289 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
Fusio
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
When omnigeneity fails
A generic non-symmetric magnetic field does not confine magnetized charged
particles for long times due to secular magnetic drifts. Stellarator magnetic
fields should be omnigeneous (that is, designed such that the secular drifts
vanish), but perfect omnigeneity is technically impossible. There always are
small deviations from omnigeneity that necessarily have large gradients. The
amplification of the energy flux caused by a deviation of size is
calculated and it is shown that the scaling with of the
amplification factor can be as large as linear. In opposition to common wisdom,
most of the transport is not due to particles trapped in ripple wells, but to
the perturbed motion of particles trapped in the omnigeneous magnetic wells
around their bounce points.Comment: 6 pages, 2 figure
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
A Comparison of Three QRS Detection Algorithms Over a Public Database
AbstractWe have compared three of the best QRS detection algorithms, regarding their results, to check the performance and to elucidate which get better accuracy. In the literature these algorithms were published in a theoretical way, without offering their code, so it is difficult to check its real behaviour over different collections of ECG records. This work brings the community our source code of each algorithm and results of its validation over a public database. In addition, this software was developed as a framework in order to permit the inclusion of new QRS detection algorithms and also its testing over different databases
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
Ethnobotany, volatile iols and secretion tissues of Werneria poposa from Argentina
Folk medicines are gaining great importance as information sources on traditional medicinal plants. The aim of this paper is the study of a plant traditionally employed by the Puna inhabitants: Werneria poposa Phil. Morphology of its secretory tissue and other histological diagnostic features, as well as the chemical composition of its essential oil, is described. Puna inhabitants use W. poposa mainly as an infusion for mountain sickness ('soroche'), stomach and hepatic disorders and cold. It is also used externally as a hot bath or unguent for rheumatic pains or traumas. Secretory tissues are schizogenous ducts in leaves and stems. Thirty-one components were detected in the essential oil by means of GC/MS analysis. Oil was characterized by the presence of high content of ÎČ-pinene (21.7%), α-pinene (5.5%), terpinen-4-ol (5.3%), α-terpinene (5.2%), ÎČ-phellandrene + 1,8-cineole (4.8%), isopulegol (4.8%) and ÎČ-citronellal (4.6%). At the present time, W. poposa is consumed as a medicinal plant, mainly in the northwest of Argentina, not being commercialized in the urban centers.Facultad de Ciencias Naturales y Muse
Heart of Darkness: Heart Rate Variability on Patients with Risk of Suicide
Heart Rate Variability (HRV) is an emerging research field in the study of diverse pathologies, as long as it allows considering another measurement for detecting possible aggravations. The aim of this work is to study the applicability of the analysis of HRV in order to establish if a person is at risk of suffering from suicidal ideation. This work includes the development and testing of a heart rate acquisition and automatic analysis system, with friendly software for clinicians, customized to the necessities of an emergency unit. Furthermore, it includes the analysis of the obtained data with the purpose of assessing possible correlations between HRV
parameters and personality impulsive traits. 20 patients and 10 normal cases were selected to develop this pilot study. Results show significant statistical difference (p<0.05) among patients and normal cases for pNN50, IRRR, MADRR, total HRV power, Approximate Entropy and Fractal Dimension
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