Flux-surface variations of the electrostatic potential in stellarators: impact on the radial electric field and neoclassical impurity transport

Mollén, A.; Landreman, Matt; Smith, H. M.; García-Regaña, J. M.; Nunami, M.

doi:10.1088/1361-6587/aac700

Cited by 32 publications

(65 citation statements)

References 49 publications

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“…However, these conclusions, drawn from the results presented in refs. [1,20], cover still a very narrow parameter and configuration window of W7-X. In particular all plasmas studied in those references are ion root plasmas foreseen during the future W7-X operation phase OP2.…”

Section: Potential Variations In W7-x: Cerc Plasmas and Effect Of Kinmentioning

confidence: 99%

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: Potential Variations In W7-x: Cerc Plasmas and Effect Of Kinmentioning

confidence: 99%

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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“…2017; Mollén et al. 2018) for tokamaks and stellarators. Turbulent transport is also known to be affected by these variations, see for example Mollén et al.…”

Section: Introductionmentioning

confidence: 99%

Collisional transport of impurities with flux-surface varying density in stellarators

et al. 2018

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High-Z impurities in magnetic confinement devices are prone to develop density variations on the flux-surface, which can significantly affect their transport. In this paper, we generalize earlier analytic stellarator calculations of the neoclassical radial impurity flux in the mixed-collisionality regime (collisional impurities and low-collisionality bulk ions) to include the effect of such flux-surface variations. We find that only in the homogeneous density case is the transport of highly collisional impurities (in the Pfirsch-Schlüter regime) independent of the radial electric field. We study these effects for a Wendelstein 7-X (W7-X) vacuum field, with simple analytic models for the potential perturbation, under the assumption that the impurity density is given by a Boltzmann response to a perturbed potential. In the W7-X case studied, we find that larger amplitude potential perturbations cause the radial electric field to dominate the transport of the impurities. In addition, we find that classical impurity transport can be larger than the neoclassical transport in W7-X. † Email address for correspondence: bstefan@chalmers.se

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“…We optimize n z for a W7-X and an LHD model case, with the logarithmic and potential gradients shown in Fig. 1 and magnetic geometry in from LHD discharge #113208 at t = 4.64 s 25,26 . Note that the cases considered here are not experimental cases, and merely use simulated or experimental data as realistic inputs.…”

Section: Resultsmentioning

confidence: 99%

Optimization of flux-surface density variation in stellarator plasmas with respect to the transport of collisional impurities

et al. 2019

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Avoiding impurity accumulation is a requirement for steady-state stellarator operation. The accumulation of impurities can be heavily affected by variations in their density on the flux-surface. Using recently derived semi-analytic expressions for the transport of a collisional impurity species with high-Z and flux-surface densityvariation in the presence of a low-collisionality bulk ion species, we numerically optimize the impurity density-variation on the flux-surface to minimize the radial peaking factor of the impurities. These optimized density-variations can reduce the core impurity density by 0.75 Z (with Z the impurity charge number) in the Large Helical Device case considered here, and by 0.89 Z in a Wendelstein 7-X standard configuration case. On the other hand, when the same procedure is used to find density-variations that maximize the peaking factor, it is notably increased compared to the case with no density-variation. This highlights the potential importance of measuring and controlling these variations in experiments.

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Flux-surface variations of the electrostatic potential in stellarators: impact on the radial electric field and neoclassical impurity transport

Cited by 32 publications

References 49 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

Collisional transport of impurities with flux-surface varying density in stellarators

Optimization of flux-surface density variation in stellarator plasmas with respect to the transport of collisional impurities

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