The role of neutral friction in governing parallel flows in the HSX stellarator

Abstract

Experimentally measured parallel ion flows in the standard quasihelically symmetric configuration of the HSX stellarator show a large discrepancy with previous neoclassical calculations, whereas flow in a symmetry degraded configuration is in reasonably good agreement with neoclassical theory. To examine this discrepancy, the neoclassical transport code PENTA has been modified to include collisions with background neutrals. It has been found that the flow damping due to ion-neutral friction significantly modifies the ion parallel flows in the standard configuration. This makes the modeling of the ion parallel flow agree better with the experimental measurements. In the symmetry degraded configuration, higher neoclassical viscosity makes the flow damping arising from neutral friction less important. The bootstrap current was found to be insensitive to the neutral friction as the decrease in ion flow was matched by an increase in the electron flow.

Parallel flow driven by electron cyclotron heating in the helically symmetric experiment

“…The parallel neoclassical viscosity of the QHS configuration is known to be smaller than that of the mirror configuration due to the presence of helical symmetry in the former and not in the latter; thus it can be expected that the parallel flow velocity in the QHS configuration is greater than that observed in the mirror configuration. However, the parallel flow observed in the QHS configuration [11,14] is smaller than that observed in the mirror configuration.…”

“…Figure 6 shows the calculated and observed flow velocity profiles in the QHS and mirror configurations. The neutral damping is also an important factor in evaluations of the plasma flow [14], and the hatched regions in figure 6 correspond to the uncertainty regarding the neutral density. The reference value of the neutral density is calculated using the DEGAS code [34].…”

“…Recently, spontaneous flows have been observed in electron cyclotron heating (ECH) plasmas in many tokamaks and helical devices such as the JT-60U, the large helical device and the helically symmetric experiment (HSX), even though ECH is not considered to act as a momentum source. To establish the underlying mechanism behind these observations, many experimental [6][7][8][9][10][11] and theoretical [12][13][14] studies have been undertaken. In this work, we indicate that ECH can induce the electromagnetic force and it acts as a momentum source in the HSX plasma.…”

Estimation of the <i>j </i>× <i>B </i>Force Produced by Electron Cyclotron Heating in HSX Plasma

“…However, a smaller toroidal flow has been author's e-mail: yamamoto_y@p-grp.nucleng.kyoto-u.ac.jp * ) This article is based on the presentation at the 27th International Toki Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference (APPTC2018). observed in the QHS configuration [6,7], a result that has not been understood well yet.…”

Stepped pressure equilibrium with relaxed flow and applications in reversed-field pinch plasmas

“…Although flow is neoclassically damped in general stellarators, the introduction of quasi-symmetry could allow undamped flow to exist [27,28,29]. This has been confirmed experimentally on the HSX stellarator [30,31]. Such equilibria are intrinsically 3D and with flow.…”