527 research outputs found
Global effects on neoclassical transport in the pedestal with impurities
We present a numerical study of collisional transport in a tokamak pedestal
in the presence of non-trace impurities, using the radially global
neoclassical solver PERFECT [M. Landreman et al. 2014 Plasma Phys. Control.
Fusion 56 045005]. It is known that in a tokamak core with non-trace impurities
present the radial impurity flux opposes the bulk ion flux to provide an
ambipolar particle transport, with the electron transport being negligibly
small. However, in a sharp density pedestal with sub-sonic ion flows the
electron transport can be comparable to the ion and impurity flows.
Furthermore, the neoclassical particle transport is not intrinsically
ambipolar, and the non-ambipolarity of the fluxes extends outside the pedestal
region by the radial coupling of the perturbations. The neoclassical momentum
transport, which is finite in the presence of ion orbit-width scale profile
variations, is significantly enhanced when impurities are present in non-trace
quantities, even if the total parallel mass flow is dominated by the bulk ions
Consistent multiphase-field theory for interface driven multidomain dynamics
We present a new multiphase-field theory for describing pattern formation in
multi-domain and/or multi-component systems. The construction of the free
energy functional and the dynamic equations is based on criteria that ensure
mathematical and physical consistency. We first analyze previous
multiphase-field theories, and identify their advantageous and disadvantageous
features. On the basis of this analysis, we introduce a new way of constructing
the free energy surface, and derive a generalized multiphase description for
arbitrary number of phases (or domains). The presented approach retains the
variational formalism; reduces (or extends) naturally to lower (or higher)
number of fields on the level of both the free energy functional and the
dynamic equations; enables the use of arbitrary pairwise equilibrium
interfacial properties; penalizes multiple junctions increasingly with the
number of phases; ensures non-negative entropy production, and the convergence
of the dynamic solutions to the equilibrium solutions; and avoids the
appearance of spurious phases on binary interfaces. The new approach is tested
for multi-component phase separation and grain coarsening
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
Free energy of the bcc-liquid interface and the Wulff shape as predicted by the Phase-Field Crystal model
The Euler-Lagrange equation of the phase-field crystal (PFC) model has been
solved under appropriate boundary conditions to obtain the equilibrium free
energy of the body centered cubic crystal-liquid interface for 18 orientations
at various reduced temperatures in the range .
While the maximum free energy corresponds to the
orientation for all values, the minimum is realized by the direction for small , and by the orientation for higher . The predicted dependence on
the reduced temperature is consistent with the respective mean field critical
exponent. The results are fitted with an eight-term Kubic harmonic series, and
are used to create stereographic plots displaying the anisotropy of the
interface free energy. We have also derived the corresponding Wulff shapes that
vary with increasing from sphere to a polyhedral form that differs
from the rhombo-dodecahedron obtained previously by growing a bcc seed until
reaching equilibrium with the remaining liquid
The importance of the classical channel in the impurity transport of optimized stellarators
In toroidal magnetic confinement devices, such as tokamaks and stellarators,
neoclassical transport is usually an order of magnitude larger than its
classical counterpart. However, when a high-collisionality species is present
in a stellarator optimized for low Pfirsch-Schl\"uter current, its classical
transport can be comparable to the neoclassical transport. In this letter, we
compare neoclassical and classical fluxes and transport coefficients calculated
for Wendelstein 7-X (W7-X) and Large Helical Device (LHD) cases. In W7-X, we
find that the classical transport of a collisional impurity is comparable to
the neoclassical transport for all radii, while it is negligible in the LHD
cases, except in the vicinity of radii where the neoclassical transport changes
sign. In the LHD case, electrostatic potential variations on the flux-surface
significantly enhance the neoclassical impurity transport, while the classical
transport is largely insensitive to this effect in the cases studied.Comment: 10 pages, 2 figure
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
–
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.Vetenskapsrådet and Marie Sklodowska Curie Actions, Cofund, Project INCA 60039
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