Large tangential electric fields in plasmas close to temperature screening

Abstract

Low-collisionality stellarator plasmas usually display a large negative radial electric field that has been expected to cause accumulation of impurities due to their high charge number. In this paper, two combined effects that can potentially modify this scenario are discussed. First, it is shown that, in low collisionality plasmas, the kinetic contribution of the electrons to the radial electric field can make it negative but small, bringing the plasma close to impurity temperature screening (i.e., to a situation in which the ion temperature gradient is the main drive of impurity transport and causes outward flux); in plasmas of very low collisionality, such as those of the Large Helical Device displaying impurity hole, screening may actually occur. Second, the component of the electric field that is tangent to the flux surface (in other words, the variation of the electrostatic potential on the flux surface), although smaller than the radial component, has recently been suggested to be an additional relevant drive for radial impurity transport. Here, it is explained that, especially when the radial electric field is small, the tangential magnetic drift has to be kept in order to correctly compute the tangential electric field, that can be larger than previously expected. This can have a strong impact on impurity transport, as we illustrate by means of simulations using the newly-developed code KNOSOS (KiNetic Orbit-averaging-SOlver for Stellarators).

References in corpus (6)

• 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
• Electrostatic potential variations along flux surfaces in stellarators
• Benchmark of the Local Drift-kinetic Models for Neoclassical Transport Simulation in Helical Plasmas
• Parallel impurity dynamics in the TJ-II stellarator

Cited by in corpus (8)

• KNOSOS: a fast orbit-averaging neoclassical code for stellarator geometry
• Turbulent impurity transport simulations in Wendelstein 7-X plasmas
• Stellarator impurity flux driven by electric fields tangent to magnetic surfaces
• Electrostatic potential variations on stellarator magnetic surfaces in low collisionality regimes
• Turbulent transport of impurities in 3D devices
• On-surface potential and radial electric field variations in electron root stellarator plasmas
• Study on impurity hole plasmas by global neoclassical simulation
• Impact of main ion pressure anisotropy on stellarator impurity transport