38 research outputs found
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
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
Comparison of particle trajectories and collision operators for collisional transport in nonaxisymmetric plasmas
In this work, we examine the validity of several common simplifying
assumptions used in numerical neoclassical calculations for nonaxisymmetric
plasmas, both by using a new continuum drift-kinetic code and by considering
analytic properties of the kinetic equation. First, neoclassical phenomena are
computed for the LHD and W7-X stellarators using several versions of the
drift-kinetic equation, including the commonly used incompressible-ExB-drift
approximation and two other variants, corresponding to different effective
particle trajectories. It is found that for electric fields below roughly one
third of the resonant value, the different formulations give nearly identical
results, demonstrating the incompressible ExB-drift approximation is quite
accurate in this regime. However, near the electric field resonance, the models
yield substantially different results. We also compare results for various
collision operators, including the full linearized Fokker-Planck operator. At
low collisionality, the radial transport driven by radial gradients is nearly
identical for the different operators, while in other cases it is found to be
important that collisions conserve momentum
Impurity transport in Alcator C-Mod in the presence of poloidal density variation induced by ion cyclotron resonance heating
Impurity particle transport in an ion cyclotron resonance heated Alcator
C-Mod discharge is studied with local gyrokinetic simulations and a theoretical
model including the effect of poloidal asymmetries and elongation. In spite of
the strong minority temperature anisotropy in the deep core region, the
poloidal asymmetries are found to have a negligible effect on the turbulent
impurity transport due to low magnetic shear in this region, in agreement with
the experimental observations. According to the theoretical model, in outer
core regions poloidal asymmetries may contribute to the reduction of the
impurity peaking, but uncertainties in atomic physics processes prevent
quantitative comparison with experiments.Comment: 32 pages, 12 figure
Micro-tearing modes in spherical and conventional tokamaks
The onset and characteristics of Micro-Tearing Modes (MTM) in the core of spherical (NSTX) and conventional tokamaks (ASDEX Upgrade and JET) are studied through local linear gyrokinetic simulations with gyro [J. Candy and E. Belli, General Atomics Report GA-A26818 In all these plasmas, finite collisionality is needed for MTMs to become unstable and the electron temperature gradient is found to be the fundamental drive. However, a significant difference is observed in the dependence of linear growth rate of MTMs on electron temperature gradient. While it varies weakly and non-monotonically in JET and ASDEX Upgrade plasmas, in NSTX it increases with the electron temperature gradient
Turbulent impurity transport simulations in Wendelstein 7-X plasmas
A study of turbulent impurity transport by means of quasilinear and nonlinear
gyrokinetic simulations is presented for Wendelstein 7-X (W7-X). The
calculations have been carried out with the recently developed gyrokinetic code
stella. Different impurity species are considered in the presence of various
types of background instabilities: ITG, TEM and ETG modes for the quasilinear
part of the work; ITG and TEM for the nonlinear results. While the quasilinear
approach allows one to draw qualitative conclusions about the sign or relative
importance of the various contributions to the flux, the nonlinear simulations
quantitatively determine the size of the turbulent flux and check the extent to
which the quasilinear conclusions hold. Although the bulk of the nonlinear
simulations are performed at trace impurity concentration, nonlinear
simulations are also carried out at realistic effective charge values, in order
to know to what degree the conclusions based on the simulations performed for
trace impurities can be extrapolated to realistic impurity concentrations. The
presented results conclude that the turbulent radial impurity transport in W7-X
is mainly dominated by ordinary diffusion, which is close to that measured
during the recent W7-X experimental campaigns. It is also confirmed that
thermo-diffusion adds a weak inward flux contribution and that, in the absence
of impurity temperature and density gradients, ITG- and TEM-driven turbulence
push the impurities inwards and outwards, respectively.Comment: 19 pages, 10 figures, 2 table
Possible mechanism responsible for observed impurity outward flow under radio frequency heating
The effect of poloidal asymmetry of impurities on impurity transport driven
by electrostatic turbulence in tokamak plasmas is analyzed. It is found that in
the presence of in-out asymmetric impurity populations the zero-flux impurity
density gradient (the so-called peaking factor) is significantly reduced. A
sign change in the impurity flux may occur if the asymmetry is sufficiently
large. This may be a contributing reason for the observed outward convection of
impurities in the presence of radio frequency heating. The effect of in-out
asymmetry is most significant in regions with low temperature gradients. In the
trapped electron mode dominated case also an up-down asymmetry can lead to a
sign change in the peaking factor from positive to negative. The effect of ion
parallel compressibility on the peaking factor is significant, and leads to
positive peaking factors in regions with high temperature gradients, even in
the presence of in-out asymmetry.Comment: 19 pages, 14 figure