245 research outputs found
Wave-driven dynamo action in spherical magnetohydrodynamic systems
Hydrodynamic and magnetohydrodynamic numerical studies of a mechanically
forced two-vortex flow inside a sphere are reported. The simulations are
performed in the intermediate regime between the laminar flow and developed
turbulence where a hydrodynamic instability is found to generate internal waves
with a characteristic m=2 zonal wave number. It is shown that this
time-periodic flow acts as a dynamo although snapshots of the flow as well as
the mean flow are not dynamos. The magnetic fields' growth rate exhibits
resonance effects depending on the wave frequency. Furthermore, a cyclic
self-killing and self-recovering dynamo based on the relative alignment of the
velocity and magnetic fields is presented. The phenomena are explained in terms
of a mixing of non-orthogonal eigenstates of the time dependent linear operator
of the magnetic induction equation. The potential relevance of this mechanism
to dynamo experiments is discussed.Comment: 11 pages, 13 figure
Erratum to: Potential of promotion of alleles by genome editing to improve quantitative traits in livestock breeding programs
Global turbulence simulations of the tokamak edge region with GRILLIX
Turbulent dynamics in the scrape-off layer (SOL) of magnetic fusion devices
is intermittent with large fluctuations in density and pressure. Therefore, a
model is required that allows perturbations of similar or even larger magnitude
to the time-averaged background value. The fluid-turbulence code GRILLIX is
extended to such a global model, which consistently accounts for large
variation in plasma parameters. Derived from the drift reduced Braginskii
equations, the new GRILLIX model includes electromagnetic and electron-thermal
dynamics, retains global parametric dependencies and the Boussinesq
approximation is not applied. The penalisation technique is combined with the
flux-coordinate independent (FCI) approach [F. Hariri and M. Ottaviani,
Comput.Phys.Commun. 184:2419, (2013); A. Stegmeir et al., Comput.Phys.Commun.
198:139, (2016)], which allows to study realistic diverted geometries with
X-point(s) and general boundary contours. We characterise results from
turbulence simulations and investigate the effect of geometry by comparing
simulations in circular geometry with toroidal limiter against realistic
diverted geometry at otherwise comparable parameters. Turbulence is found to be
intermittent with relative fluctuation levels of up to 40% showing that a
global description is indeed important. At the same time via direct comparison,
we find that the Boussinesq approximation has only a small quantitative impact
in a turbulent environment. In comparison to circular geometry the fluctuations
are reduced in diverted geometry, which is related to a different zonal flow
structure. Moreover, the fluctuation level has a more complex spatial
distribution in diverted geometry. Due to local magnetic shear, which differs
fundamentally in circular and diverted geometry, turbulent structures become
strongly distorted in the perpendicular direction and are eventually damped
away towards the X-point
Fast transport simulations with higher-fidelity surrogate models for ITER
A fast and accurate turbulence transport model based on quasilinear
gyrokinetics is developed. The model consists of a set of neural networks
trained on a bespoke quasilinear GENE dataset, with a saturation rule
calibrated to dedicated nonlinear simulations. The resultant neural network is
approximately eight orders of magnitude faster than the original GENE
quasilinear calculations. ITER predictions with the new model project a fusion
gain in line with ITER targets. While the dataset is currently limited to the
ITER baseline regime, this approach illustrates a pathway to develop
reduced-order turbulence models both faster and more accurate than the current
state-of-the-art
Core transport and pedestal characteristics of nitrogen seeded H-mode discharges in ASDEX Upgrade
Direct observation of the turbulent emf and transport of magnetic field in a liquid sodium experiment
International audienceFor the first time, we have directly measured the transport of a vector magnetic field by isotropic turbulence in a high Reynolds number liquid metal flow. In analogy with direct measurements of the turbulent Reynolds stress (turbulent viscosity) that governs momentum transport, we have measured the turbulent electromotive force (emf) by simultaneously measuring three components of velocity and magnetic fields, and computed the correlations that lead to mean-field current generation. Furthermore, we show that this turbulent emf tends to oppose and cancel out the local current, acting to increase the effective resistivity of the medium, i.e., it acts as an enhanced magnetic diffusivity. This has important implications for turbulent transport in astrophysical objects, particularly in dynamos and accretion disks
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