296 research outputs found
Identification of vortexes obstructing the dynamo mechanism in laboratory experiments
The magnetohydrodynamic dynamo effect explains the generation of
self-sustained magnetic fields in electrically conducting flows, especially in
geo- and astrophysical environments. Yet the details of this mechanism are
still unknown, e.g., how and to which extent the geometry, the fluid topology,
the forcing mechanism and the turbulence can have a negative effect on this
process. We report on numerical simulations carried out in spherical geometry,
analyzing the predicted velocity flow with the so-called Singular Value
Decomposition, a powerful technique that allows us to precisely identify
vortexes in the flow which would be difficult to characterize with conventional
spectral methods. We then quantify the contribution of these vortexes to the
growth rate of the magnetic energy in the system. We identify an axisymmetric
vortex, whose rotational direction changes periodically in time, and whose
dynamics are decoupled from those of the large scale background flow, is
detrimental for the dynamo effect. A comparison with experiments is carried
out, showing that similar dynamics were observed in cylindrical geometry. These
previously unexpected eddies, which impede the dynamo effect, offer an
explanation for the experimental difficulties in attaining a dynamo in
spherical geometry.Comment: 25 pages, 12 figures, submitted to Physics of Fluid
Fe-Si-B čestice praha studirane EBSD analizom područja kod velikog uvećanja
We produced amorphous Fe–Si–B soft-magnetic powder using water atomisation. During annealing the powder particles developed a nanocrystalline structure, and annealing at over 700°C led to the formation of ferrite and boride phases. Here we present a high-magnification electron backscatter diffraction (EBSD) mapping analysis of the powder particles, in combination with a field-emission-gun scanning electron microscopy (FEGSEM) analysis. Some of the problems associated with the preparation of the powder particles for the EBSD analysis as well as the drift problems occurring during the EBSD mapping are reported.Amorfni Fe-Si-B magnetni prahovi su bili proizvedeni vodenom atomizacijom. Za vrijeme procesa žarenja čestice praha dobiju nanokristaliničnu strukturu. Žarenje iznad 700 °C prouzroči nastanak feritne i boridne faze. U ovom članku je prikazana EBSD analiza područja kod velikog uvećanja prašnih čestica u kombinaciji s FESEM. Istaknuti su problemi pripreme čestica praha za EBSD analizu kao i problem pomicanja uzorka tokom EBSD analize
Self-Consitent Computation of Transport Barrier Formation by Fluid Drift Turbulence in Tokamak Geometry
Acceleration of particles in imbalanced magnetohydrodynamic turbulence
The present work investigates the acceleration of test particles in balanced
and imbalanced Alfv\'{e}nic turbulence, relevant to the solar-wind problem.
These turbulent states, obtained numerically by prescribing the injection rates
for the ideal invariants, are evolved dynamically with the particles. While the
energy spectrum for balanced and imbalanced states is known, the impact made on
particle heating is a matter of debate, with different considerations giving
different results. By performing direct numerical simulations, resonant and
non-resonant particle accelerations are automatically considered and the
correct turbulent phases are taken into account. For imbalanced turbulence, it
is found that the acceleration rate of charged particles is reduced and the
heating rate diminished. This behaviour is independent of the particle
gyroradius, although particles that have a stronger adiabatic motion (smaller
gyroradius) tend to experience a larger heating.Comment: 5 pages 6 figure
Microstructure of NiTi orthodontic wires observations using transmission electron microscopy
This work presents the results of the microstructure observation of six different types of NiTi orthodontic wires by using Transmission Electron Microscopy (TEM). Within these analyses the chemical compositions of each wire were observed in different places by applying the EDS detector. Namely, the chemical composition in the orthodontic wires is very important because it shows the dependence between the phase temperatures and mechanical properties. Microstructure observations showed that orthodontic wires consist of nano-sized grains containing precipitates of Ti<sub>2</sub>Ni and/or TiC. The first precipitated Ti<sub>2</sub>Ni are rich in Ti, while the precipitated TiC is rich in C. Further investigation showed that there was a difference in average grain size in the NiTi matrix. The sizes of grains in orthodontic wires are in the range from approximately 50 to 160 nm and the sizes of precipitate are in the range from 0,3 μm to 5 μm
Bringing global gyrokinetic turbulence simulations to the transport timescale using a multiscale approach
The vast separation dividing the characteristic times of energy confinement
and turbulence in the core of toroidal plasmas makes first-principles
prediction on long timescales extremely challenging. Here we report the
demonstration of a multiple-timescale method that enables coupling global
gyrokinetic simulations with a transport solver to calculate the evolution of
the self-consistent temperature profile. This method, which exhibits resiliency
to the intrinsic fluctuations arising in turbulence simulations, holds
potential for integrating nonlocal gyrokinetic turbulence simulations into
predictive, whole-device models.Comment: 7 pages, 3 figure
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
Pair Plasma Instability in Homogeneous Magnetic Guide Fields
Pair plasmas, collections of both matter and antimatter particles of equal mass, represent a paradigm for the study of basic plasma science, and many open questions exist regarding these unique systems. They are found in many astrophysical settings, such as gamma-ray bursts, and have recently also been produced in carefully designed laboratory experiments. A central research topic in plasma physics is instability; however, unlike their more common ion–electron siblings, pair plasmas are generally thought to be stable to cross field pressure gradients in homogeneous magnetic fields. It is shown here by means of kinetic full-f simulations that, when a pressure gradient is first established, the Gradient-driven Drift Coupling mode is destabilized and becomes turbulent. Force balance is eventually achieved by a combination of flattened pressure profiles due to turbulent transport and establishment of a magnetic field gradient, saturating the growth. During the unstable phase, key physics can be captured by a δf gyrokinetic description, where it is shown analytically and numerically that parallel particle motion results in a coupling of all electromagnetic field components. A fluid model derived therefrom accurately predicts linear eigenmodes and is used to resolve global profile effects. For laser-based electron–positron plasma experiments, prompt instability is predicted with growth times much shorter than plasma lifetimes. Similarly, growth rates are calculated for the planned APEX experiment as well as gamma-ray burst scenarios, suggesting that the instability may contribute to the early evolution of these systems.</p
Applications of large eddy simulation methods to gyrokinetic turbulence
The Large Eddy Simulation (LES) approach - solving numerically the large
scales of a turbulent system and accounting for the small-scale influence
through a model - is applied to nonlinear gyrokinetic systems that are driven
by a number of different microinstabilities. Comparisons between modeled, lower
resolution, and higher resolution simulations are performed for an experimental
measurable quantity, the electron density fluctuation spectrum. Moreover, the
validation and applicability of LES is demonstrated through a series of
diagnostics based on the free energetics of the system.Comment: 14 pages, 9 figure
Gyrokinetic simulations of microturbulence in tokamak plasmas presenting an electron internal transport barrier, and development of a global version of the GENE code
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