Magnetic island evolution in hot ion plasmas

Ishizawa, A.; Waelbroeck, F. L.; Fitzpatrick, Richard; Horton, W.; Nakajima, N.

doi:10.1063/1.4739291

Cited by 18 publications

(19 citation statements)

References 37 publications

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“…The resulting expression of the tearing instability index in the subsonic regime ω < k // C s , appropriate to the experiments of this paper, is presented in [46] and has been extensively studied numerically in [47] validating, for small as well as large islands the generic expression:…”

Section: Further Discussion Of the Instability Conditions For Ntms In...mentioning

confidence: 99%

Effects of central electron cyclotron power on plasma rotation and on triggerless onset of NTMs in the TCV tokamak

et al. 2015

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Experiments in the TCV tokamak show that high power central electron cyclotron heating (ECH) and current drive (ECCD) produce significant direct modification of the plasma rotation profile, as well as an effect on the equilibrium current density profile. In a regime of unsteady rotation, these effects contribute to the onset of neoclassical tearing instabilities, in the absence of triggers such as sawteeth, edge localised modes (ELMS) or relevant ‘error’ fields. In turn the growing tearing modes' breaking axisymmetry provides a nonlinear magnetic torque which converts the power absorption in a co-directed rotation with a flattening of the profile at the rational surfaces. The experimental results are presented and discussed in the context of theoretical models of neoclassical toroidal viscosity and ion inertial effects.

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Section: Further Discussion Of the Instability Conditions For Ntms In...mentioning

confidence: 99%

Effects of central electron cyclotron power on plasma rotation and on triggerless onset of NTMs in the TCV tokamak

et al. 2015

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“…16 This technique of imposing a magnetic island has been used to determine the electrostatic potential in rotating islands and analyse the adiabatic response of ions, 17 to determine the impact of small rotating islands on the bootstrap current, 18 to study the impact of a static island on ITG turbulence, 12,13,16 to prove the existence of a vortex mode related to the presence of the island, 15 and to analyse the stability of magnetic islands in reduced two-fluid models. 19 This approach, where the magnetic island is externally imposed, is based on the time scale separation between ITG and tearing modes. Essentially, the total magnetic field (including island) is imposed as B ¼ B 0 þ dB, where B 0 is the background magnetic field and dB is the perturbation.…”

Section: Description Of the Model Normalizations Used In This Pamentioning

confidence: 99%

Verification of a magnetic island in gyro-kinetics by comparison with analytic theory

et al. 2015

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A rotating magnetic island is imposed in the gyrokinetic code GKW, when finite differences are used for the radial direction, in order to develop the predictions of analytic tearing mode theory and understand its limitations. The implementation is verified against analytics in sheared slab geometry with three numerical tests that are suggested as benchmark cases for every code that imposes a magnetic island. The convergence requirements to properly resolve physics around the island separatrix are investigated. In the slab geometry, at low magnetic shear, binormal flows inside the island can drive Kelvin-Helmholtz instabilities which prevent the formation of the steady state for which the analytic theory is formulated. V C 2015 AIP Publishing LLC.In a tokamak, a magnetic equilibrium consisting of nested non-axisymmetric flux surfaces exhibits a current singularity layer where the safety factor is rational, i.e., where q ¼ m/n, with m and n two integers, respectively, the poloidal and toroidal mode numbers of a radial magnetic perturbation dB r ðh; uÞ, called tearing mode. 1-3 The surfaces given by the relation q ¼ m/n are called resonant surfaces. This singularity in the parallel current is clearly nonphysical and must be regularized by non-ideal, typically collisional, effects leading to magnetic re-connection, 4 which represents the destruction of the resonant surfaces into island chains. The formation of magnetic islands represents a change in the magnetic topology. These resistive instabilities are found in both astrophysical and laboratory plasmas. 5 In the case of laboratory plasmas and in particular, in the case of tokamaks, the presence of magnetic islands can have strong consequences for the core confinement, 6 due to the fact that particles can travel across the island following the perturbed field lines which connect one side of the island with the other. This mechanism is usually referred to as parallel streaming and is the term we use in the following. This increase of the (radial) transport leads to the flattening of the radial profiles and therefore to a loss of energy which eventually represents a loss of core confinement.Because of the impact that magnetic islands have on confinement, it is essential to analyse their stability. The nonlinear evolution of the perturbation leading to the formation of magnetic islands is given by the generalized Rutherford equations 6where r s is the radial position of the resonant surface, s R is the characteristic resistive time, D 0 is the matching parameter from the external solution, f 0 is the position of the island in the binormal direction rf and the functions D and F t represent, respectively, the free energy available for reconnection and the acceleration in the binormal direction of the island. In these equations, we have introduced also the radial halfwidth w and the rotation frequency x of the island in the binormal direction. The functions D and F t can be written in terms of the non inductive parallel current J n:i: k as follows:where X is a flux label...

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“…In order to investigate the self-consistent couplings of ITG turbulence and tearing modes in a simple, tractable form, we employ a simple five-field two-fluid model [21,40,41] in a Cartesian slab geometry [21,[41][42][43][44] similar to that used in previous studies, considered together only in Ishizawa et al [21,41]. The simulation radial and poloidal domains span the range −a ≤ x ≤ a and −b/2 ≤ y < b/2 respectively, with Dirichlet boundary conditions on all fluctuating fields at x = ±a and periodic boundary conditions at y = ±b/2.…”

Section: A Model Overviewmentioning

confidence: 99%

Dynamics of ion temperature gradient turbulence and transport with a static magnetic island

et al. 2016

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Understanding the interaction mechanisms between large-scale magnetohydrodynamic instabilities and small-scale drift-wave microturbulence is essential for predicting and optimizing the performance of magnetic confinement based fusion energy experiments. We report progress on understanding these interactions using both analytic theory and numerical simulations performed with the BOUT++ [B. Dudson et al., Comput. Phys. Comm. 180, 1467(2009] framework. This work focuses upon the dynamics of the ion temperature gradient instability in the presence of a background static magnetic island, using a weakly electromagnetic two-dimensional five-field fluid model. It is found that the island width must exceed a threshold size (comparable to the turbulent correlation length in the no-island limit) to significantly impact the turbulence dynamics, with the primary impact being an increase in turbulent fluctuation and heat flux amplitudes. The turbulent radial ion energy flux is shown to localize near the X-point, but does so asymmetrically in the poloidal dimension. An effective turbulent resistivity which acts upon the island outer layer is also calculated, and shown to always be significantly (10x -100x) greater than the collisional resistivity used in the simulations.

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Magnetic island evolution in hot ion plasmas

Cited by 18 publications

References 37 publications

Effects of central electron cyclotron power on plasma rotation and on triggerless onset of NTMs in the TCV tokamak

Effects of central electron cyclotron power on plasma rotation and on triggerless onset of NTMs in the TCV tokamak

Verification of a magnetic island in gyro-kinetics by comparison with analytic theory

Dynamics of ion temperature gradient turbulence and transport with a static magnetic island

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