Threshold for neoclassical magnetic islands in a low collision frequency tokamak

Wilson, H. R.; Connor, J. W.; Hastie, R. J.; Hegna, C. C.

doi:10.1063/1.871830

Cited by 287 publications

(506 citation statements)

References 18 publications

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“…In particular, it was realized that the propagation of small islands through the plasma would induce resonant current perturbations affecting mode stability [10,11]. Depending on the direction of propagation of the initial, small, island, the current induced by the ion polarization drift could be stabilizing or destabilizing.…”

Section: Introductionmentioning

confidence: 99%

Observation of spontaneous neoclassical tearing modes

Fredrickson¹

2002

Physics of Plasmas

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We present data in this paper from the Tokamak Fusion Test Reactor (TFTR) which challenges the commonly held belief that extrinsic MHD events such as sawteeth or ELMs are required to provide the seed islands that trigger Neoclassical Tearing Modes (NTMs). While sawteeth are reported to provide the trigger for most of the NTMs on DIII-D and ASDEX-U, the majority of NTMs seen in TFTR occur in plasmas without sawteeth, that is which are above the β threshold for sawtooth stabilization. Examples of NTMs appearing in the absence of any detectable extrinsic MHD activity will be shown. Conversely, large n=1 modes in plasmas above the NTM β threshold generally do not triggerNTMs. An alternative mechanism for generating seed islands will be discussed.2

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Section: Introductionmentioning

confidence: 99%

Observation of spontaneous neoclassical tearing modes

Fredrickson¹

2002

Physics of Plasmas

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“…25 In this model, the predicted ion polarization induced threshold is on the order of the ion banana width in very collisionless plasmas. The effect of this work is not included in the present simulations.…”

Section: Discussionmentioning

confidence: 99%

Computational modeling of neoclassical and resistive magnetohydrodynamic tearing modes in tokamaks

Gianakon¹,

Hegna²,

Callen³

1996

Physics of Plasmas

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Numerical studies of the nonlinear evolution of magnetohydrodynamic-type tearing modes in three-dimensional toroidal geometry with neoclassical effects are presented. The inclusion of neoclassical physics introduces an additional free-energy source for the nonlinear formation of magnetic islands through the effects of a bootstrap current in Ohm's law. The neoclassical tearing mode is demonstrated to be destabilized in plasmas which are otherwise ⌬Ј stable, albeit once an island width threshold is exceeded. The plasma pressure dynamics and neoclassical tearing growth is shown to be sensitive to the choice of the ratio of the parallel to perpendicular diffusivity ( ʈ / Ќ ). The study is completed with a demonstration and theoretical comparison of the threshold for single helicity neoclassical magnetohydrodynamic tearing modes, which is described based on parameter scans of the local pressure gradient, the ratio of perpendicular to parallel pressure diffusivities Ќ / ʈ , and the magnitude of an initial seed magnetic perturbation.

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“…The polarization current also plays a role, possibly stabilizing NTMs even when there is temperature flattening inside the island. 19 Understanding the role of competing stabilizing and destabilizing effects for seed islands will be the subject of future research. For our second example of heat flow dynamics in tokamaks, we explore the localized wall heat load that results in a NIMROD simulation of DIII-D discharge 87009.…”

Section: Application Of Nonlocal Closures In Tokamaksmentioning

confidence: 99%

Nonlocal closures for plasma fluid simulations

et al. 2004

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The application of fluid models in studies of transport and macroscopic stability of magnetized, nearly collisionless plasmas requires closure relations that are inherently nonlocal. Such closures address the fact that particles are capable of carrying information over macroscopic parallel scale lengths. In this work, generalized closures that embody Landau, collisional and particle-trapping physics are derived and discussed. A gyro/bounce-averaged drift kinetic equation is solved via an expansion in eigenfunctions of the pitch-angle scattering operator and the resulting system of algebraic equations is solved by integrating along characteristics. The desired closure moments take the form of integral equations involving perturbations in the flow and temperature along magnetic field lines. Implementation of the closures in massively parallel plasma fluid simulation codes is also discussed. This implementation includes the use of a semi-implicit time advance of the fluid equations to stabilize the dominant closure terms which are introduced explicitly. Application of the nonlocal, parallel heat flow closure, q ʈ , in studies of temperature flattening across helical magnetic islands in toroidal geometry reveal a scaling of temperature versus critical island width for flattening of Tϳw d Ϫ1.5 . This result predicts more robust flattening at small island widths when compared to the diffusive scaling, Tϳw d Ϫ1.7 , which assumes a Braginskii-type parallel heat conductivity. Preliminary application of q ʈ to tokamak disruption simulations shows qualitative agreement of wall heat loads with experimental observations, smooth distribution in toroidal angle, and striation in the poloidal direction along the wall.

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Threshold for neoclassical magnetic islands in a low collision frequency tokamak

Cited by 287 publications

References 18 publications

Observation of spontaneous neoclassical tearing modes

Observation of spontaneous neoclassical tearing modes

Computational modeling of neoclassical and resistive magnetohydrodynamic tearing modes in tokamaks

Nonlocal closures for plasma fluid simulations

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