyond the critical velocity for island suppression), the stress is dominated by the Maxwell and the Reynolds terms, and the exchange of forces is taking place in a narrow region around the point where the plasma flow velocity matches the Alfvén speed. © 1995 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1656-1660 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: "Pressure driven tokamaks'' are special tokamaks for which the rate of injection of energy and mass (e.g., by neutral beams) is so large that no drive for the toroidal current is needed. Examples of pressure driven tokamak equilibria are found numerically; for these examples, both the poloidal and the toroidal magnetic fields vanish in a region around the plasma center. Thus, the ratio between the plasma pressure and the magnetic field pressure is large, namely of order unity. Therefore, pressure driven tokamaks appear attractive for fusion reactors; it is, however, an open question whether there exist magnetohydrodynamically stable pressure driven equilibria. © 1996 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1495-1499 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The validity of a series expansion proposed previously [T. H. Jensen and M. S. Chu, Phys. Fluids 27, 2881 (1984)] for describing general Taylor configurations of magnetized plasmas has been reexamined because an apparent paradox was realized. From analyses of simple cases which can be dealt with mostly analytically, it is concluded that the paradox is a Gibbs phenomenon, and that the series expansion is valid. © 1999 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 2997-3000 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A feedback system, which essentially makes a resistive wall appear ideally conducting, is discussed. Such a system applied to a resistive wall surrounding a plasma will stabilize certain modes which would be unstable in the absence of the feedback system. The system discussed is similar to the "intelligent shell" by Bishop [Plasma Phys. Controlled Fusion 31, 1179 (1989)]; it utilizes a number of autonomous subsystems, each covering only a fraction of the resistive wall. A model example discussed suggests that only relatively few autonomous subsystems are needed and that the requirements of the electronics appear modest. © 1997 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2236-2241 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of a subsonic toroidal flow on the linear magnetohydrodynamic stability of a tokamak plasma surrounded by an external resistive wall is studied. A complex non-self-adjoint eigenvalue problem for the stability of general kink and tearing modes is formulated, solved numerically, and applied to high β tokamaks. Results indicate that toroidal plasma flow, in conjunction with dissipation in the plasma, can open a window of stability for the position of the external wall. In this window, stable plasma beta values can significantly exceed those predicted by the Troyon scaling law with no wall. Computations utilizing experimental data indicate good agreement with observations. © 1995 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 2757-2761 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The "intelligent shell" [C. M. Bishop, Plasma Phys. Contr. Fusion 31, 1179 (1989)] utilizes a feedback system intended to make a resistive wall appear perfectly conducting to a plasma. It can thus be used for stabilizing modes of the plasma which are unstable when the plasma is surrounded by a resistive wall, but stable if the wall were perfectly conducting. Several concepts of magnetic confinement, such as reversed field pinches, spheromaks, and tokamaks may benefit from an intelligent shell. The paper addresses the question of the dependency of the stabilizing effect on the gain and bandwidth of the feedback circuits (assumed linear). A simple model for the phenomena involved is made and solved numerically for certain parameter values. A characteristic time of the model is a resistive time τ of the wall; the calculations suggest that an upper cutoff frequency of ∼50/τ and sufficient gain provides a stabilization similar to that of ideal circuits with infinite bandwidth and gain. Under laboratory circumstances with τ∼10−3 s it is thus practical to obtain mass produced components which make the circuits as effective as ideal circuits. © 1999 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1650-1656 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The subject of magnetic islands, due to error fields in devices such as tokamaks, is of importance for understanding energy and angular momentum confinement observed in existing devices and for a rational determination of the accuracy required for the coils of future large devices. Simple arguments show that, for parameter values of interest, the viscous drag on plasma inside an island, caused by a toroidal flow of plasma outside the island, can affect the islands significantly. This subject is studied using a numerical implementation of a simple, almost ideal magnetohydrodynamical, two-dimensional model. A simplified version of the model is also studied analytically. The study suggests that the islands shrink somewhat as the drag force is applied and that there is an upper limit to the total drag force that a surface of islands can sustain; when this maximum drag force is approached, large gradients in the current density are formed, which may give rise to instabilities that make invalid the two-dimensional assumption.
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