19 research outputs found
Condensation of microturbulence-generated shear flows into global modes
In full flux-surface computer studies of tokamak edge turbulence, a spectrum
of shear flows is found to control the turbulence level and not just the
conventional (0,0)-mode flows. Flux tube domains too small for the large
poloidal scale lengths of the continuous spectrum tend to overestimate the
flows, and thus underestimate the transport. It is shown analytically and
numerically that under certain conditions dominant (0,0)-mode flows independent
of the domain size develop, essentially through Bose-Einstein condensation of
the shear flows.Comment: 5 pages, 4 figure
Transport control by coherent zonal flows in the core/edge transitional regime
3D Braginskii turbulence simulations show that the energy flux in the
core/edge transition region of a tokamak is strongly modulated - locally and on
average - by radially propagating, nearly coherent sinusoidal or solitary zonal
flows. The flows are geodesic acoustic modes (GAM), which are primarily driven
by the Stringer-Winsor term. The flow amplitude together with the average
anomalous transport sensitively depend on the GAM frequency and on the magnetic
curvature acting on the flows, which could be influenced in a real tokamak,
e.g., by shaping the plasma cross section. The local modulation of the
turbulence by the flows and the excitation of the flows are due to wave-kinetic
effects, which have been studied for the first time in a turbulence simulation.Comment: 5 pages, 5 figures, submitted to PR
Quasi-Two-Dimensional Dynamics of Plasmas and Fluids
In the lowest order of approximation quasi-twa-dimensional dynamics of planetary atmospheres and of plasmas in a magnetic field can be described by a common convective vortex equation, the Charney and Hasegawa-Mirna (CHM) equation. In contrast to the two-dimensional Navier-Stokes equation, the CHM equation admits "shielded vortex solutions" in a homogeneous limit and linear waves ("Rossby waves" in the planetary atmosphere and "drift waves" in plasmas) in the presence of inhomogeneity. Because of these properties, the nonlinear dynamics described by the CHM equation provide rich solutions which involve turbulent, coherent and wave behaviors. Bringing in non ideal effects such as resistivity makes the plasma equation significantly different from the atmospheric equation with such new effects as instability of the drift wave driven by the resistivity and density gradient. The model equation deviates from the CHM equation and becomes coupled with Maxwell equations. This article reviews the linear and nonlinear dynamics of the quasi-two-dimensional aspect of plasmas and planetary atmosphere starting from the introduction of the ideal model equation (CHM equation) and extending into the most recent progress in plasma turbulence.U. S. Department of Energy DE-FG05-80ET-53088Ministry of Education, Science and Culture of JapanFusion Research Cente
Overcoming of One More Pitfall in Boundary Element Calculations with Computer Simulations of Ion-Selective Electrode Response
Computer simulations
of ion-selective membrane electrodes using
diffusion layer models based on finite-differences principle for calculating
diffusion processes in both phases and taking into account the local
ion exchange equilibrium at the interface are successfully used for
clarifying and even predicting the influence of different diffusion
factors on several time-dependent characteristics of electrodes. It
is shown here that a well-established approach based on the assumption
of the constant concentration of the interfering ion in the sample
solution fails for solutions containing strongly interfering ions
where the concentration of the interfering ion in the boundary layer
of the solution can be far lower in comparison with its concentration
in the bulk. The limitation is demonstrated by a drastic discrepancy
between experimental and calculated curves for the dependence of potential
on time. This limitation can be overcome by taking into account the
change of the interfering ion concentration in the boundary layer
in accordance with the electroneutrality condition. A good agreement
between simulation results and experimental data is demonstrated
An Interface Equilibria-Triggered Time-Dependent Diffusion Model of the Boundary Potential and Its Application for the Numerical Simulation of the Ion-Selective Electrode Response in Real Systems
A simple
dynamic model of the phase boundary potential of ion-selective
electrodes is presented. The model is based on the calculations of
the concentration profiles of the components in membrane and sample
solution phases by means of the finite difference method. The fundamental
idea behind the discussed model is that the concentration gradients
in both membrane and sample solution phases determine only the diffusion
of the components inside the corresponding phases but not the transfer
across the interface. The transfer of the components across the interface
at any time is determined by the corresponding local interphase equilibria.
According to the presented model, each new calculation cycle begins
with the correction of the components’ concentrations in the
near-boundary (first) layers of the membrane and solution, based on
the constants of the interphase equilibria and the concentrations
established at a given time as a result of diffusion. The corrected
concentrations of the components in the boundary layers indicate the
start of a new cycle every time with respect to the calculations of
diffusion processes inside each phase from the first layer to the
second one, and so on. In contrast to the well-known Morf’s
model, the above-mentioned layers do not comprise an imaginary part
and are entirely localized in the corresponding phases, and this allows
performing the calculations of the equilibrium concentrations by taking
into account material balance for each component. The model remains
operational for any realistic scenarios of the electrode functioning.
The efficiency and predictive ability of the proposed model are confirmed
by comparing the results of calculations with the experimental data
on the dynamics of the potential change of a picrate-selective electrode
in nitrate solutions when determining the selectivity coefficients
using the methods recommended by IUPAC