115 research outputs found
Electrostatic potential variations on stellarator magnetic surfaces in low collisionality regimes
The component of the neoclassical electrostatic potential that is
non-constant on the magnetic surface, that we denote by , can
affect radial transport of highly charged impurities, and this has motivated
its inclusion in some modern neoclassical codes. The number of neoclassical
simulations in which is calculated is still scarce, partly
because they are usually demanding in terms of computational resources,
especially at low collisionality. In this paper the size, the scaling with
collisionality and with aspect ratio, and the structure of on
the magnetic surface are analytically derived in the , and
superbanana-plateau regimes of stellarators close to omnigeneity; i. e.
stellarators that have been optimized for neoclassical transport. It is found
that the largest that the neoclassical equations admit scales
linearly with the inverse aspect ratio and with the size of the deviation from
omnigeneity. Using a model for a perturbed omnigeneous configuration, the
analytical results are verified and illustrated with calculations by the code
KNOSOS. The techniques, results and numerical tools employed in this paper can
be applied to neoclassical transport problems in tokamaks with broken
axisymmetry.Comment: 30 pages, 12 figures, 1 table. Published versio
Stellarator impurity flux driven by electric fields tangent to magnetic surfaces
The control of impurity accumulation is one of the main challenges for future
stellarator fusion reactors. The standard argument to explain this accumulation
relies on the, in principle, large inward pinch in the neoclassical impurity
flux caused by the typically negative radial electric field in stellarators.
This simplified interpretation was proven to be flawed by Helander et al.
[Phys. Rev. Lett. 118, 155002 (2017)], who showed that in a relevant regime
(low-collisionality main ions and collisional impurities) the radial electric
field does not drive impurity transport. In that reference, the effect of the
component of the electric field that is tangent to the magnetic surface was not
included. In this Letter, an analytical calculation of the neoclassical radial
impurity flux incorporating such effect is given, showing that it can be very
strong for highly charged impurities and that, once it is taken into account,
the dependence of the impurity flux on the radial electric field reappears.
Realistic examples are provided in which the inclusion of the tangential
electric field leads to impurity expulsion.Comment: 10 pages, 1 figure. Published versio
Nonlinear gyrokinetic PIC simulations in stellarators with the code EUTERPE
In this work, the first nonlinear particle-in-cell simulations carried out in
a stellarator with the global gyrokinetic code EUTERPE using realistic plasma
parameters are reported. Several studies are conducted with the aim of enabling
reliable nonlinear simulations in stellarators with this code. First, EUTERPE
is benchmarked against ORB5 in both linear and nonlinear settings in a tokamak
configuration. Next, the use of noise control and stabilization tools, a
Krook-type collision operator, markers weight smoothing and heating sources is
investigated. It is studied in detail how these tools influence the linear
growth rate of instabilities in both tokamak and stellarator geometries and
their influence on the linear zonal flow evolution in a stellarator. Then, it
is studied how these tools allow improving the quality of the results in a set
of nonlinear simulations of electrostatic turbulence in a stellarator
configuration.
Finally, these tools are applied to a W7-X magnetic configuration using
experimental plasma parameters.Comment: 24 pages, 19 figure
Turbulent transport of impurities in 3D devices
A large diffusive turbulent contribution to the radial impurity transport in
Wendelstein 7-X (W7-X) plasmas has been experimentally inferred during the
first campaigns and numerically confirmed by means of gyrokinetic simulations
with the code stella. In general, the absence of strong impurity accumulation
during the initial W7-X campaigns is attributed to this diffusive term. In the
present work the diffusive contribution is also calculated in other stellarator
plasmas. In particular, the diffusion (D) and convection (V) coefficients of
carbon and iron impurities produced by ion-temperature-gradient (ITG)
turbulence are obtained for W7-X, LHD, TJ-II and NCSX. The results show that,
although the size of D and V can differ across the four devices, inward
convection is found for all of them. For W7-X, TJ-II and NCSX the two
coefficients are comparable and the turbulent peaking factor is surprisingly
similar. In LHD, appreciably weaker diffusive and convective impurity transport
and significantly larger turbulent peaking factor are predicted. All this
suggests that ITG turbulence, although not strongly, would lead to negative
impurity density gradients in stellarators. Then, considering mixed ITG/Trapped
Electron Mode (TEM) turbulence for the specific case of W7-X, it has been
quantitatively assessed to what degree pellet fueled reduced turbulence
scenarios feature reduced turbulent transport of impurities as well. The
results for trace iron impurities show that, although their turbulent transport
is not entirely suppressed, a significant reduction of V and a stronger
decrease of D are found. Although the diffusion is still above neoclassical
levels, the neoclassical convection would gain under such conditions a greater
specific weight on the dynamics of impurities in comparison with standard ECRH
scenarios.Comment: 16 pages, 8 figure
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
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