11,048 research outputs found
Optimizing stellarators for large flows
Plasma flow is damped in stellarators because they are not intrinsically
ambipolar, unlike tokamaks, in which the flux-surface averaged radial electric
current vanishes for any value of the radial electric field. Only
quasisymmetric stellarators are intrinsically ambipolar, but exact
quasisymmetry is impossible to achieve in non-axisymmetric toroidal
configurations. By calculating the violation of intrinsic ambipolarity due to
deviations from quasisymmetry, one can derive criteria to assess when a
stellarator can be considered quasisymmetric in practice, i.e. when the flow
damping is weak enough. Let us denote by a small parameter that
controls the size of a perturbation to an exactly quasisymmetric magnetic
field. Recently, it has been shown that if the gradient of the perturbation is
sufficiently small, the flux-surface averaged radial electric current scales as
for any value of the collisionality. It was also argued that when
the gradient of the perturbation is large, the quadratic scaling is replaced by
a more unfavorable one. In this paper, perturbations with large gradients are
rigorously treated. In particular, it is proven that for low collisionality a
perturbation with large gradient yields, at best, an deviation
from quasisymmetry. Heuristic estimations in the literature incorrectly
predicted an deviation.Comment: 24 pages, 2 figures. To appear in Plasma Physics and Controlled
Fusio
Flow damping in stellarators close to quasisymmetry
Quasisymmetric stellarators are a type of optimized stellarators for which
flows are undamped to lowest order in an expansion in the normalized Larmor
radius. However, perfect quasisymmetry is impossible. Since large flows may be
desirable as a means to reduce turbulent transport, it is important to know
when a stellarator can be considered to be sufficiently close to quasisymmetry.
The answer to this question depends strongly on the size of the spatial
gradients of the deviation from quasisymmetry and on the collisionality regime.
Recently, formal criteria for closeness to quasisymmetry have been derived in a
variety of situations. In particular, the case of deviations with large
gradients was solved in the regime. Denoting by a parameter
that gives the size of the deviation from quasisymmetry, it was proven that
particle fluxes do not scale with , as typically claimed, but
with . It was also shown that ripple wells are not necessarily the main
cause of transport. This paper reviews those works and presents a new result in
another collisionality regime, in which particles trapped in ripple wells are
collisional and the rest are collisionless.Comment: 14 pages, 2 figures. To appear in Plasma Physics and Controlled
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The effect of tangential drifts on neoclassical transport in stellarators close to omnigeneity
In general, the orbit-averaged radial magnetic drift of trapped particles in
stellarators is non-zero due to the three-dimensional nature of the magnetic
field. Stellarators in which the orbit-averaged radial magnetic drift vanishes
are called omnigeneous, and they exhibit neoclassical transport levels
comparable to those of axisymmetric tokamaks. However, the effect of deviations
from omnigeneity cannot be neglected in practice. For sufficiently low
collision frequencies (below the values that define the regime), the
components of the drifts tangential to the flux surface become relevant. This
article focuses on the study of such collisionality regimes in stellarators
close to omnigeneity when the gradient of the non-omnigeneous perturbation is
small. First, it is proven that closeness to omnigeneity is required to
preserve radial locality in the drift-kinetic equation for collisionalities
below the regime. Then, it is shown that neoclassical transport is
determined by two layers in phase space. One of the layers corresponds to the
regime and the other to the superbanana-plateau regime. The
importance of the superbanana-plateau layer for the calculation of the
tangential electric field is emphasized, as well as the relevance of the latter
for neoclassical transport in the collisionality regimes considered in this
paper. In particular, the tangential electric field is essential for the
emergence of a new subregime of superbanana-plateau transport when the radial
electric field is small. A formula for the ion energy flux that includes the
regime and the superbanana-plateau regime is given. The energy
flux scales with the square of the size of the deviation from omnigeneity.
Finally, it is explained why below a certain collisionality value the
formulation presented in this article ceases to be valid.Comment: 36 pages. Version to be published in Plasma Physics and Controlled
Fusio
Dynamics of zonal flow-like structures in the edge of the TJ-II stellarator
The dynamics of fluctuating electric field structures in the edge of the
TJ-II stellarator, that display zonal flow-like traits, is studied. These
structures have been shown to be global and affect particle transport
dynamically [J.A. Alonso et al., Nucl. Fus. 52 063010 (2012)]. In this article
we discuss possible drive (Reynolds stress) and damping (Neoclassical
viscosity, geodesic transfer) mechanisms for the associated ExB velocity. We
show that: (a) while the observed turbulence-driven forces can provide the
necessary perpendicular acceleration, a causal relation could not be firmly
established, possibly because of the locality of the Reynolds stress
measurements, (b) the calculated neoclassical viscosity and damping times are
comparable to the observed zonal flow relaxation times, and (c) although an
accompanying density modulation is observed to be associated to the zonal flow,
it is not consistent with the excitation of pressure side-bands, like those
present in geodesic acoustic oscillations, caused by the compression of the ExB
flow field
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
Has a Higgs-flavon with a GeV mass been detected at the LHC13?
Higgs-flavon fields appear as a part of the Froggatt-Nielsen (FN) mechanism,
which attempts to explain the hierarchy of Yukawa couplings. We explore the
possibility that the 750 GeV diphoton resonance recently reported at the LHC13,
could be identified with a low-scale Higgs-flavon field and find the
region of the parameter space consistent with CMS and ATLAS data. It is found
that the extra vector-like fermions of the ultraviolet completion of the FN
mechanism are necessary in order to reproduce the observed signal. We consider
a standard model (SM) extension that contains two Higgs doublets (a standard
one and an inert one) and one complex FN singlet. The inert doublet includes a
stable neutral boson, which provides a viable dark matter candidate, while the
mixing of the standard doublet and the FN singlet induces flavor violation in
the Higgs sector at the tree-level. Constraints on the parameters of the model
are derived from the LHC Higgs data, which include the search for the lepton
flavor violating decay of the SM Higgs boson . It is also
found that in some region of the parameter space the model may give rise to a
large branching ratio for the decay, of the order of 0.1, which
could be searched for at the LHC.Comment: 18 pages, 7 Figures, includes updated files to match published
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