Moderation of neoclassical impurity accumulation in high temperature plasmas of helical devices

Velasco, J. L.; Calvo, I.; Satake, S.; Alonso, A.; Nunami, M.; Yokoyama, M.; Sato, Masahiko; Estrada, T.; Fontdecaba, J. M.; Liniers, M.; McCarthy, K. J.; Medina, F.; Ochando, M. A.; Parra, Félix I.; Sugama, H.; Zhezhera, A.; Team, Lhd; Team, Tj‐Ii

doi:10.1088/0029-5515/57/1/016016

Cited by 25 publications

(37 citation statements)

References 61 publications

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“…Apart from these works, others have looked into the screening of impurities in stellarators, like ref. [21] where high T i plasmas with negative but small |E r | are shown to coexist with outward impurity flow. Finally, ref.…”

Section: Introductionmentioning

confidence: 95%

On-surface potential and radial electric field variations in electron root stellarator plasmas

García-Regaña

Estrada

Calvo

et al. 2018

Plasma Phys. Control. Fusion

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In the present work we report recent radial electric field measurements carried out with the Doppler reflectometry system in the TJ-II stellarator. The study focuses on the fact that, under some conditions, the radial electric field measured at different points over the same flux surface shows significantly different values. A numerical analysis is carried out considering the contribution arising from the radial dependence of Φ 1 as a possible correction term to the total radial electric field. Here Φ 1 is the neoclassical electrostatic potential variation over the surface. The comparison shows good agreement in some aspects, like the conditions under which this correction is large (electron-root conditions) or negligible (ion-root conditions). But it disagrees in others like the sign of the correction. The results are discussed together with the underlying reasons of this partial disagreement. In addition, motivated by the recent installation of the dual Doppler reflectometry system in Wendelstein 7-X (W7-X), Φ 1 estimations for W7-X are revisited considering Core-Electron-Root-Plasma (CERC) plasmas from its first experimental campaign. The simulations show larger values of Φ 1 under electron-root conditions than under ion root ones. The contribution from the kinetic electron response is shown to become important at some radii. All this results in a potential variation size noticeably larger than estimated in our previous work in W7-X [1] for other plasma parameters and another configuration.

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Section: Introductionmentioning

confidence: 95%

On-surface potential and radial electric field variations in electron root stellarator plasmas

García-Regaña

Estrada

Calvo

et al. 2018

Plasma Phys. Control. Fusion

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“…First, as we will discuss throughout the next sections, the standard ion root description neglects the contribution of the electrons to the ambipolarity equation. Once included, E r is still negative but gets smaller in magnitude, and then T i is able to compete with it in driving impurity transport; for extreme cases, screening may occur, as first discussed in [13]. Second, previous calculations of ϕ 1 [5,6] neglect the tangential magnetic drift in the drift-kinetic equation, and therefore the contribution of the superbanana-plateau regime to bulk ion transport and the contribution of the superbanana-plateau layer to the variation of the ion bulk density and of the electrostatic potential on the flux surface.…”

Section: Introductionmentioning

confidence: 95%

Large tangential electric fields in plasmas close to temperature screening

Velasco¹,

Calvo²,

García-Regaña³

et al. 2018

Plasma Phys. Control. Fusion

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Low-collisionality stellarator plasmas usually display a large negative radial electric field that has been expected to cause accumulation of impurities due to their high charge number. In this paper, two combined effects that can potentially modify this scenario are discussed. First, it is shown that, in low collisionality plasmas, the kinetic contribution of the electrons to the radial electric field can make it negative but small, bringing the plasma close to impurity temperature screening (i.e., to a situation in which the ion temperature gradient is the main drive of impurity transport and causes outward flux); in plasmas of very low collisionality, such as those of the Large Helical Device displaying impurity hole [1,2], screening may actually occur. Second, the component of the electric field that is tangent to the flux surface (in other words, the variation of the electrostatic potential on the flux surface), although smaller than the radial component, has recently been suggested to be an additional relevant drive for radial impurity transport. Here, it is explained that, especially when the radial electric field is small, the tangential magnetic drift has to be kept in order to correctly compute the tangential electric field, that can be larger than previously expected. This can have a strong impact on impurity transport, as we illustrate by means of simulations using the newly-developed code KNOSOS (KiNetic Orbit-averaging-SOlver for Stellarators).

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“…1Recently, both the prevalence of the radial electric field in the transport of impurities and the absence of impurity screening in three-dimensional magnetic fields have been brought into question for several collisionality regimes (Velasco et al. 2017; Helander et al. 2017).…”

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confidence: 99%

Electrostatic potential variations on stellarator magnetic surfaces in low collisionality regimes

Calvo¹,

Velasco²,

Parra³

et al. 2018

J. Plasma Phys.

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The component of the neoclassical electrostatic potential that is non-constant on the magnetic surface, that we denote byφ, 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 whichφ 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 ofφ on the magnetic surface are analytically derived in the 1/ν, √ ν and superbanana-plateau regimes of stellarators close to omnigeneity; i. e. stellarators that have been optimized for neoclassical transport. It is found that the largestφ 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. † Recently, both the prevalence of the radial electric field in the transport of impurities and the absence of impurity screening in three-dimensional magnetic fields have been brought into question for several collisionality regimes Helander et al. 2017).

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Moderation of neoclassical impurity accumulation in high temperature plasmas of helical devices

Cited by 25 publications

References 61 publications

On-surface potential and radial electric field variations in electron root stellarator plasmas

On-surface potential and radial electric field variations in electron root stellarator plasmas

Large tangential electric fields in plasmas close to temperature screening

Electrostatic potential variations on stellarator magnetic surfaces in low collisionality regimes

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