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

Abstract

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 ${\Phi }_1$ as a possible correction term to the total radial electric field. Here ${\Phi }_1$ 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), ${\Phi }_1$ estimations for W7-X are revisited considering Core-Electron-Root-Plasma (CERC) plasmas from its first experimental campaign. The simulations show larger values of ${\Phi }_1$ 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 \cite{Regana_nf_57_056004_2017} for other plasma parameters and another configuration.

arXiv comment23 pages, 10 figures

References in corpus (10)

• Linearized model Fokker-Planck collision operators for gyrokinetic simulations. I. Theory
• Comparison of particle trajectories and collision operators for collisional transport in nonaxisymmetric plasmas
• The effect of tangential drifts on neoclassical transport in stellarators close to omnigeneity
• Impurity transport in a mixed-collisionality stellarator plasma
• Vanishing neoclassical viscosity and physics of the shear layer in stellarators
• Stellarators close to quasisymmetry
• Electrostatic potential variations along flux surfaces in stellarators
• Electrostatic potential variations on stellarator magnetic surfaces in low collisionality regimes
• Large tangential electric fields in plasmas close to temperature screening
• Damping of radial electric field fluctuations in the TJ-II stellarator

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• Fast simulations for large aspect ratio stellarators with the neoclassical code KNOSOS