35,283 research outputs found
Nonlinear stabilization of tokamak microturbulence by fast ions
Nonlinear electromagnetic stabilization by suprathermal pressure gradients
found in specific regimes is shown to be a key factor in reducing tokamak
microturbulence, augmenting significantly the thermal pressure electromagnetic
stabilization. Based on nonlinear gyrokinetic simulations investigating a set
of ion heat transport experiments on the JET tokamak, described by Mantica et
al. [Phys. Rev. Lett. 107 135004 (2011)], this result explains the
experimentally observed ion heat flux and stiffness reduction. These findings
are expected to improve the extrapolation of advanced tokamak scenarios to
reactor relevant regimes.Comment: 5 pages, 5 figure
First principles of modelling the stabilization of microturbulence by fast ions
The observation that fast ions stabilize ion-temperature-gradient-driven
microturbulence has profound implications for future fusion reactors. It is
also important in optimizing the performance of present-day devices. In this
work, we examine in detail the phenomenology of fast ion stabilization and
present a reduced model which describes this effect. This model is derived from
the high-energy limit of the gyrokinetic equation and extends the existing
"dilution" model to account for nontrivial fast ion kinetics. Our model
provides a physically-transparent explanation for the observed stabilization
and makes several key qualitative predictions. Firstly, that different classes
of fast ions, depending on their radial density or temperature variation, have
different stabilizing properties. Secondly, that zonal flows are an important
ingredient in this effect precisely because the fast ion zonal response is
negligible. Finally, that in the limit of highly-energetic fast ions, their
response approaches that of the "dilution" model; in particular, alpha
particles are expected to have little, if any, stabilizing effect on plasma
turbulence. We support these conclusions through detailed linear and nonlinear
gyrokinetic simulations.Comment: 29 pages, 10 figures, 3 table
Self-consistent modeling of laminar electrohydrodynamic plumes from ultrasharp needles in cyclohexane
This paper presents a self-consistent model of electrohydrodynamic (EHD) laminar plumes produced by electron injection
from ultra-sharp needle tips in cyclohexane. Since the density of electrons injected into the liquid is well described by the
Fowler-Nordheim field emission theory, the injection law is not assumed. Furthermore, the generation of electrons in
cyclohexane and their conversion into negative ions is included in the analysis. Detailed steady-state characteristics of EHD
plumes under weak injection and space-charge limited injection are studied. It is found that the plume characteristics far from
both electrodes and under weak injection can be accurately described with an asymptotic simplified solution proposed by
Vazquez et al. Physics of Fluids 12, 2809 (2000) when the correct longitudinal electric field distribution and liquid velocity
radial profile are used as input. However, this asymptotic solution deviates from the self-consistently calculated plume
parameters under space-charge limited injection since it neglects the radial variations of the electric field produced by a highdensity
charged core. In addition, no significant differences in the model estimates of the plume are found when the
simulations are obtained either with the Finite Element Method or with a diffusion-free particle method. It is shown that the
model also enables the calculation of the current-voltage (IV) characteristic of EHD laminar plumes produced by electron
field emission, with good agreement with measured values reported in the literature.Ministerio de Economía y Competitividad FIS2014-54539-P
Magnetic compressibility and ion-temperature-gradient-driven microinstabilities in magnetically confined plasmas
The electromagnetic theory of the strongly driven ion-temperature-gradient
(ITG) instability in magnetically confined toroidal plasmas is developed.
Stabilizing and destabilizing effects are identified, and a critical
(the ratio of the electron to magnetic pressure) for stabilization
of the toroidal branch of the mode is calculated for magnetic equilibria
independent of the coordinate along the magnetic field. Its scaling is
where is the characteristic electron
temperature gradient length, and the major radius of the torus. We
conjecture that a fast particle population can cause a similar stabilization
due to its contribution to the equilibrium pressure gradient. For sheared
equilibria, the boundary of marginal stability of the electromagnetic
correction to the electrostatic mode is also given. For a general magnetic
equilibrium, we find a critical length (for electromagnetic stabilization) of
the extent of the unfavourable curvature along the magnetic field. This is a
decreasing function of the local magnetic shear
Can Deflagration-Detonation-Transitions occur in Type Ia Supernovae?
The mechanism for deflagration-detonation-transition (DDT) by turbulent
preconditioning, suggested to explain the possible occurrence of delayed
detonations in Type Ia supernova explosions, is argued to be conceptually
inconsistent. It relies crucially on diffusive heat losses of the burned
material on macroscopic scales. Regardless of the amplitude of turbulent
velocity fluctuations, the typical gradient scale for temperature fluctuations
is shown to be the laminar flame width or smaller, rather than the factor of
thousand more required for a DDT. Furthermore, thermonuclear flames cannot be
fully quenched in regions much larger than the laminar flame width as a
consequence of their simple ``chemistry''. Possible alternative explosion
scenarios are briefly discussed.Comment: 8 pages, uses aastex; added references. Accepted by ApJ Letter
Revisiting the stability of spatially heterogeneous predator-prey systems under eutrophication
We employ partial integro-differential equations to model trophic interaction
in a spatially extended heterogeneous environment. Compared to classical
reaction-diffusion models, this framework allows us to more realistically
describe the situation where movement of individuals occurs on a faster time
scale than the demographic (population) time scale, and we cannot determine
population growth based on local density. However, most of the results reported
so far for such systems have only been verified numerically and for a
particular choice of model functions, which obviously casts doubts about these
findings. In this paper, we analyse a class of integro-differential
predator-prey models with a highly mobile predator in a heterogeneous
environment, and we reveal the main factors stabilizing such systems. In
particular, we explore an ecologically relevant case of interactions in a
highly eutrophic environment, where the prey carrying capacity can be formally
set to 'infinity'. We investigate two main scenarios: (i) the spatial gradient
of the growth rate is due to abiotic factors only, and (ii) the local growth
rate depends on the global density distribution across the environment (e.g.
due to non-local self-shading). For an arbitrary spatial gradient of the prey
growth rate, we analytically investigate the possibility of the predator-prey
equilibrium in such systems and we explore the conditions of stability of this
equilibrium. In particular, we demonstrate that for a Holling type I (linear)
functional response, the predator can stabilize the system at low prey density
even for an 'unlimited' carrying capacity. We conclude that the interplay
between spatial heterogeneity in the prey growth and fast displacement of the
predator across the habitat works as an efficient stabilizing mechanism.Comment: 2 figures; appendices available on request. To appear in the Bulletin
of Mathematical Biolog
Angular momentum transport modeling: achievements of a gyrokinetic quasi-linear approach
QuaLiKiz, a model based on a local gyrokinetic eigenvalue solver is expanded
to include momentum flux modeling in addition to heat and particle fluxes.
Essential for accurate momentum flux predictions, the parallel asymmetrization
of the eigenfunctions is successfully recovered by an analytical fluid model.
This is tested against self-consistent gyrokinetic calculations and allows for
a correct prediction of the ExB shear impact on the saturated potential
amplitude by means of a mixing length rule. Hence, the effect of the ExB shear
is recovered on all the transport channels including the induced residual
stress. Including these additions, QuaLiKiz remains ~10 000 faster than
non-linear gyrokinetic codes allowing for comparisons with experiments without
resorting to high performance computing. The example is given of momentum pinch
calculations in NBI modulation experiments
Diamagnetic Suppression of Component Magnetic Reconnection at the Magnetopause
We present particle-in-cell simulations of collisionless magnetic
reconnection in a system (like the magnetopause) with a large density asymmetry
across the current layer. In the presence of an ambient component of the
magnetic field perpendicular to the reconnection plane the gradient creates a
diamagnetic drift that advects the X-line with the electron diamagnetic
velocity. When the relative drift between the ions and electrons is of the
order the Alfven speed the large scale outflows from the X-line necessary for
fast reconnection cannot develop and the reconnection is suppressed. We discuss
how these effects vary with both the plasma beta and the shear angle of the
reconnecting field and discuss observational evidence for diamagnetic
stabilization at the magnetopause.Comment: 10 pages, 10 figures; accepted by JGR; agu2001.cls and agu.bst
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