Neoclassical plateau regime transport in a tokamak pedestal

Abstract

In subsonic tokamak pedestals the radial scale of plasma profiles can be comparable to the ion poloidal Larmor radius, thereby making the radial electrostatic field so strong that the E × B drift has to be retained in the ion kinetic equation in the same order as the parallel streaming. The modifications of neoclassical plateau regime transport -such as the ion heat flux, and the poloidal ion and impurity flowsare evaluated in the presence of a strong radial electric field. The altered poloidal ion flow can lead to a significant increase in the bootstrap current in the pedestal where the spatial profile variation is strong because of the enhanced coefficient of the ion temperature gradient term near the electric field minimum. Unlike the banana regime, orbit squeezing does not affect the plateau regime results.

The effect of the radial electric field on neoclassical flows in a tokamak pedestal

“…The difference between the solid and dashed line profiles in Figures 1c, d, e, and f is quite clear. The effect of the pedestal electric field [5,6] dramatically improves the agreement and is large enough to explain the observed discrepancies pointed out in previous work [1]. However, as we show in Figure 1f, the E r effect in the banana regime can sometimes result in peak magnitudes that are still less than the magnitude of the peak in the measured velocity profile.…”

“…This discrepancy indicates that the net flow of bulk ions is substantially less than, or even in the opposite direction to, the conventional neoclassical result. A similar change in the background ion flow is predicted by new first-principle studies of banana [5] and plateau [6] regime pedestals, which attribute the new features to a strong radial electric field. Here we demonstrate that these new theoretical results are quantitatively consistent with the experimental observations in C-Mod.…”

“…Reexamination of banana [5] and plateau [6] regime model predictions in the pedestal shows that ion heat flux and poloidal flow, as well as the neoclassical polarization and the zonal flow [9,10], are substantially modified compared to their core counterparts. In particular, it has been pointed out that the modifications to the background ion flow [5] are of the correct sign required to explain the discrepancy between the impurity flow measurements in Alcator C-Mod [1] and conventional neoclassical results [2--4,7].…”

“…In this paper we employ B 2 to avoid ambiguity, when applying the analytical results of Refs. [5,6] to the practical case of the Alcator C-Mod pedestal, where B varies poloidally. In section 4 we compare predictions of (5) and (8) with the experimental data.…”

“…Then, after briefly describing in section 3 the experimental technique used to obtain the impurity flow data [1], in section 4 we compare this data with analytical results of Refs. [5,6]. In the last section we discuss the main factors that may affect the accuracy of the analysis and comment on a further consequence of the pedestal modification of the bulk ion flow.…”