986 research outputs found
Impurity transport in temperature gradient driven turbulence
In the present paper the transport of impurities driven by trapped electron
(TE) mode turbulence is studied. Non-linear (NL) gyrokinetic simulations using
the code GENE are compared with results from quasilinear (QL) gyrokinetic
simulations and a computationally efficient fluid model. The main focus is on
model comparisons for electron temperature gra- dient driven turbulence
regarding the sign of the convective impurity velocity (pinch) and the impurity
density gradient R/LnZ (peaking factor) for zero impurity flux. In particular,
the scaling of the impurity peaking factors with impurity charge Z and with
driving temper- ature gradient is investigated and compared with the results
for Ion Temperature Gradient (ITG) driven turbulence. In addition, the impurity
peaking is compared to the main ion peaking obtained by a self-consistent fluid
calculation of the density gradients corresponding to zero particle fluxes.
For the scaling of the peaking factor with impurity charge Z, a weak
dependence is obtained from NL GENE and fluid simulations. The QL GENE results
show a stronger dependence for low Z impurities and overestimates the peaking
factor by up to a factor of two in this region. As in the case of ITG dominated
turbulence, the peaking factors saturate as Z increases, at a level much below
neoclassical predictions. However, the scaling with Z is weak or reversed as
compared to the ITG case.
The scaling of impurity peaking with the background temperature gradients is
found to be weak in the NL GENE and fluid simulations. The QL results are also
here found to significantly overestimate the peaking factor for low Z values.
For the parameters considered, the background density gradient for zero
particle flux is found to be slightly larger than the corresponding impurity
zero flux gradient.Comment: 23 pages, 13 figures. Submitted to AIP: Physics of Plasma
Statistical description of turbulent transport for flux driven toroidal plasmas
A novel methodology to analyze non-Gaussian probability distribution
functions (PDFs) of intermittent turbulent transport in global full-f
gyrokinetic simulations is presented. In this work, the Auto-Regressive
Integrated Moving Average (ARIMA) model is applied to time series data of
intermittent turbulent heat transport to separate noise and oscillatory trends,
allowing for the extraction of non-Gaussian features of the PDFs. It was shown
that non-Gaussian tails of the PDFs from first principles based gyrokinetic
simulations agree with an analytical estimation based on a two fluid model.Comment: arXiv admin note: text overlap with arXiv:1008.321
Amorphous metallizations for high-temperature semiconductor device applications
The initial results of work on a class of semiconductor metallizations which appear to hold promise as primary metallizations and diffusion barriers for high temperature device applications are presented. These metallizations consist of sputter-deposited films of high T sub g amorphous-metal alloys which (primarily because of the absence of grain boundaries) exhibit exceptionally good corrosion-resistance and low diffusion coefficients. Amorphous films of the alloys Ni-Nb, Ni-Mo, W-Si, and Mo-Si were deposited on Si, GaAs, GaP, and various insulating substrates. The films adhere extremely well to the substrates and remain amorphous during thermal cycling to at least 500 C. Rutherford backscattering and Auger electron spectroscopy measurements indicate atomic diffussivities in the 10 to the -19th power sq cm/S range at 450 C
Particle transport in density gradient driven TE mode turbulence
The turbulent transport of main ion and trace impurities in a tokamak device
in the presence of steep electron density gradients has been studied. The
parameters are chosen for trapped electron (TE) mode turbulence, driven
primarily by steep electron density gradients relevant to H-mode physics, but
with a transition to temperature gradient driven turbulence as the density
gradient flattens. Results obtained through non-linear (NL) and quasilinear
(QL) gyrokinetic simulations using the GENE code are compared with results
obtained from a fluid model. Main ion and impurity transport is studied by
examining the balance of convective and diffusive transport, as quantified by
the density gradient corresponding to zero particle flux (peaking factor).
Scalings are obtained for the impurity peaking with the background electron
density gradient and the impurity charge number. It is shown that the impurity
peaking factor is weakly dependent on impurity charge and significantly smaller
than the driving electron density gradient.Comment: 11 pages, 6 figures. Submitted to Nuclear Fusion SP
Constructing a regular histogram - a comparison of methods
Even for a well-trained statistician the construction of a histogram for a given
real-valued data set is a difficult problem. It is even more difficult to construct
a fully automatic procedure which specifies the number and widths of the bins in
a satisfactory manner for a wide range of data sets. In this paper we compare
several histogram construction methods by means of a simulation study. The study
includes plug-in methods, cross-validation, penalized maximum likelihood and the
taut string procedure. Their performance on different test beds is measured by the
Hellinger distance and the ability to identify the modes of the underlying density
Interaction between turbulence and neoclassical effects in global gyrokinetic simulations with GENE
Two-axis magnetic field sensor
A ferromagnetic thin-film based magnetic field sensor with first and second sensitive direction sensing structures each having a nonmagnetic intermediate layer with two major surfaces on opposite sides thereof having a magnetization reference layer on one and an anisotropic ferromagnetic material sensing layer on the other having a length in a selected length direction and a smaller width perpendicular thereto and parallel to the relatively fixed magnetization direction. The relatively fixed magnetization direction of said magnetization reference layer in each is oriented in substantially parallel to the substrate but substantially perpendicular to that of the other. An annealing process is used to form the desired magnetization directions
Nonlinear structures in interchange mode turbulence
Generation of linearly unstable streamers and nonlinearly saturated zonal flows have been investigated using two-dimensional simulations of interchange mode turbulence. It has been shown that nonlinear diamagnetic interactions tend to inhibit spectral cascades towards larger length scales and consequently suppress the nonlinear excitation of zonal flows. The latter has also been found to be in qualitative agreement with a theoretical analysis based on the reductive perturbation method
Zonal flow generation in ion temperature gradient mode turbulence
In the present work the zonal flow (ZF) growth rate in toroidal
ion-temperature-gradient (ITG) mode turbulence including the effects of
elongation is studied analytically. The scaling of the ZF growth with plasma
parameters is examined for typical tokamak parameter values. The physical model
used for the toroidal ITG driven mode is based on the ion continuity and ion
temperature equations whereas the ZF evolution is described by the vorticity
equation. The results indicate that a large ZF growth is found close to
marginal stability and for peaked density profiles and these effects may be
enhanced by elongation.Comment: 20 pages, 5 figure
Zonal flow generation in collisionless trapped electron mode turbulence
In the present work the generation of zonal flows in collisionless trapped
electron mode (TEM) turbulence is studied analytically. A reduced model for TEM
turbulence is utilized based on an advanced fluid model for reactive drift
waves. An analytical expression for the zonal flow growth rate is derived and
compared with the linear TEM growth, and its scaling with plasma parameters is
examined for typical tokamak parameter values.Comment: 20 pages, 4 figure
- …