Radio Frequency Induced and Neoclassical Asymmetries and their Effects on Turbulent Impurity Transport in a Tokamak

Pusztai, István; Landreman, Matt; Mollén, A.; Kazakov, Yevgen; Fülöp, Tünde

doi:10.1002/ctpp.201410012

Cited by 5 publications

(7 citation statements)

References 29 publications

(42 reference statements)

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“…We observe that in the presence of global effects the poloidal variation of the density perturbation is not restricted to an up-down asymmetry, it can be more significant, and can develop rapid radial variations (in accordance with previous numerical results 27,30 ). The poloidal asymmetries observed here arise in the presence of finite inverse aspect ratio and finite orbit width effects, and are not closely related to those predicted by the analytical theories in 19,28,29 , since those require significant poloidal variations in the ion-impurity friction.…”

Section: Discussionsupporting

confidence: 92%

Global effects on neoclassical transport in the pedestal with impurities

Pusztai¹,

Buller²,

Landreman³

2016

Plasma Phys. Control. Fusion

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We present a numerical study of collisional transport in a tokamak pedestal in the presence of non-trace impurities, using the radially global δf neoclassical solver Perfect [M. Plasma Phys. Control. Fusion 56 045005]. It is known that in a tokamak core with non-trace impurities present the radial impurity flux opposes the bulk ion flux to provide an ambipolar particle transport, with the electron transport being negligibly small. However, in a sharp density pedestal with sub-sonic ion flows the electron transport can be comparable to the ion and impurity flows. Furthermore, the neoclassical particle transport is not intrinsically ambipolar, and the non-ambipolarity of the fluxes extends outside the pedestal region by the radial coupling of the perturbations. The neoclassical momentum transport, which is finite in the presence of ion orbit-width scale profile variations, is significantly enhanced when impurities are present in non-trace quantities, even if the total parallel mass flow is dominated by the bulk ions.

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Section: Discussionsupporting

confidence: 92%

Global effects on neoclassical transport in the pedestal with impurities

Pusztai¹,

Buller²,

Landreman³

2016

Plasma Phys. Control. Fusion

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“…In this case, figure 9 suggests an inward shift of the HFS E r well with respect to the LFS one, opposite to the shift obtained when the temperature profiles are aligned as in figures 5(a) and 6(a). It is interesting to note that a slight inward shift of the HFS E r well with respect to the LFS one qualitatively agrees with potential asymmetries found by the code PERFECT [12] in the case of weak ion temperature gradients (see figures 1 and 2 of [46]). Figures 5(b) and 6 (b) show that when the HFS and LFS E r wells are aligned, substantial poloidal asymmetries in T z are observed as far in as ρ ≈ 0.97.…”

Section: Simplified Theoretical Considerationssupporting

confidence: 80%

Inboard and outboard radial electric field wells in the H- and I-mode pedestal of Alcator C-Mod and poloidal variations of impurity temperature

et al. 2014

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Abstract. We present inboard (HFS) and outboard (LFS) radial electric field (E r ) and impurity temperature (T z ) measurements in the I-mode and H-mode pedestal of Alcator C-Mod. These measurements reveal strong E r wells at the HFS and the LFS midplane in both regimes and clear pedestals in T z , which are of similar shape and height for the HFS and LFS. While the H-mode E r well has a radially symmetric structure, the E r well in I-mode is asymmetric, with a stronger ExB shear layer at the outer edge of the E r well, near the separatrix. Comparison of HFS and LFS profiles indicates that impurity temperature and plasma potential are not simultaneously flux functions. Uncertainties in radial alignment after mapping HFS measurements along flux surfaces to the LFS do not, however, allow direct determination as to which quantity varies poloidally and to what extent. Radially aligning HFS and LFS measurements based on the T z profiles would result in substantial inboard-outboard variations of plasma potential and electron density. Aligning HFS and LFS E r wells instead also approximately aligns the impurity poloidal flow profiles, while resulting in a LFS impurity temperature exceeding the HFS values in the region of steepest gradients by up to 70%. Considerations based on a simplified form of total parallel momentum balance and estimates of parallel and perpendicular heat transport time scales seem to favor an approximate alignment of the E r wells and a substantial poloidal asymmetry in impurity temperature.

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“…In words, δ is the largest value of the δ X profiles corresponding to T a (ψ) and η a (ψ) for all species. Note that while n and Φ can vary on shorter length scales if the corresponding η has a slow radial variation, the DKE (1) itself is based on an expansion in ρ/L X = (B p /B)δ for all plasma parameters, so no plasma profile ations on a flux surface, which naturally develop for impurities in the pedestal [21][22][23][24][25] . Such poloidal variations have been implemented in the stellarator code Sfincs 26,27 that shares a very similar numerical framework to Perfect.…”

Section: Global δF Drift-kineticsmentioning

confidence: 99%

Neoclassical flows in deuterium–helium plasma density pedestals

Buller¹,

Pusztai²,

Newton³

et al. 2017

Plasma Phys. Control. Fusion

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In tokamak transport barriers, the radial scale of profile variations can be comparable to a typical ion orbit width, which makes the coupling of the distribution function across flux surfaces important in the collisional dynamics. We use the radially global steady-state neoclassical δf code Perfect to calculate poloidal and toroidal flows, and radial fluxes, in the pedestal. In particular, we have studied the changes in these quantities as the plasma composition is changed from a deuterium bulk species with a helium impurity to a helium bulk with a deuterium impurity, under specific profile similarity assumptions. The poloidally resolved radial fluxes are not divergence-free in isolation in the presence of sharp radial profile variations, which leads to the appearance of poloidal return-flows. These flows exhibit a complex radial-poloidal structure that extends several orbit widths into the core and is sensitive to abrupt radial changes in the ion temperature gradient. We find that a sizable neoclassical toroidal angular momentum transport can arise in the radially global theory, in contrast to the local.

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Radio Frequency Induced and Neoclassical Asymmetries and their Effects on Turbulent Impurity Transport in a Tokamak

Cited by 5 publications

References 29 publications

Global effects on neoclassical transport in the pedestal with impurities

Global effects on neoclassical transport in the pedestal with impurities

Inboard and outboard radial electric field wells in the H- and I-mode pedestal of Alcator C-Mod and poloidal variations of impurity temperature

Neoclassical flows in deuterium–helium plasma density pedestals

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