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 [1,2], 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).

Global Effects on the Variation of Ion Density and Electrostatic Potential on the Flux Surface in Helical Plasmas

“…From the top, each row corresponds to r/a = 0.2, 0.4, 0.6, and 0.8, respectively. The leftmost column is the result of the model (I) calculated with EUTERPE in [10]. The next from the leftmost column is the result of the same model obtained with the local version of FORTEC-3D.…”

“…where n 0 = n i0 n e0 . This expression is used in several studies to compute Φ 1 (e.g., [10,11,15]). Some assumptions in the argument above may be questionable, and models based on more relaxed assumptions have also been investigated [12,16].…”

“…1): while plasma A corresponds to a standard ion root plasma, plasma B, of which density is ten times lower than that of plasma A, is closer to an impurity hole plasma. The profiles of those plasmas correspond to the plasma A and B in reference [10], respectively, which investigated the effect of the tangential magnetic drift on Φ 1 given by (17) using the codes EUTERPE and KNOSOS. The study showed that the profile of Φ 1 was largely modified by the inclusion of the tangential magnetic drift.…”

“…However, it has been shown that the non-uniform part of the electrostatic potential on the flux surface, Φ 1 ≡ Φ − Φ 0 (r), may also have significant impact on impurity transport [6]. Several studies have investigated and confirmed that the effect of Φ 1 may have substantial impact on impurity transport, and the results tend to indicate that Φ 1 acts in such a way that impurity flux is driven more inwardly rather than outwardly [10][11][12]. However, all the studies have been conducted with local simulation codes, and no global calculation has been performed yet.…”

“…Furthermore, each study uses different approximations and the results vary accordingly as well. Among them, it has been shown that even the tangential part of the magnetic drift radically changes the profile of Φ 1 and the necessity of global calculation has been suggested [10]. Thus, we have applied a global neoclassical simulation code FORTEC-3D to evaluate Φ 1 for the first time and have compared the global result with previous local simulation results.…”