727 research outputs found
Turbulent transport of impurities and their effect on energy confinement
By presenting linear and nonlinear gyrokinetic studies, based on a balanced
neutral beam injection deuterium discharge from the DIII-D tokamak, we
demonstrate that impurities alter the scaling of the transport on the charge
and mass of the main species, and even more importantly, they can dramatically
change the energy transport even in relatively small quantities. A poloidally
varying equilibrium electrostatic potential can lead to a strong reduction or
sign change of the impurity peaking factor due to the combined effect of the
in-out impurity density asymmetry and the EXB drift of impurities. We present
an approximate expression for the impurity peaking factor and demonstrate that
impurity peaking is not significantly affected by impurity self-collisions.Comment: Accepted for publication in Plasma Physics and Controlled Fusio
Impurity transport in trapped electron mode driven turbulence
Trapped electron mode turbulence is studied by gyrokinetic simulations with
the GYRO code and an analytical model including the effect of a poloidally
varying electrostatic potential. Its impact on radial transport of high-Z trace
impurities close to the core is thoroughly investigated and the dependence of
the zero-flux impurity density gradient (peaking factor) on local plasma
parameters is presented. Parameters such as ion-to-electron temperature ratio,
electron temperature gradient and main species density gradient mainly affect
the impurity peaking through their impact on mode characteristics. The poloidal
asymmetry, the safety factor and magnetic shear have the strongest effect on
impurity peaking, and it is shown that under certain scenarios where trapped
electron modes are dominant, core accumulation of high-Z impurities can be
avoided. We demonstrate that accounting for the momentum conservation property
of the impurity-impurity collision operator can be important for an accurate
evaluation of the impurity peaking factor.Comment: 30 pages, 10 figure
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Phase field approach to heterogeneous crystal nucleation in alloys
We extend the phase field model of heterogeneous crystal nucleation developed recently [L. GrĂĄnĂĄsy, T. Pusztai, D. Saylor, and J. A. Warren, Phys. Rev. Lett. 98, 035703 (2007)] to binary alloys. Three approaches are considered to incorporate foreign walls of tunable wetting properties into phase field simulations: a continuum realization of the classical spherical cap model (called Model A herein), a non-classical approach (Model B) that leads to ordering of the liquid at the wall, and to the appearance of a surface spinodal, and a non-classical model (Model C) that allows for the appearance of local states at the wall that are accessible in the bulk phases only via thermal fluctuations. We illustrate the potential of the presented phase field methods for describing complex polycrystalline solidification morphologies including the shish-kebab structure, columnar to equiaxed transition, and front-particle interaction in binary alloys
Diffusion-controlled anisotropic growth of stable and metastable crystal polymorphs in the phase-field crystal model
The official published version of the article can be accessed from the link below - Copyright @ 2009 APSWe use a simple density functional approach on a diffusional time scale, to address freezing to the body-centered cubic (bcc), hexagonal close-packed (hcp), and face-centered cubic (fcc) structures. We observe faceted equilibrium shapes and diffusion-controlled layerwise crystal growth consistent with two-dimensional nucleation. The predicted growth anisotropies are discussed in relation with results from experiment and atomistic simulations. We also demonstrate that varying the lattice constant of a simple cubic substrate, one can tune the epitaxially growing body-centered tetragonal structure between bcc and fcc, and observe a Mullins-Sekerka-Asaro-Tiller-Grinfeld-type instability.This work has been supported by the EU FP7
Collaborative Project ENSEMBLE under Grant
Agreement NMP4-SL-2008-213669, the Hungarian
Academy of Sciences under contract OTKA-K-62588, the Academy of Finland via its COMP CoE grant, and by Tekes via its MASIT33 project. A. J. acknowledges financial
support from the Finnish Academy of Science and Letters. T. P. acknowledges support from the Bolyai JaÂŽnos Grant
Advanced operator-splitting-based semi-implicit spectral method to solve the binary phase-field crystal equations with variable coefficients
We present an efficient method to solve numerically the equations of dissipative dynamics of the binary phase-field crystal model proposed by Elder et al. [Phys. Rev. B 75, 064107 (2007)] characterized by variable coefficients. Using the operator splitting method, the problem has been decomposed into sub-problems that can be solved more efficiently. A combination of non-trivial splitting with spectral semi-implicit solution leads to sets of algebraic equations of diagonal matrix form. Extensive testing of the method has been carried out to find the optimum balance among errors associated with time integration, spatial discretization, and splitting. We show that our method speeds up the computations by orders of magnitude relative to the conventional explicit finite difference scheme, while the costs of the pointwise implicit solution per timestep remains low. Also we show that due to its numerical dissipation, finite differencing can not compete with spectral differencing in terms of accuracy. In addition, we demonstrate that our method can efficiently be parallelized for distributed memory systems, where an excellent scalability with the number of CPUs is observed
Edge momentum transport by neutrals: an interpretive numerical framework
Due to their high cross-field mobility, neutrals can contribute to momentum transport even at
the low relative densities found inside the separatrix and they can generate intrinsic rotation.
We use a charge-exchange dominated solution to the neutral kinetic equation, coupled to
neoclassical ions, to evaluate the momentum transport due to neutrals. Numerical solutions
to the drift-kinetic equation allow us to cover the full range of collisionality, including the
intermediate levels typical of the tokamak edge. In the edge there are several processes likely
to contribute to momentum transport in addition to neutrals. Therefore, we present here an
interpretive framework that can evaluate the momentum transport through neutrals based
on radial plasma profiles. We demonstrate its application by analysing the neutral angular
momentum flux for an L-mode discharge in the ASDEX Upgrade tokamak. The magnitudes of
the angular momentum fluxes we find here due to neutrals of 0.6
â
2
Nm
are comparable to the
net torque on the plasma from neutral beam injection, indicating the importance of neutrals for
rotation in the edge.VetenskapsrÄdet and Marie Sklodowska Curie Actions, Cofund, Project INCA 60039
Effective Governance of Global Financial Markets:An Evolutionary Plan for Reform
Runaway electrons, which are generated in a plasma where the induced electric field exceeds a certain critical value, can reach very high energies in the MeV range. For such energetic electrons, radiative losses will contribute significantly to the momentum space dynamics. Under certain conditions, due to radiative momentum losses, a non-monotonic feature - a âbump' - can form in the runaway electron tail, creating a potential for bump-on-tail-type instabilities to arise. Here, we study the conditions for the existence of the bump. We derive an analytical threshold condition for bump appearance and give an approximate expression for the minimum energy at which the bump can appear. Numerical calculations are performed to support the analytical derivation
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