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  • 11
    Electronic Resource
    Electronic Resource
    Angular momentum injection into a Penning–Malmberg trap (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 917-930 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is demonstrated using conventional fluid theory that angular momentum can be injected into a single component plasma confined in a Penning–Malmberg trap via an externally generated, oscillating, nonaxisymmetric, electric field. The torque exerted on the plasma by the electric field is a highly nonmonotonic function of the plasma angular rotation velocity. The torque vs angular velocity curve is dominated by sharp resonances at which the angular phase velocity of a particular poloidal harmonic of the external field matches the plasma angular rotation velocity. The torque exerted on the plasma by a given poloidal harmonic is negative when the field rotates faster than the plasma, and vice versa. This rather surprising behavior is shown to be entirely consistent with a standard result in hydrodynamic theory, but is generally not observed in present-day experiments. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872208
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  • 12
    Electronic Resource
    Electronic Resource
    Stabilization of the resistive wall mode using a fake rotating shell (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2641-2652 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Tokamak plasma performance can, in theory, be greatly improved if the so-called "resistive wall mode'' is stabilized. This can be achieved by spinning the plasma rapidly, but such a scheme would be difficult to implement in a reactor setting. A more promising approach is to apply external feedback in order to make a resistive shell placed around the plasma act like a perfect conductor. A scheme is outlined by which a network of feedback controlled conductors surrounding the plasma can be made to act like a rotating shell. This fake rotating shell combined with a stationary conventional shell (e.g., the vacuum vessel) can completely stabilize the resistive wall mode. The gain, bandwidth, current, and power requirements of the feedback amplifiers are extremely modest. A previously proposed stabilization scheme (the intelligent shell) is also investigated, and is compared with the fake rotating shell concept. The main disadvantage of the former scheme is that it requires a high gain. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871521
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  • 13
    Electronic Resource
    Electronic Resource
    Feedback stabilization of the resistive shell mode in a tokamak fusion reactor (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 2519-2531 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Stabilization of the "resistive shell mode" is vital to the success of the "advanced tokamak" concept. The most promising reactor relevant approach is to apply external feedback using, for instance, the previously proposed "fake rotating shell" scheme [R. Fitzpatrick and T. H. Jensen, Phys. Plasmas 3, 2641 (1996)]. This scheme, like other simple feedback schemes, only works if the feedback controlled conductors are located inside the "critical radius" at which a perfectly conducting shell is just able to stabilize the ideal external kink mode. In general, this is not possible in a reactor, since engineering constraints demand that any feedback controlled conductors be placed outside the neutron shielding blanket (i.e., relatively far from the edge of the plasma). It is demonstrated that the fake rotating shell feedback scheme can be modified so that it works even when the feedback controlled conductors are located well beyond the critical radius. The gain, bandwidth, current, and total power requirements of such a feedback system for a reactor sized plasma are estimated to be less than 100, a few Hz, a fews tens of kA, and a few MW, respectively. These requirements could easily be met using existing technology. It is concluded that feedback stabilization of the resistive shell mode is possible in a tokamak fusion reactor. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872231
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  • 14
    Electronic Resource
    Electronic Resource
    Stability of coupled tearing and twisting modes in tokamaks (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 3308-3336 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A dispersion relation is derived for resistive modes of arbitrary parity in a tokamak plasma. At low mode amplitude, tearing and twisting modes which have nonideal magnetohydrodynamical (MHD) behavior at only one rational surface at a time in the plasma are decoupled via sheared rotation and diamagnetic flows. At higher amplitude, more unstable "compound'' modes develop which have nonideal behavior simultaneously at many surfaces. Such modes possess tearing parity layers at some of the nonideal surfaces, and twisting parity layers at others, but mixed parity layers are generally disallowed. At low mode number, "compound'' modes are likely to have tearing parity layers at all of the nonideal surfaces in a very low-β plasma, but twisting parity layers become more probable as the plasma β is increased. At high mode number, unstable twisting modes which exceed a critical amplitude drive conventional magnetic island chains on alternate rational surfaces, to form an interlocking structure in which the O points and X points of neighboring chains line up.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870482
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  • 15
    Electronic Resource
    Electronic Resource
    Formation and locking of the "slinky mode" in reversed-field pinches (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1168-1193 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The formation and breakup of the "slinky mode" in a Reversed-Field Pinch (RFP) is investigated analytically. The slinky mode is a toroidally localized, coherent interference pattern in the magnetic field, which corotates with the plasma at the reversal surface. This mode forms, via a series of bifurcations, as a result of the nonlinear coupling of multiple m=1 core tearing modes. The slinky mode breaks up via a second series of bifurcations. However, the typical mode amplitude below which slinky breakup is triggered is much smaller than that above which slinky formation occurs. Analytic expressions for the slinky formation and breakup thresholds are obtained in all regimes of physical interest. The locking of the slinky mode to a static error field is also investigated analytically. Either the error field arrests the rotation of the plasma at the reversal surface before the formation of the slinky mode, so that the mode subsequently forms as a nonrotating mode, or the slinky mode forms as a rotating mode and subsequently locks to the error field. Analytic expressions for the locking and unlocking thresholds are obtained in all regimes of physical interest. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873361
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  • 16
    Electronic Resource
    Electronic Resource
    Effect of a nonuniform resistive wall on the stability of tokamak plasmas (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 2931-2939 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of a nonuniform resistive wall on the stability of plasma magnetohydrodynamical (MHD) modes is examined. For the case of a tokamak plasma interacting with a wall possessing toroidally nonuniform electrical resistance, the kink mode dispersion relation is found to reduce to a surprisingly simple form, provided that the scale of variation of the resistance is sufficiently large. The influence of a wall with toroidal gaps on tokamak plasma stability is investigated in some detail. Under some circumstances, kink modes are found to "explode'' through the gaps with ideal growth rates. A similar investigation is made for a modular wall constructed of alternate thick and thin sections.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870533
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  • 17
    Electronic Resource
    Electronic Resource
    The effect of a partial resistive shell on the magnetohydrodynamical stability of tokamak plasmas (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 4043-4068 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A comprehensive theory is developed to determine the effect of a partial resistive shell on the growth rate of the external kink mode in a low-β, large aspect-ratio, circular flux-surface tokamak. In most cases, it is possible to replace a partial shell by a complete "effective shell" of somewhat larger radius. In fact, the radius of the effective shell can be used to parametrize the ability of a partial shell to moderate the growth of the external kink mode. It is necessary to draw a distinction between "resonant shells," for which the eddy currents excited in the shell are able to flow in unidirectional continuous loops around the plasma, and "nonresonant shells," for which this is not possible. As a general rule, resonant shells perform better than similar nonresonant shells. The theory is used to derive some general rules regarding the design of incomplete passive stabilizing shells. The theory is also employed to determine the effectiveness of two realistic feedback stabilization schemes for the resistive shell mode, both of which only require a relatively small number of independent feedback controlled conductors external to the plasma. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872525
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  • 18
    Electronic Resource
    Electronic Resource
    Helical temperature perturbations associated with tearing modes in tokamak plasmas (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 825-838 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An investigation is made into the electron temperature perturbations associated with tearing modes in tokamak plasmas. It is found that there is a critical magnetic island width below which the conventional picture where the temperature is flattened inside the separatrix is invalid. This effect comes about because of the stagnation of magnetic field lines in the vicinity of the rational surface and the finite parallel thermal conductivity of the plasma. Islands whose widths lie below the critical value are not destabilized by the perturbed bootstrap current, unlike conventional magnetic islands. This effect may provide an explanation for some puzzling experimental results regarding error field-induced magnetic reconnection. The critical island width is found to be fairly substantial in conventional tokamak plasmas, provided that the long mean-free path nature of parallel heat transport and the anomalous nature of perpendicular heat transport are taken into account in the calculation. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871434
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  • 19
    Electronic Resource
    Electronic Resource
    Control of tearing modes in toroidal fusion experiments using "designer" error fields (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2760-2770 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is demonstrated that a magnetic island chain formed by a saturated tearing instability in a toroidal magnetic fusion device can lock to a special class of externally generated magnetic perturbation in a stabilizing phase. The theoretical apparatus needed to design such perturbations is outlined. These special perturbations—which are termed "designer" error fields—could be used to control the amplitudes of tearing modes in toroidal magnetic fusion experiments without the requirement of fast phase modulation. This type of control would be far more feasible in a reactor environment than conventional active feedback control via external magnetic perturbations. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1365956
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  • 20
    Electronic Resource
    Electronic Resource
    Nonlinear dynamics of feedback modulated magnetic islands in toroidal plasmas (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4983-4995 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An analysis is presented of the dynamics of a helical magnetic island chain embedded in a toroidal plasma, in the presence of an externally imposed, rotating, magnetic perturbation of the same helicity. Calculations are carried out in the large aspect-ratio, zero-β, resistive magnetohydrodynamical limit, and incorporate a realistic treatment of plasma viscosity. There are three regimes of operation, depending on the modulation frequency (i.e., the difference in rotation frequency between the island chain and the external perturbation). For slowly modulated islands, the perturbed velocity profile extends across the whole plasma. For strongly modulated islands, the perturbed velocity profile is localized around the island chain, but remains much wider than the chain. Finally, for very strongly modulated islands, the perturbed velocity profile collapses to a boundary layer on the island separatrix, plus a residual profile which extends a few island widths beyond the separatrix. Analytic expressions are obtained for the perturbed velocity profile, the island equation of motion, and the island width evolution equation in each of these three regimes. The ion polarization correction to the island width evolution equation, which has previously been reported to be stabilizing, is found to be destabilizing in all three regimes. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1318360
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