Electrostatic potential variations on stellarator magnetic surfaces in low collisionality regimes

Abstract

The component of the neoclassical electrostatic potential that is non-constant on the magnetic surface, that we denote by $\tilde{\phi }$, 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 which $\tilde{\phi }$ 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 of $\tilde{\phi }$ on the magnetic surface are analytically derived in the $1/\nu$, $\sqrt{\nu }$ and superbanana-plateau regimes of stellarators close to omnigeneity; i. e. stellarators that have been optimized for neoclassical transport. It is found that the largest $\tilde{\phi }$ 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.

arXiv comment30 pages, 12 figures, 1 table. Published version

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Cited by in corpus (11)

• KNOSOS: a fast orbit-averaging neoclassical code for stellarator geometry
• Turbulent impurity transport simulations in Wendelstein 7-X plasmas
• Robust stellarator optimization via flat mirror magnetic fields
• Turbulent transport of impurities in 3D devices
• On-surface potential and radial electric field variations in electron root stellarator plasmas
• Finite orbit width effects in large aspect ratio stellarators
• An adjoint method for neoclassical stellarator optimization
• Impact of main ion pressure anisotropy on stellarator impurity transport
• Fast simulations for large aspect ratio stellarators with the neoclassical code KNOSOS
• Optimization of flux-surface density variation in stellarator plasmas with respect to the transport of collisional impurities
• A model for the fast evaluation of prompt losses of energetic ions in stellarators