Kinetic theory of tearing instabilities

Drake, J. F.; Lee, Y. C.

doi:10.1063/1.862017

Cited by 459 publications

(466 citation statements)

References 8 publications

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“…First results in this area have aleady been obtained in the 1970's by several authors. [8][9][10][11] Moreover, it has recently been proposed that in finite β ITG/TEM turbulence, linearly stable microtearing modes can be excited via a nonlinear coupling to zonal modes. [5] Interestingly, this can explain both the occurrence of stochastic fields [6] and the quadratic scaling of the magnetic transport which contradicts standard quasilinear transport models.…”

Section: Introductionmentioning

confidence: 99%

Gyrokinetic prediction of microtearing turbulence in standard tokamaks

et al. 2012

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First global gyrokinetic simulations of microtearing instabilities in ASDEX Upgrade geometry provide increasing evidence for the existence of these modes in standard tokamaks. It is found that even in only moderately large devices, nonlocal effects like profile shearing are negligible, supporting the use of an efficient flux-tube approach. Nonlinear gyrokinetic simulations show that the resulting level of magnetic electron heat flux can be experimentally relevant.

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Section: Introductionmentioning

confidence: 99%

Gyrokinetic prediction of microtearing turbulence in standard tokamaks

et al. 2012

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“…The standard approach involves an asymptotic matching between an outer MHD region with a kinetic description within the resonance layer 22,23 . The solution for the outer region features a discontinuity in the first derivative of the perturbed magnetic fieldB z across the resonance layer, which captures the destabilizing influence of magnetic shear that drives tearing.…”

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confidence: 99%

Role of electron physics in the development of turbulent magnetic reconnection in collisionless plasmas

et al. 2011

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Magnetic reconnection releases energy explosively as field lines break and reconnect in plasmas ranging from the Earth's magnetosphere to solar eruptions and astrophysical applications. Collisionless kinetic simulations have shown that this process involves both ion and electron kinetic-scale features, with electron current layers forming nonlinearly during the onset phase and playing an important role in enabling field lines to break 1-4 . In larger two-dimensional studies, these electron current layers become highly extended, which can trigger the formation of secondary magnetic islands 5-10 , but the influence of realistic three-dimensional dynamics remains poorly understood. Here we show that, for the most common type of reconnection layer with a finite guide field, the three-dimensional evolution is dominated by the formation and interaction of helical magnetic structures known as flux ropes. In contrast to previous theories 11 , the majority of flux ropes are produced by secondary instabilities within the electron layers. New flux ropes spontaneously appear within these layers, leading to a turbulent evolution where electron physics plays a central role.Thin current layers are the preferred locations for magnetic reconnection to develop. The most common configuration in nature is guide-field geometry, where the rotation of magnetic field across the layer is less than 180 • . Present theoretical ideas of how reconnection proceeds in these configurations are deeply rooted in early analytical work 11 that, if correct, would imply a direct transition to three-dimensional (3D) turbulence due to a broad spectrum of interacting tearing instabilities. At the core of this idea is the notion that a spectrum of tearing instabilities develops across the initial current sheet for perturbations satisfying the local resonance condition. As these modes grow, the resulting magnetic islands would overlap, leading to stochastic magnetic-field lines and a turbulent evolution. Recently, this type of scenario was proposed as a mechanism for accelerating energetic particles during reconnection 12 . Similar ideas for generating turbulence have been studied in fusion plasmas 13 using resistive magnetohydrodynamics (MHD) and two-fluid 14 models. Alternatively, other researchers have imposed turbulent fluctuations within MHD models in an attempt to understand the consequences 15 . In either case, these results are not applicable to the highly collisionless environment of the magnetosphere, where reconnection is initiated within kinetic ion-scale current layers. The ability to study the self-consistent generation of turbulence during magnetic reconnection with first-principles 3D simulations has only become feasible in the past year.1 Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA, 2 University of California San Diego, La Jolla, California 92093, USA. *e-mail: daughton@lanl.gov. tearing instability gives rise to flux ropes as illustrated by an isosurface of the particle density coloured by the magnitude of the curr...

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“…1). 19 As noted above, these simulations were run in the local limit which is valid only for asymptotically small ρ * =ρ s /a→0. At r/a=0.6 in this NSTX discharge ρ * ≈1/130 so that the simulated domain width (L x =80 ρ s ) spans ~60% of the minor radius.…”

Section: Exb Shearmentioning

confidence: 99%

“…By microtearing we refer to resonant electromagnetic modes with tearing parity (flux-surface averaged 〈A || 〉 is finite) on a rational flux surface, q=m/n, (where q is the safety factor) at high toroidal (n) and poloidal (m) mode number that are stable to conventional resistive tearing instability [15,16]. Because of the strong stabilizing influence of field line bending (represented by a large negative tearing parameter that asymptotes to Δ′=-2m/r at high m [17]), some other mechanism must be responsible for instability, such as the time-dependent thermal force [18][19][20][21][22][23][24][25][26][27][28] or trapped-passing particle interaction [29][30][31]. Either mechanism requires finite electron collisionality (ν e ) as well as sufficient electron temperature gradient (∇T e ) and electron beta (β e ) for instability to ensue.…”

Section: Introductionmentioning

confidence: 99%

Simulation Of Microtearing Turbulence In NSTX

Guttenfelder¹,

Kaye²,

Nevins³

et al. 2012

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Thermal energy confinement times in NSTX dimensionless parameter scans increase with decreasing collisionality. While ion thermal transport is neoclassical, the source of anomalous electron thermal transport in these discharges remains unclear, leading to considerable uncertainty when extrapolating to future ST devices at much lower collisionality.Linear gyrokinetic simulations find microtearing modes to be unstable in high collisionality discharges. First non-linear gyrokinetic simulations of microtearing turbulence in NSTX show they can yield experimental levels of transport. Magnetic flutter is responsible for almost all the transport (~98%), perturbed field line trajectories are globally stochastic, and a test particle stochastic transport model agrees to within 25% of the simulated transport. Most significantly, microtearing transport is predicted to increase with electron collisionality, consistent with the observed NSTX confinement scaling. While this suggests microtearing modes may be the source of electron thermal transport, the predictions are also very sensitive to electron temperature gradient, indicating the scaling of the instability threshold is important. In addition, microtearing turbulence is susceptible to suppression via sheared E×B flows as experimental values of E×B shear (comparable to the linear growth rates) dramatically reduce the transport below experimental values. Refinements in numerical resolution and physics model assumptions are expected to minimize the apparent discrepancy. In cases where the predicted transport is strong, calculations suggest that a proposed polarimetry diagnostic may be sensitive to the magnetic perturbations associated with the unique structure of microtearing turbulence.

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Kinetic theory of tearing instabilities

Cited by 459 publications

References 8 publications

Gyrokinetic prediction of microtearing turbulence in standard tokamaks

Gyrokinetic prediction of microtearing turbulence in standard tokamaks

Role of electron physics in the development of turbulent magnetic reconnection in collisionless plasmas

Simulation Of Microtearing Turbulence In NSTX

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