52 research outputs found
Nonlinear growth of zonal flows by secondary instability in general magnetic geometry
We present a theory of the nonlinear growth of zonal flows in magnetized
plasma turbulence, by the mechanism of secondary instability. The theory is
derived for general magnetic geometry, and is thus applicable to both tokamaks
and stellarators. The predicted growth rate is shown to compare favorably with
nonlinear gyrokinetic simulations, with the error scaling as expected with the
small parameter of the theory.Comment: New J. Phys. 201
The anisotropic redistribution of free energy for gyrokinetic plasma turbulence in a Z-pinch
For a Z-pinch geometry, we report on the nonlinear redistribution of free
energy across scales perpendicular to the magnetic guide field, for a turbulent
plasma described in the framework of gyrokinetics. The analysis is performed
using a local flux-surface approximation, in a regime dominated by
electrostatic fluctuations driven by the entropy mode, with both ion and
electron species being treated kinetically. To explore the anisotropic nature
of the free energy redistribution caused by the emergence of zonal flows, we
use a polar coordinate representation for the field-perpendicular directions
and define an angular density for the scale flux. Positive values for the
classically defined (angle integrated) scale flux, which denote a direct energy
cascade, are shown to be also composed of negative angular sections, a fact
that impacts our understanding of the backscatter of energy and the way in
which it enters the modeling of sub-grid scales for turbulence. A definition
for the flux of free energy across each perpendicular direction is introduced
as well, which shows that the redistribution of energy in the presence of zonal
flows is highly anisotropic
Gyrokinetic Large Eddy Simulations
The Large Eddy Simulation (LES) approach is adapted to the study of plasma
microturbulence in a fully three-dimensional gyrokinetic system. Ion
temperature gradient driven turbulence is studied with the {\sc GENE} code for
both a standard resolution and a reduced resolution with a model for the
sub-grid scale turbulence. A simple dissipative model for representing the
effect of the sub-grid scales on the resolved scales is proposed and tested.
Once calibrated, the model appears to be able to reproduce most of the features
of the free energy spectra for various values of the ion temperature gradient
Free energy cascade in gyrokinetic turbulence
In gyrokinetic theory, the quadratic nonlinearity is known to play an
important role in the dynamics by redistributing (in a conservative fashion)
the free energy between the various active scales. In the present study, the
free energy transfer is analyzed for the case of ion temperature gradient
driven turbulence. It is shown that it shares many properties with the energy
transfer in fluid turbulence. In particular, one finds a forward (from large to
small scales), extremely local, and self-similar cascade of free energy in the
plane perpendicular to the background magnetic field. These findings shed light
on some fundamental properties of plasma turbulence, and encourage the
development of large eddy simulation techniques for gyrokinetics.Comment: 4 pages, 2 Postscript figure
Global gyrokinetic simulations of ITG turbulence in the configuration space of the Wendelstein 7-X stellarator
We study the effect of turbulent transport in different magnetic
configurations of the Weldenstein 7-X stellarator. In particular, we performed
direct numerical simulations with the global gyrokinetic code GENE-3D, modeling
the behavior of Ion Temperature Gradient turbulence in the Standard,
High-Mirror, and Low-Mirror configurations of W7-X. We found that the
Low-Mirror configuration produces more transport than both the High-Mirror and
the Standard configurations. By comparison with radially local simulations, we
have demonstrated the importance of performing global nonlinear simulations to
predict the turbulent fluxes quantitatively
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
Assessing global ion thermal confinement in critical-gradient-optimized stellarators
We investigate the confinement properties of two recently devised
quasi-helically symmetric stellarator configurations, HSK and QSTK. Both have
been optimized for large critical gradients of the ion temperature gradient
mode, which is an important driver of turbulent transport in magnetic
confinement fusion devices. To predict the resulting core plasma profiles, we
utilize an advanced theoretical framework based on the gyrokinetic codes GENE
and GENE-3D, coupled to the transport code TANGO. Compared to the HSX
stellarator, both HSK and QSTK achieve significantly higher core-to-edge
temperature ratios, partly thanks to their smaller aspect ratios, with the
other part due to more detailed shaping of the magnetic geometry achieved
during optimization. The computed confinement time, however, is less sensitive
to core temperature than edge temperature, simply due to the disproportionate
influence the edge has on stored plasma energy. We therefore emphasize the
possible benefits of further optimizing turbulence in the outer core region,
and the need to include accurate modelling of confinement in the edge region in
order to assess overall plasma performance of turbulence optimized
stellarators
Gyrokinetic GENE simulations of DIII-D near-edge L-mode plasmas
We present gyrokinetic simulations with the GENE code addressing the
near-edge region of an L-mode plasma in the DIII-D tokamak. At radial position
, simulations with the ion temperature gradient increased by
above the nominal value give electron and ion heat fluxes that are in
simultaneous agreement with the experiment. This gradient increase is
consistent with the combined statistical and systematic uncertainty of
the Charge Exchange Recombination Spectroscopy (CER) measurements at the level. Multi-scale simulations are carried out with realistic mass
ratio and geometry for the first time in the near-edge. These multi-scale
simulations suggest that the highly unstable ion temperature gradient (ITG)
modes of the flux-matched ion-scale simulations suppress electron-scale
transport, such that ion-scale simulations are sufficient at this location. At
radial position , nonlinear simulations show a hybrid state of ITG
and trapped electron modes~(TEMs), which was not expected from linear
simulations. The nonlinear simulations reproduce the total experimental heat
flux with the inclusion of shear effects and an
increase in the electron temperature gradient by . This gradient
increase is compatible with the combined statistical and systematic uncertainty
of the Thomson scattering data at the level. These results are
consistent with previous findings that gyrokinetic simulations are able to
reproduce the experimental heat fluxes by varying input parameters close to
their experimental uncertainties, pushing the validation frontier closer to the
edge region.Comment: 14 pages, 17 figures, published in Physics of Plasma
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