Compressible impurity flow in the TJ-II stellarator

Arévalo, Juan Miguel Insúa; Alonso, A.; McCarthy, K. J.; Velasco, J. L.; García-Regaña, J. M.; Landreman, Matt

doi:10.1088/0029-5515/54/1/013008

Cited by 15 publications

(29 citation statements)

References 32 publications

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“…In that reference the asymmetry of the impurity density was assessed by comparing the inboard and outboard parallel velocities of C 6+ . Whereas the model was not able to explain the sign of the measured flow variation, the predicted density asymmetry is qualitatively consistent with the 2D emissivity pattern shown here (see figures 5 and 6 and figure 7 in reference [20]). …”

Section: Summary and Discussionsupporting

confidence: 83%

Parallel impurity dynamics in the TJ-II stellarator

Alonso

Velasco

Calvo

et al. 2016

Plasma Phys. Control. Fusion

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Abstract. We review in a tutorial fashion some of the causes of impurity density variations along field lines and radial impurity transport in the moment approach framework. An explicit and compact form of the parallel inertia force valid for arbitrary toroidal geometry and magnetic coordinates is derived and shown to be non-negligible for typical TJ-II plasma conditions. In the second part of the article, we apply the fluid model including main ion-impurity friction and inertia to observations of asymmetric emissivity patterns in neutral beam heated plasmas of the TJ-II stellarator. The model is able to explain qualitatively several features of the radiation asymmetry, both in stationary and transient conditions, based on the calculated in-surface variations of the impurity density.

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Section: Summary and Discussionsupporting

confidence: 83%

Parallel impurity dynamics in the TJ-II stellarator

Alonso

Velasco

Calvo

et al. 2016

Plasma Phys. Control. Fusion

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“…At higher densities, systematic variations of flow were observed within a magnetic surface (figure 2), which are interpreted as being due to particle number conservation in the presence of impurity density variations along the surface. Modelling based on the above-mentioned mechanisms (see [9] and references therein) resulted in density variations of 20-30% for these higher density NBI plasmas, and return parallel flows of the correct size (∼5 km s −1 ). However, the calculated flow correction had the opposite sign of the observations.…”

Section: Particle Energy and Impurity Transportmentioning

confidence: 95%

Transport, stability and plasma control studies in the TJ-II stellarator

Sánchez¹,

Alegre²,

Alonso³

et al. 2015

Nucl. Fusion

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The main TJ-II results since 2012 are presented in this overview. Impurity confinement is studied showing an isotopic dependence of impurity confinement time, asymmetries in parallel impurity flows in TJ-II ion-root plasmas and impurity density asymmetries within a flux surface. In addition, first observations of electrostatic potential variations within the same magnetic flux surface are presented. Evidence of the impact of three-dimensional magnetic structures on plasma confinement and L-H transitions is also presented. The leading role of the plasma turbulence is emphasized by the observed temporal ordering of the limit cycle oscillations at the L-I-H transition. Comparative studies between tokamaks and stellarators have provided direct experimental evidence for the importance of multi-scale physics to unravel the impact of the isotope effect on transport. Novel solutions for plasma facing components based on the recently installed Li-liquid limiters (LLLs) have been developed on TJ-II, showing the self-screening effect of evaporating liquid lithium, protecting plasma-facing components against heat loads, and tritium inventory control. Regarding plasma stability, magnetic well scan experiments show that traditional stability criteria, on which

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“…It is well-known (Galeev & Sagdeev 1979;Ho & Kulsrud 1987) that, for sufficiently low ion collisionality, tangential drifts have to be kept in the stellarator drift-kinetic equation. If the radial electric field is not small and the aspect ratio is large, one can show that the drift-kinetic equation is radially local and that the tangential magnetic drift † As a matter of fact, this has brought up the relevance of the measurement ofφ (Pedrosa et al 2015) and its impact on impurity density inhomogeneities (Arévalo et al 2014;Alonso et al 2016).…”

Section: Introductionmentioning

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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Compressible impurity flow in the TJ-II stellarator

Cited by 15 publications

References 32 publications

Parallel impurity dynamics in the TJ-II stellarator

Parallel impurity dynamics in the TJ-II stellarator

Transport, stability and plasma control studies in the TJ-II stellarator

Electrostatic potential variations on stellarator magnetic surfaces in low collisionality regimes

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