The effect of background flow shear on gyrokinetic turbulence in the cold ion limit

Ball, Justin; Brunner, S.; McMillan, B. F.

doi:10.1088/1361-6587/ab1320

Cited by 6 publications

(11 citation statements)

References 55 publications

(151 reference statements)

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“…The parallel flow is one of the main physical features of neoclassical corrections. Therefore to estimate the effect of these corrections, we will use previous studies on the parallel velocity gradient (PVG) instability [81][82][83][84][85][86]. The PVG growth rate is…”

Section: E × B Shearmentioning

confidence: 99%

Toroidal and slab ETG instability dominance in the linear spectrum of JET-ILW pedestals

et al. 2020

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Local linear gyrokinetic simulations show that electron temperature gradient (ETG) instabilities are the fastest growing modes for k y ρ i ≳ 0.1 in the steep gradient region for a JET pedestal discharge (92174) where the electron temperature gradient is steeper than the ion temperature gradient. Here, k y is the wavenumber in the direction perpendicular to both the magnetic field and the radial direction, and ρ i is the ion gyroradius. At k y ρ i ≳ 1, the fastest growing mode is often a novel type of toroidal ETG instability. This toroidal ETG mode is driven at scales as large as k y ρ i ∼ (ρ i /ρ e )L Te /R 0 ∼ 1 and at a sufficiently large radial wavenumber that electron finite Larmor radius effects become important; that is, K x ρ e ∼ 1, where K x is the effective radial wavenumber. Here, ρ e is the electron gyroradius, R 0 is the major radius of the last closed flux surface, and 1/L Te is an inverse length proportional to the logarithmic gradient of the equilibrium electron temperature. The fastest growing toroidal ETG modes are often driven far away from the outboard midplane. In this equilibrium, ion temperature gradient instability is subdominant at all scales and kinetic ballooning modes are shown to be suppressed by E × B shear. ETG modes are very resilient to E × B shear. Heuristic quasilinear arguments suggest that the novel toroidal ETG instability is important for transport.

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Section: E × B Shearmentioning

confidence: 99%

Toroidal and slab ETG instability dominance in the linear spectrum of JET-ILW pedestals

et al. 2020

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“…Though not the most realistic, these simplifications are often used together to facilitate the study of plasma dynamics [37][38][39][40][41]. We will start from the derivation in [42], which models parallel velocity gradient (PVG) turbulence in a slab geometry. By assuming the cold ion limit T i ≪ Z i T e and adiabatic electrons, it rigorously derives two coupled fluid equations that exactly correspond to the full electrostatic, collisionless gyrokinetic model.…”

Section: Benchmarkingmentioning

confidence: 99%

“…No fluid closure is required. We will start from equations ( 16) and ( 17) in [42], but include the modifications to the parallel streaming term arising from non-uniform magnetic shear (i.e. equation ( 23)).…”

Section: Benchmarkingmentioning

confidence: 99%

Local gyrokinetic simulations of tokamaks with non-uniform magnetic shear

Ball

Brunner

2022

Plasma Phys. Control. Fusion

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In this work, we modify the standard ﬂux tube simulation domain to include arbitrary ion gyroradius-scale variation in the radial proﬁle of the safety factor. To determine how to appropriately include such a modiﬁcation, we add a strong ion gyroradius-scale source (inspired by Electron Cyclotron Current Drive) to the Fokker-Planck equation, then perform a multi-scale analysis that distinguishes the fast electrons driven by the source from the slow bulk thermal electrons. This allows us to systematically derive the needed changes to the gyrokinetic model. We ﬁnd new terms that adjust the ion and electron parallel streaming to be along the modiﬁed ﬁeld lines. These terms have been successfully implemented in a gyrokinetic code (while retaining the typical Fourier representation), which enables ﬂux tube studies of non-monotonic safety factor proﬁles and the associated proﬁle shearing. As an illustrative example, we investigate tokamaks with positive versus negative triangularity plasma shaping and ﬁnd that the importance of proﬁle shearing is not signiﬁcantly aﬀected by the change in shape.

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“…At finite shear an imposed Δy corresponds to a simple radial shift of the lowest order rational surfaces, so without loss of generality Δy can be set to zero, but this is no longer true when ŝ = 0 [6]. Accordingly, we have implemented this shifted parallel boundary condition in GENE and benchmarked the changes against analytical ITG and parallel velocity gradient instabilities in the cold ion and shearless limit [27,30]. Including Δy is essential to properly model physically possible magnetic topologies.…”

Section: Introductionmentioning

confidence: 99%

Ultra long turbulent eddies, magnetic topology, and the triggering of internal transport barriers in tokamaks

2022

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Local nonlinear gyrokinetic simulations of tokamak plasmas demonstrate that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns when the magnetic shear is very small. By accurately modeling different field line topologies (e.g. low-order rational, almost rational, or irrational value of the safety factor), we show that the self-interaction of such ``ultra long’’ eddies can dramatically reduce heat transport. This extreme sensitivity of turbulent transport to the safety factor reveals novel strategies to improve confinement, constitutes experimentally testable predictions, and illuminates past observations of internal transport barriers.

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The effect of background flow shear on gyrokinetic turbulence in the cold ion limit

Cited by 6 publications

References 55 publications

Toroidal and slab ETG instability dominance in the linear spectrum of JET-ILW pedestals

Toroidal and slab ETG instability dominance in the linear spectrum of JET-ILW pedestals

Local gyrokinetic simulations of tokamaks with non-uniform magnetic shear

Ultra long turbulent eddies, magnetic topology, and the triggering of internal transport barriers in tokamaks

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